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Java-Concurrency-Lock.html
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Java-Concurrency-Lock.html
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(max-width:767px){.header-innerr{width:auto;padding:10px;margin-bottom:-20px}}</style><div class="container sidebar-position-left page-post-detail"><div class="headband"></div><header id="header" class="header"><div class="header-inner"><div class="site-brand-wrapper"><div class="site-meta"><link href="https://fonts.font.im/css?family=Merienda" rel="stylesheet"><div class="custom-logo-site-title"></div><h1 class="site-subtitle" itemprop="description"></h1></div><div class="site-nav-toggle"><button><span class="btn-bar"></span> <span class="btn-bar"></span> <span class="btn-bar"></span></button></div></div><nav class="site-nav"><div class="site-search"><div class="popup search-popup local-search-popup"><div class="local-search-header clearfix"><span class="search-icon"><i class="fa fa-search"></i> </span><span class="popup-btn-close"><i class="fa fa-times-circle"></i></span><div class="local-search-input-wrapper"><input autocomplete="off" placeholder="Search" spellcheck="false" type="text" id="local-search-input"></div></div><div id="local-search-result"></div></div></div></nav></div><div class="header-innerr"></div></header><main id="main" class="main"><div class="main-inner"><div class="content-wrap"><div id="content" class="content"><div id="posts" class="posts-expand"><article class="post post-type-normal" itemscope itemtype="http://schema.org/Article"><link itemprop="mainEntityOfPage" href="http://mrbird.cc/Java-Concurrency-Lock.html"><span hidden itemprop="author" itemscope itemtype="http://schema.org/Person"><meta itemprop="name" content="MrBird"><meta itemprop="description" content=""><meta itemprop="image" content="/images/blogImage.jpg"></span><span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization"><meta itemprop="name" content="MrBird"></span><header class="post-header"><h2 class="post-title" itemprop="name headline">Java Concurrency Lock</h2><div class="post-meta"><span class="post-time"><span class="post-meta-item-icon"><i class="fa fa-calendar-o"></i> </span><span class="post-meta-item-text"></span> <time title="创建于" itemprop="dateCreated datePublished" datetime="2019-03-16T09:37:15+08:00">2019-03-16 </time></span><span></span> <span class="post-meta-divider">|</span> <span class="page-pv"><i class="fa fa-laptop"></i> Visit count <span class="busuanzi-value" id="busuanzi_value_page_pv"></span></span></div></header><div class="post-body" itemprop="articleBody"><p>Lock锁是用来控制多个线程访问共享资源的方式，一般来说，一个锁能够防止多个线程同时访问共享资源（但是有些锁可以允许多个线程并发的访问共享资源，比如读写锁）。在Lock接口出现之前，Java程序是靠synchronized关键字实现锁功能的，而Java SE 5之后，并发包中新增了Lock接口（以及相关实现类）用来实现锁功能，它提供了与synchronized关键字类似的同步功能，只是在使用时需要显式地获取和释放锁。虽然它缺少了（通过synchronized块或者方法所提供的）隐式获取释放锁的便捷性，但是却拥有了锁获取与释放的可操作性、可中断的获取锁以及超时获取锁等多种synchronized关键字所不具备的同步特性。</p><a id="more"></a><h2 id="ReentrantLock"><a href="#ReentrantLock" class="headerlink" title="ReentrantLock"></a>ReentrantLock</h2><p>ReentrantLock字面上意思就是可重入锁（又称为递归锁），表示该锁能够支持一个线程对资源的重复加锁。定义一个ReentrantLock：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ReentrantLock lock = <span class="keyword">new</span> ReentrantLock();</span><br></pre></td></tr></table></figure><p></p><p>默认无参构造函数创建的是非公平锁，构造函数重载方法<code>ReentrantLock(boolean fair)</code>支持传入<code>true</code>创建公平锁。公平锁的意思是多线程在获取锁的时候是公平的，也就是等待时间最长的线程最优先获取锁，类似FIFO。</p><p>使用ReentrantLock可以实现和synchronized一样的功能：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ReentrantLockTest</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> ReentrantLock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> IntStream.range(<span class="number">0</span>, <span class="number">2</span>).forEach(i -> <span class="keyword">new</span> Thread(ReentrantLockTest::needLock).start());</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">needLock</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"开始工作"</span>);</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">5</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><div class="note danger"><p>确保在finally里释放锁，否则容易造成死锁。</p></div><p>上面例子同一时刻只能有一个线程可以获得锁<code>lock</code>，别的线程必须等待锁被释放（<code>unlock</code>）才能开始竞争获取锁。程序运行结果如下所示：</p><p><img src="img/fasdfasdf.gif" alt="sdfsdaaa.gif"></p><p><code>needLock</code>方法和下面通过synchronized关键字实现锁方法效果是一样的：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">needLockBySync</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">synchronized</span> (ReentrantLockTest.class) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"开始工作"</span>);</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">5</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>为什么<code>ReentrantLock</code>又称为递归锁呢？这是因为：一个线程如果获取了某个方法的锁，这个方法内部即使调用了别的需要获取锁的方法，那么这个线程不需要再次等待获取锁，可以直接进去。说着可能有点抽象，下面举个例子：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>{</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Lock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> IntStream.rangeClosed(<span class="number">0</span>,<span class="number">1</span>).forEach(i-> <span class="keyword">new</span> Thread(Test::method1, String.valueOf(i)).start());</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">method1</span><span class="params">()</span> </span>{</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" invoked method1"</span>);</span><br><span class="line"> method2();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">method2</span><span class="params">()</span> </span>{</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" invoked method2"</span>);</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>程序运行结果：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">0 invoked method1</span><br><span class="line">0 invoked method2</span><br><span class="line">1 invoked method1</span><br><span class="line">1 invoked method2</span><br></pre></td></tr></table></figure><p></p><p>上面例子中，method1和method2都加了锁，线程0获取到了method1的锁后，内部可以直接调用method2，无需重新获取锁对象。<code>synchronized</code>也具有相同的特性。</p><p><code>ReentrantLock</code>可以对一个方法不限次的重复加锁，但解锁次数必须和加锁次数一致，否则锁永远不会被释放，别的线程将无法获取该方法的锁，比如：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Test</span> </span>{</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Lock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> IntStream.rangeClosed(<span class="number">0</span>,<span class="number">1</span>).forEach(i-> <span class="keyword">new</span> Thread(Test::method1, String.valueOf(i)).start());</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">method1</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="comment">// 加锁4次</span></span><br><span class="line"> lock.lock();</span><br><span class="line"> lock.lock();</span><br><span class="line"> lock.lock();</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" invoked method1"</span>);</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> <span class="comment">// 解锁3次</span></span><br><span class="line"> lock.unlock();</span><br><span class="line"> lock.unlock();</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>程序运行结果： <img src="img/QQ20200226-112236@2x.png" alt="QQ20200226-112236@2x"> <img src="img/QQ20200226-112323@2x.png" alt="QQ20200226-112323@2x"></p><p>线程1一直处于WAITING状态，因为线程0加锁了4次，但只释放了3次锁，所以线程1一直无法获取到锁。</p><p><code>lock</code>方法是不可被打断的，即调用线程的<code>interrupt</code>方法不起作用：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ReentrantLockTest</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> ReentrantLock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(ReentrantLockTest::testLockUnInterruptibly);</span><br><span class="line"> thread1.start();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"></span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(ReentrantLockTest::testLockUnInterruptibly);</span><br><span class="line"> thread2.start();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"></span><br><span class="line"> thread2.interrupt();</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">testLockUnInterruptibly</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock(); <span class="comment">// 不可以被打断</span></span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"开始工作"</span>);</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) { }</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>运行结果:</p><p><img src="img/QQ截图20190517140102.png" alt="QQ截图20190517140102.png"></p><p><img src="img/QQ截图20190517140138.png" alt="QQ截图20190517140138.png"></p><p>thread2(Thread-1)依旧在继续等待获取锁，没有被打断。</p><p>ReentrantLock提供了可打断获取锁的方法<code>lockInterruptibly</code>：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ReentrantLockTest</span> </span>{</span><br><span class="line"></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> ReentrantLock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(ReentrantLockTest::testLockInterruptibly);</span><br><span class="line"> thread1.start();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"></span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(ReentrantLockTest::testLockInterruptibly);</span><br><span class="line"> thread2.start();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"></span><br><span class="line"> thread2.interrupt();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">testLockInterruptibly</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lockInterruptibly(); <span class="comment">// 可以被打断</span></span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"开始工作"</span>);</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) { }</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p><img src="img/sfasdfasfdasdfa.png" alt="QQ截图20190517140645.png"></p><p>thread2在等待获取锁时被打断，抛出<code>InterruptedException</code>异常。</p><p>ReentrantLock的<code>tryLock</code>方法用于尝试获取锁，返回boolean类型，表示获取锁成功与否：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ReentrantLockTest</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">final</span> ReentrantLock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(ReentrantLockTest::testTryLock, <span class="string">"thread1"</span>);</span><br><span class="line"> thread1.start();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"></span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(ReentrantLockTest::testTryLock, <span class="string">"thread2"</span>);</span><br><span class="line"> thread2.start();</span><br><span class="line"> }</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">testTryLock</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">if</span> (lock.tryLock()) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"开始工作"</span>);</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) { }</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">else</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"没有获取到锁"</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p><img src="img/QQ截图20190517142035.png" alt="QQ截图20190517142035.png"></p><p>thread1抢到锁后进入死循环，一直不释放锁。thread2尝试获取锁失败后直接放弃。</p><p><code>tryLock</code>的重载方法<code>tryLock(long timeout, TimeUnit unit)</code>可以设置尝试获取锁的时间范围，超过这个时间没有获取到锁则返回false。</p><p>ReentrantLock一些别的方法：</p><table><tr><th>方法</th><th>含义</th></tr><tr><td><code>getQueueLength()</code></td><td>等待获取锁线程数量</td></tr><tr><td><code>hasQueuedThreads()</code></td><td>是否有在等待获取锁的线程</td></tr><tr><td><code>hasQueuedThread(Thread thread)</code></td><td>等待获取锁的线程队列里是包含指定的线程</td></tr><tr><td><code>isLocked</code></td><td>当前锁是否被任意一个线程获取到了</td></tr></table><h2 id="Spin-Lock"><a href="#Spin-Lock" class="headerlink" title="Spin Lock"></a>Spin Lock</h2><p>JUC中并没有自旋锁对应的类，而所谓的自旋锁就是：尝试获取锁的线程不会马上阻塞，而是采用循环的方式去尝试获取锁。这种方式的好处是可以减少线程上下文切换的消耗，缺点是循环会消耗CPU资源。一个经典的自旋锁例子就是unsafe类里的CAS思想：</p><p><img src="img/QQ20200226-113140@2x.png" alt="QQ20200226-113140@2x"></p><p>我们可以利用CAS实现一个自旋锁：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">SpinLock</span> </span>{</span><br><span class="line"></span><br><span class="line"> AtomicReference<Thread> reference = <span class="keyword">new</span> AtomicReference<>();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">lock</span><span class="params">()</span> </span>{</span><br><span class="line"> Thread thread = Thread.currentThread();</span><br><span class="line"> System.out.println(thread.getName() + <span class="string">"尝试获取锁"</span>);</span><br><span class="line"> <span class="keyword">while</span> (!reference.compareAndSet(<span class="keyword">null</span>, thread)) {</span><br><span class="line"> <span class="comment">// 自旋锁就是利用CAS思想制造循环，block住代码</span></span><br><span class="line"> }</span><br><span class="line"> System.out.println(thread.getName() + <span class="string">"获取到了锁"</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">unlock</span><span class="params">()</span> </span>{</span><br><span class="line"> Thread thread = Thread.currentThread();</span><br><span class="line"> reference.compareAndSet(thread, <span class="keyword">null</span>);</span><br><span class="line"> System.out.println(thread.getName() + <span class="string">"释放锁"</span>);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> SpinLock lock = <span class="keyword">new</span> SpinLock();</span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"做某事..."</span>);</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">5</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }, <span class="string">"线程1"</span>).start();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"做某事..."</span>);</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">5</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }, <span class="string">"线程2"</span>).start();</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>程序输出如下：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">线程1尝试获取锁</span><br><span class="line">线程1获取到了锁</span><br><span class="line">线程1做某事...</span><br><span class="line">线程2尝试获取锁</span><br><span class="line">线程1释放锁</span><br><span class="line">线程2获取到了锁</span><br><span class="line">线程2做某事...</span><br><span class="line">线程2释放锁</span><br></pre></td></tr></table></figure><p></p><h2 id="ReadWriteLock"><a href="#ReadWriteLock" class="headerlink" title="ReadWriteLock"></a>ReadWriteLock</h2><p>ReadWriteLock为读写锁。ReentrantLock为排他锁，同一时刻只允许一个线程进行访问，而读写锁在同一时刻可以允许多个读线程访问，但是在写线程访问时，所有的读线程和其他写线程均被阻塞。读写锁维护了一对锁，一个读锁和一个写锁，通过分离读锁和写锁，使得并发性相比一般的排他锁有了很大提升。</p><p>简而言之，ReadWriteLock包含读写锁，遵循以下规则：</p><ul><li><p>写的时候不能读</p></li><li><p>写的时候不能写</p></li><li><p>读的时候不能写</p></li><li><p>读的时候可以读</p></li></ul><p>ReadWriteLock为接口，我们使用它的实现类ReentrantReadWriteLock创建读写锁实例：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">ReentrantReadWriteLock lock = <span class="keyword">new</span> ReentrantReadWriteLock(<span class="keyword">true</span>);</span><br><span class="line">ReentrantReadWriteLock.ReadLock readLock = lock.readLock();</span><br><span class="line">ReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();</span><br></pre></td></tr></table></figure><p></p><p>使用读写锁创建一个读写的例子：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ReadWriteLockTest</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> ReentrantReadWriteLock lock = <span class="keyword">new</span> ReentrantReadWriteLock(<span class="keyword">true</span>);</span><br><span class="line"> <span class="comment">// 读锁</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> ReentrantReadWriteLock.ReadLock readLock = lock.readLock();</span><br><span class="line"> <span class="comment">// 写锁</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> ReentrantReadWriteLock.WriteLock writeLock = lock.writeLock();</span><br><span class="line"> <span class="comment">// 存放数据</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> List<Long> data = <span class="keyword">new</span> ArrayList<>();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> write();</span><br><span class="line"> }</span><br><span class="line"> }, <span class="string">"writer"</span>).start();</span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> read();</span><br><span class="line"> }</span><br><span class="line"> }, <span class="string">"reader"</span>).start();</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">write</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> writeLock.lock(); <span class="comment">// 写锁</span></span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> <span class="keyword">long</span> value = System.currentTimeMillis();</span><br><span class="line"> data.add(value);</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" 写入value: "</span> + value);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> writeLock.unlock(); <span class="comment">// 释放写锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">read</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> readLock.lock(); <span class="comment">// 获取读锁</span></span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> String value = data.stream().map(String::valueOf).collect(Collectors.joining(<span class="string">","</span>));</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">"读取data: "</span> + value);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> readLock.unlock(); <span class="comment">// 释放读锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p><img src="img/xxxxxxxxx.gif" alt="xxxxxxxxx.gif"></p><p>ReentrantReadWriteLock还包含了一些别的实用方法：</p><p><img src="img/QQ截图20190517153100.png" alt="QQ截图20190517153100.png"></p><p>功能和上面介绍的ReentrantLock差不多，方法名见名知其意，不再演示了。</p><h2 id="JDK8-StampedLock"><a href="#JDK8-StampedLock" class="headerlink" title="JDK8 StampedLock"></a>JDK8 StampedLock</h2><p>JDK8 新增了一个锁StampedLock，它是对ReadWriteLock的改进。</p><p>使用ReadWriteLock的时候，当读线程数量远大于写线程数量的时候就会出现“写饥饿”现象。因为锁大概率都被读线程抢走了，写线程很难抢到锁，这将使得读写效率非常低下。</p><p>JDK8的StampedLock就是为了解决这个问题而设计的，StampedLock包含<strong>乐观锁</strong>和<strong>悲观锁</strong>：</p><ul><li><p>乐观锁：每次去拿数据的时候，并不获取锁对象，而是判断标记位（stamp）是否有被修改，如果有修改就再去读一次。</p></li><li><p>悲观锁：每次拿数据的时候都去获取锁。</p></li></ul><p>通过乐观锁，当写线程没有写数据的时候，标志位stamp并没有改变，所以即使有再多的读线程在读取数据，它们都可以直接去读数据，而无需获取锁，这就不会使得写线程抢不到锁了。</p><div class="note info"><p>stamp类似一个时间戳的作用，每次写的时候对其+1来改变被操作对象的stamp值。</p></div><p>下面我们通过一个例子来模拟写饥饿的情况：创建20个线程，其中19个线程用于读数据，1个线程用于写数据：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">StampedLockTest</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> StampedLock lock = <span class="keyword">new</span> StampedLock();</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> List<Long> data = <span class="keyword">new</span> ArrayList<>();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> ExecutorService executorService = Executors.newFixedThreadPool(<span class="number">20</span>);</span><br><span class="line"></span><br><span class="line"> Runnable read = StampedLockTest::read;</span><br><span class="line"> Runnable write = StampedLockTest::write;</span><br><span class="line"></span><br><span class="line"> IntStream.range(<span class="number">0</span>, <span class="number">19</span>).forEach(i -> executorService.submit(read));</span><br><span class="line"> executorService.submit(write);</span><br><span class="line"></span><br><span class="line"> executorService.shutdown();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">read</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">long</span> stamped = -<span class="number">1</span>;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> stamped = lock.readLock(); <span class="comment">// 获取悲观锁，阻塞写线程</span></span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> String collect = data.stream().map(String::valueOf).collect(Collectors.joining(<span class="string">","</span>));</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" read value: "</span> + collect);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlockRead(stamped);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">write</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">long</span> stamped = -<span class="number">1</span>;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> stamped = lock.writeLock();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> <span class="keyword">long</span> value = System.currentTimeMillis();</span><br><span class="line"> data.add(value);</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" write value: "</span> + value);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlockWrite(stamped);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>上面例子通过StampedLock调用<code>writeLock</code>、<code>unlockWrite</code>、<code>readLock</code>和<code>unlockRead</code>的时候都会导致StampedLock的stamp值的变化，即每次+1，直到加到最大值，然后从0重新开始。</p><p>上面程序运行结果如下： <img src="img/QQ截图20190517155259.png" alt="QQ截图20190517155259.png"></p><p>可以看到写线程最后才抢到锁并写入数据。</p><p>我们通过乐观锁来改善这个例子：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">StampedLockTest2</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> StampedLock lock = <span class="keyword">new</span> StampedLock();</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> List<Long> data = <span class="keyword">new</span> ArrayList<>();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> ExecutorService executorService = Executors.newFixedThreadPool(<span class="number">20</span>);</span><br><span class="line"></span><br><span class="line"> Runnable read = StampedLockTest2::read;</span><br><span class="line"> Runnable write = StampedLockTest2::write;</span><br><span class="line"></span><br><span class="line"> IntStream.range(<span class="number">0</span>, <span class="number">19</span>).forEach(i -> executorService.submit(read));</span><br><span class="line"> executorService.submit(write);</span><br><span class="line"></span><br><span class="line"> executorService.shutdown();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">read</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">long</span> stamped = lock.tryOptimisticRead(); <span class="comment">// 获取乐观锁</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">// 直接读取值</span></span><br><span class="line"> String collect = data.stream().map(String::valueOf).collect(Collectors.joining(<span class="string">","</span>));</span><br><span class="line"></span><br><span class="line"> <span class="comment">// 如果戳被改变，方法返回false，说明stamped被修改过了（被write方法修改过了，有新的数据写入），</span></span><br><span class="line"> <span class="comment">// 那么重新获取锁并去读取值，否则直接使用上面读取的值。</span></span><br><span class="line"> <span class="keyword">if</span> (!lock.validate(stamped)) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> stamped = lock.readLock();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> collect = data.stream().map(String::valueOf).collect(Collectors.joining(<span class="string">","</span>));</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlockRead(stamped);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" read value: "</span> + collect);</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">write</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">long</span> stamped = -<span class="number">1</span>;</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> stamped = lock.writeLock();</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> <span class="keyword">long</span> value = System.currentTimeMillis();</span><br><span class="line"> data.add(value);</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" write value: "</span> + value);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlockWrite(stamped);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>我们重点关注<code>read</code>方法。<code>read</code>方法一开始通过调用StampedLock的<code>tryOptimisticRead</code>方法来获取标志位stamp，获取乐观锁并不会真正的去获取锁（所以不会阻塞写操作），然后直接去读数据。接着通过StampedLock的<code>validate</code>方法判断标志位stamp是否被修改了（<code>write</code>方法里会修改标志位的值），如果方法返回true，则说明数据没有被修改过，直接使用前面读取的数据即可；否则需要去获取锁重新去读数据，阻止写操作。</p><p>上面例子运行结果如下：</p><p><img src="img/QQ截图20190517160314.png" alt="QQ截图20190517160314.png"></p><p>可以看到，写操作一开始就抢到了锁，并写入了数据。</p><p><strong>简而言之，StampedLock解决了在没有新数据写入时，由于过多读操作抢夺锁而使得写操作一直获取不到锁无法写入新数据的问题。</strong></p><h2 id="Condition"><a href="#Condition" class="headerlink" title="Condition"></a>Condition</h2><p>Condition接口提供了类似Object的<code>wait</code>、<code>notify</code>和<code>notifyAll</code>方法，与Lock配合可以实现生产/消费模式，但是这两者在使用方式以及功能特性上还是有差别的。</p><p>使用Codition实现一个生产消费的例子：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ConditionTest</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> ReentrantLock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Condition condition = lock.newCondition();</span><br><span class="line"> <span class="comment">// 初始数据</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> data = <span class="number">0</span>;</span><br><span class="line"> <span class="comment">// 是否被消费</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">volatile</span> <span class="keyword">boolean</span> consumed = <span class="keyword">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> produceData();</span><br><span class="line"> }</span><br><span class="line"> },<span class="string">"producer"</span>).start();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> consumeData();</span><br><span class="line"> }</span><br><span class="line"> },<span class="string">"consumer"</span>).start();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">produceData</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock(); <span class="comment">// 获取锁</span></span><br><span class="line"> <span class="keyword">while</span> (!consumed) { <span class="comment">// 判断数据是否被消费</span></span><br><span class="line"> condition.await(); <span class="comment">// 如果没有被消费则进入等待</span></span><br><span class="line"> }</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> data++;</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" produce data = "</span> + data);</span><br><span class="line"> consumed = <span class="keyword">false</span>; <span class="comment">// 生产完数据将消费标识置为false</span></span><br><span class="line"> condition.signal(); <span class="comment">// 解除await，用于通知消费者可以开始消费了</span></span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock(); <span class="comment">// 释放锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">consumeData</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock(); <span class="comment">// 获取锁</span></span><br><span class="line"> <span class="keyword">while</span> (consumed) { <span class="comment">// 判断数据是否被消费</span></span><br><span class="line"> condition.await(); <span class="comment">// 如果被消费了则进入等待</span></span><br><span class="line"> }</span><br><span class="line"> TimeUnit.MICROSECONDS.sleep(<span class="number">500</span>);</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" consume data = "</span> + data);</span><br><span class="line"> consumed = <span class="keyword">true</span>; <span class="comment">// 消费完将消费标识置为true</span></span><br><span class="line"> condition.signal(); <span class="comment">// 解除await，用于通知生产者可以开始生产了</span></span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock(); <span class="comment">// 释放锁</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>上面例子中，通过<code>consumed</code>判断数据是否被消费。<code>produceData</code>方法在获取锁后，判断数据是否被消费，如果没有被消费，则调用Condition的<code>await</code>方法进入等待，直到Condition对象的<code>signal</code>方法被调用；<code>consumeData</code>方法逻辑和<code>produceData</code>一致。</p><p>Condition核心用法就是通过<code>await</code>方法让线程进入阻塞等待状态，通过<code>signal</code>解除阻塞状态。上面的例子运行效果如下所示：</p><p><img src="img/eososos.gif" alt="eososos.gif"></p><p>对应上面的例子我们可以思考下面三个问题：</p><ol><li><p>是否可以只使用Lock而不使用Condition？</p></li><li><p>生产者抢到了锁进入await，并没有释放锁，为什么消费者可以获得锁？</p></li><li><p>是否可以只使用Condition不使用Lock？</p></li></ol><p>对于第一个问题：<strong>是否可以只使用Lock而不使用Condition？</strong></p><p>虽然我们可以定义公平的ReentrantLock，但是实际上并不能确保100%公平，只是尽可能的公平。生产消费模型必须为生产者生成完了数据通知消费者消费，消费者消费完了通知生产者生产，这是环环相扣的，不允许出现别的情况。</p><p>对于第二个问题：<strong>生产者抢到了锁进入await，并没有释放锁，为什么消费者可以获得锁？</strong></p><p>假如一开始<code>produceData</code>方法先通过<code>lock.lock()</code>获取到了锁，consumed初始值为false，所以接着方法会调用<code>condition.await()</code>进入阻塞等待。<code>await</code>方法会使得当前线程<strong>释放锁对象</strong>，然后进入休眠状态，直到发生下面三种情况之一才会被解除休眠：</p><ol><li><p>Condition的<code>signal</code>方法被调用；</p></li><li><p>Condition的<code>signalAll</code>方法被调用；</p></li><li><p>其他线程调用了当前线程的<code>interrupt</code>方法。</p></li></ol><p>对于第三个问题：<strong>是否可以只使用Condition不使用Lock？</strong></p><p>既然<code>await</code>会使得线程进入阻塞等待状态，那么是否可以直接使用<code>await</code>，而不使用Lock呢？我们改造上面的例子，去掉获取和释放锁的相关代码：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">ConditionTest</span> </span>{</span><br><span class="line"> <span class="comment">// 公平锁</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> ReentrantLock lock = <span class="keyword">new</span> ReentrantLock(<span class="keyword">true</span>);</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> Condition condition = lock.newCondition();</span><br><span class="line"> <span class="comment">// 初始数据</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> data = <span class="number">0</span>;</span><br><span class="line"> <span class="comment">// 是否被消费</span></span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">volatile</span> <span class="keyword">boolean</span> consumed = <span class="keyword">false</span>;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> <span class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> produceData();</span><br><span class="line"> }</span><br><span class="line"> }).start();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> consumeData();</span><br><span class="line"> }</span><br><span class="line"> }).start();</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">produceData</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">while</span> (!consumed) {</span><br><span class="line"> condition.await();</span><br><span class="line"> }</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> data++;</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" produce data = "</span> + data);</span><br><span class="line"> consumed = <span class="keyword">false</span>;</span><br><span class="line"> condition.signal();</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">consumeData</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">while</span> (consumed) {</span><br><span class="line"> condition.await();</span><br><span class="line"> }</span><br><span class="line"> TimeUnit.MICROSECONDS.sleep(<span class="number">500</span>);</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" consume data = "</span> + data);</span><br><span class="line"> consumed = <span class="keyword">true</span>;</span><br><span class="line"> condition.signal();</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p><img src="img/QQ截图20190517152744.png" alt="QQ截图20190517152744.png"></p><p>可以看到，程序抛出<code>IllegalMonitorStateException</code>异常，所以Condition必须配合Lock使用。</p><p>正如前面说的，Condition的功能类似于Object对象的<code>wait</code>和<code>notify</code>方法，下面我们使用Object对象的<code>wait</code>和<code>notify</code>方法实现一个类似上面生产消费的功能：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">WaitNotifyExample</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> data = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">volatile</span> <span class="keyword">boolean</span> used = <span class="keyword">false</span>;</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">static</span> Object MONITOR = <span class="keyword">new</span> Object();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> produceData();</span><br><span class="line"> }</span><br><span class="line"> }, <span class="string">"thread1"</span>).start();</span><br><span class="line"></span><br><span class="line"> <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">while</span> (<span class="keyword">true</span>) {</span><br><span class="line"> consumeData();</span><br><span class="line"> }</span><br><span class="line"> }, <span class="string">"thread2"</span>).start();</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">produceData</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">synchronized</span> (MONITOR) {</span><br><span class="line"> <span class="keyword">while</span> (!used) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> MONITOR.wait();</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> data++;</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" 生产data = "</span> + data);</span><br><span class="line"> used = <span class="keyword">false</span>;</span><br><span class="line"> MONITOR.notifyAll();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">consumeData</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">synchronized</span> (MONITOR) {</span><br><span class="line"> <span class="keyword">while</span> (used) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> MONITOR.wait();</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> TimeUnit.SECONDS.sleep(<span class="number">1</span>);</span><br><span class="line"> } <span class="keyword">catch</span> (InterruptedException e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> }</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" 消费data = "</span> + data);</span><br><span class="line"> used = <span class="keyword">true</span>;</span><br><span class="line"> MONITOR.notifyAll();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p></p><p>效果如下所示：</p><p><img src="img/baoningn.gif" alt="baoningn.gif"></p><p><code>Condition</code>还可以绑定多个条件，并唤醒指定的线程，举个三个线程循环干活的例子：</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">public</span> <span class="class"><span class="keyword">class</span> <span class="title">Loop</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">volatile</span> String value = <span class="string">"a"</span>;</span><br><span class="line"> <span class="keyword">private</span> Lock lock = <span class="keyword">new</span> ReentrantLock();</span><br><span class="line"> <span class="comment">// 条件1</span></span><br><span class="line"> <span class="keyword">private</span> Condition condition1 = lock.newCondition();</span><br><span class="line"> <span class="comment">// 条件2</span></span><br><span class="line"> <span class="keyword">private</span> Condition condition2 = lock.newCondition();</span><br><span class="line"> <span class="comment">// 条件3</span></span><br><span class="line"> <span class="keyword">private</span> Condition condition3 = lock.newCondition();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">printA</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">while</span> (!value.equals(<span class="string">"a"</span>)) {</span><br><span class="line"> condition1.await();</span><br><span class="line"> }</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" print a"</span>);</span><br><span class="line"> value = <span class="string">"b"</span>;</span><br><span class="line"> condition2.signal();</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">printB</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">while</span> (!value.equals(<span class="string">"b"</span>)) {</span><br><span class="line"> condition2.await();</span><br><span class="line"> }</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" print b"</span>);</span><br><span class="line"> value = <span class="string">"c"</span>;</span><br><span class="line"> condition3.signal();</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">printC</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> lock.lock();</span><br><span class="line"> <span class="keyword">while</span> (!value.equals(<span class="string">"c"</span>)) {</span><br><span class="line"> condition3.await();</span><br><span class="line"> }</span><br><span class="line"> System.out.println(Thread.currentThread().getName() + <span class="string">" print c"</span>);</span><br><span class="line"> value = <span class="string">"a"</span>;</span><br><span class="line"> condition1.signal();</span><br><span class="line"> } <span class="keyword">catch</span> (Exception e) {</span><br><span class="line"> e.printStackTrace();</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> lock.unlock();</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">static</span> <span class="keyword">void</span> <span class="title">main</span><span class="params">(String[] args)</span> </span>{</span><br><span class="line"> Loop loop = <span class="keyword">new</span> Loop();</span><br><span class="line"> IntStream.rangeClosed(<span class="number">0</span>, <span class="number">2</span>).forEach(i -> <span class="keyword">new</span> Thread(loop::printA, <span class="string">"线程A"</span>).start());</span><br><span class="line"> IntStream.rangeClosed(<span class="number">0</span>, <span class="number">2</span>).forEach(i -> <span class="keyword">new</span> Thread(loop::printB, <span class="string">"线程B"</span>).start());</span><br><span class="line"> IntStream.rangeClosed(<span class="number">0</span>, <span class="number">2</span>).forEach(i -> <span class="keyword">new</span> Thread(loop::printC, <span class="string">"线程C"</span>).start());</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>程序输出如下：</p><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">线程A print a</span><br><span class="line">线程B print b</span><br><span class="line">线程C print c</span><br><span class="line">线程A print a</span><br><span class="line">线程B print b</span><br><span class="line">线程C print c</span><br><span class="line">线程A print a</span><br><span class="line">线程B print b</span><br><span class="line">线程C print c</span><br></pre></td></tr></table></figure><p></p><h2 id="synchronized和Lock区别"><a href="#synchronized和Lock区别" class="headerlink" title="synchronized和Lock区别"></a>synchronized和Lock区别</h2><p>synchronized和Lock都是用于控制多个线程访问共享资源的方式，但他们还是有区别的，主要可以从下面几个方面比较：</p><p><strong>1.构成不一样</strong></p><p><code>synchronized</code>是Java关键字，属于JVM层面，底层是由monitorenter和monitorexit指令完成（查看字节码证实）：</p><p><img src="img/QQ20200228-141512@2x.png" alt="QQ20200228-141512@2x"></p><p>而Lock是JUC下的Java接口类，是API层面的🔐。</p><p><strong>2.使用方式不同</strong></p><p><code>synchronized</code>不需要我们手动释放锁，当<code>synchronized</code>代码执行完后，当前线程会自动释放锁；</p><p><code>ReentrantLock</code>需要手动释放锁，不然会造成死锁。</p><p><strong>3.可中断性</strong></p><p><code>synchronized</code>是不可中断的，除非同步方法内抛出异常或者程序正常运行完成； <code>ReentrantLock</code>是可以中断的，比如<code>lockInterruptibly()</code>方法。</p><p><strong>4.公平否</strong></p><p><code>synchronized</code>是非公平锁；</p><p><code>ReentrantLock</code>可以通过构造方法<code>ReentrantLock(boolean fair)</code>设置公平与否。</p><p><strong>5.灵活性</strong></p><p><code>synchronized</code>不可以设置条件；</p><p><code>ReentrantLock</code>可以通过condition绑定多条件，精确唤醒指定线程。</p><script>$(".post-body 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