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<input autocomplete="off" placeholder="搜索..." spellcheck="false" type="text" id="local-search-input"></div></div><div id="local-search-result"></div></div></div></nav></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"><div class="post-block"><link itemprop="mainEntityOfPage" href="https://nc2era.com/3b701acb.html"><span hidden itemprop="author" itemscope itemtype="http://schema.org/Person"><meta itemprop="name" content="AloofJr"><meta itemprop="description" content><meta itemprop="image" content="/images/avatar.gif"></span><span hidden itemprop="publisher" itemscope itemtype="http://schema.org/Organization"><meta itemprop="name" content="从入门到放弃"></span><header class="post-header"><h2 class="post-title" itemprop="name headline">【从入门到放弃-Java】并发编程-锁-synchronized</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">发表于</span> <time title="创建于" itemprop="dateCreated datePublished" datetime="2019-07-14T17:38:00+08:00">2019-07-14</time></span> <span class="post-comments-count"><span class="post-meta-divider">|</span><span class="post-meta-item-icon"><i class="fa fa-comment-o"></i></span><a href="/3b701acb.html#comments" itemprop="discussionUrl"><span class="post-comments-count valine-comment-count" data-xid="/3b701acb.html" itemprop="commentCount"></span></a></span> <span class="post-meta-divider">|</span><span class="page-pv"><i class="fa fa-file-o"></i> 浏览<span class="busuanzi-value" id="busuanzi_value_page_pv"></span></span><div class="post-wordcount"><span class="post-meta-item-icon"><i class="fa fa-file-word-o"></i></span> <span class="post-meta-item-text">字数统计:</span> <span title="字数统计">2.3k</span> <span class="post-meta-divider">|</span><span class="post-meta-item-icon"><i class="fa fa-clock-o"></i></span> <span class="post-meta-item-text">阅读时长 ≈</span> <span title="阅读时长">10</span></div></div></header><div class="post-body" itemprop="articleBody"><h2 id="简介"><a href="#简介" class="headerlink" title="简介"></a>简介</h2><p>上篇<a href="https://nc2era.com/3567c420.html">【从入门到放弃-Java】并发编程-线程安全</a>中，我们了解到，可以通过加锁机制来保护共享对象，来实现线程安全。</p><p>synchronized是java提供的一种内置的锁机制。通过synchronized关键字同步代码块。线程在进入同步代码块之前会自动获得锁，并在退出同步代码块时自动释放锁。内置锁是一种互斥锁。</p><p>本文来深入学习下synchronized。</p><h2 id="使用"><a href="#使用" class="headerlink" title="使用"></a>使用</h2><h3 id="同步方法"><a href="#同步方法" class="headerlink" title="同步方法"></a>同步方法</h3><h4 id="同步非静态方法"><a href="#同步非静态方法" class="headerlink" title="同步非静态方法"></a>同步非静态方法</h4><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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">synchronized</span> <span class="keyword">void</span> <span class="title">add1</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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 class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Synchronized sync = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync.add1();</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> thread1.start();</span><br><span class="line"> thread2.start();</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> System.out.println(count);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>结果符合预期：synchronized作用于非静态方法，锁定的是实例对象，如上所示锁的是sync对象，因此线程能够正确的运行，count的结果总会是20000。</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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">synchronized</span> <span class="keyword">void</span> <span class="title">add1</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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 class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Synchronized sync = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Synchronized sync1 = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync1.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> thread1.start();</span><br><span class="line"> thread2.start();</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> System.out.println(count);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>结果不符合预期：如上所示，作用于非静态方法，锁的是实例化对象，因此当sync和sync1同时运行时，还是会出现线程安全问题，因为锁的是两个不同的实例化对象。</p><h4 id="同步静态方法"><a href="#同步静态方法" class="headerlink" title="同步静态方法"></a>同步静态方法</h4><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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</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">synchronized</span> <span class="keyword">void</span> <span class="title">add1</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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">synchronized</span> <span class="keyword">void</span> <span class="title">add11</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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 class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Synchronized sync = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Synchronized sync1 = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> Synchronized.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> Synchronized.add11();</span><br><span class="line"></span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> thread1.start();</span><br><span class="line"> thread2.start();</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> System.out.println(count);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>结果符合预期：锁静态方法时，锁的是类对象。因此在不同的线程中调用add1和add11依然会得到正确的结果。</p><h3 id="同步代码块"><a href="#同步代码块" class="headerlink" title="同步代码块"></a>同步代码块</h3><h4 id="锁当前实例对象"><a href="#锁当前实例对象" class="headerlink" title="锁当前实例对象"></a>锁当前实例对象</h4><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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">add1</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">synchronized</span> (<span class="keyword">this</span>) {</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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">synchronized</span> <span class="keyword">void</span> <span class="title">add11</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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 class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Synchronized sync = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Synchronized sync1 = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync1.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> thread1.start();</span><br><span class="line"> thread2.start();</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> System.out.println(count);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>结果不符合预期：当synchronized同步方法块时，锁的是实例对象时，如上示例在不同的实例中调用此方法还是会出现线程安全问题。</p><h4 id="锁其它实例对象"><a href="#锁其它实例对象" class="headerlink" title="锁其它实例对象"></a>锁其它实例对象</h4><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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</span><br><span class="line"> <span class="keyword">public</span> String lock = <span class="keyword">new</span> String();</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">add1</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">synchronized</span> (lock) {</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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">synchronized</span> <span class="keyword">void</span> <span class="title">add11</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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 class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Synchronized sync = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Synchronized sync1 = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync1.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> thread1.start();</span><br><span class="line"> thread2.start();</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> System.out.println(count);</span><br><span class="line"></span><br><span class="line"> System.out.println(sync.lock == sync1.lock);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure> <img src="/3b701acb/1.jpg"><p>结果不符合预期：当synchronized同步方法块时，锁的是其它实例对象时，如上示例在不同的实例中调用此方法还是会出现线程安全问题。</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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</span><br><span class="line"> <span class="keyword">public</span> String lock = <span class="string">""</span>;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">add1</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">synchronized</span> (lock) {</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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">synchronized</span> <span class="keyword">void</span> <span class="title">add11</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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 class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Synchronized sync = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Synchronized sync1 = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync1.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> thread1.start();</span><br><span class="line"> thread2.start();</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> System.out.println(count);</span><br><span class="line"></span><br><span class="line"> System.out.println(sync.lock == sync1.lock);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure> <img src="/3b701acb/2.jpg"><p>结果符合预期：当synchronized同步方法块时，锁的虽然是其它实例对象时，但已上实例中，因为String = “” 是存放在常量池中的，实际上锁的还是相同的对象，因此是线程安全的</p><h4 id="锁类对象"><a href="#锁类对象" class="headerlink" title="锁类对象"></a>锁类对象</h4><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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</span><br><span class="line"></span><br><span class="line"> <span class="function"><span class="keyword">private</span> <span class="keyword">void</span> <span class="title">add1</span><span class="params">()</span> </span>{</span><br><span class="line"> <span class="keyword">synchronized</span> (Synchronized.class) {</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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">synchronized</span> <span class="keyword">void</span> <span class="title">add11</span><span class="params">()</span> </span>{</span><br><span class="line"> count++;</span><br><span class="line"> System.out.println(count);</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 class="keyword">throws</span> InterruptedException </span>{</span><br><span class="line"> Synchronized sync = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Synchronized sync1 = <span class="keyword">new</span> Synchronized();</span><br><span class="line"> Thread thread1 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"> Thread thread2 = <span class="keyword">new</span> Thread(() -> {</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>; i< <span class="number">10000</span>; i++) {</span><br><span class="line"> sync1.add1();</span><br><span class="line"> }</span><br><span class="line"> });</span><br><span class="line"></span><br><span class="line"> thread1.start();</span><br><span class="line"> thread2.start();</span><br><span class="line"> Thread.sleep(<span class="number">1000</span>);</span><br><span class="line"> System.out.println(count);</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>结果符合预期：当synchronized同步方法块时，锁的是类对象时，如上示例在不同的实例中调用此方法是线程安全的。</p><h2 id="锁机制"><a href="#锁机制" class="headerlink" title="锁机制"></a>锁机制</h2><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></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">Synchronized</span> </span>{</span><br><span class="line"> <span class="keyword">private</span> <span class="keyword">static</span> <span class="keyword">int</span> count;</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">synchronized</span> (Synchronized.class) {</span><br><span class="line"> count++;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure><p>使用javap -v Synchronized.class反编译class文件。</p> <img src="/3b701acb/3.jpg"><p><strong>可以看到synchronized实际上是通过monitorenter和monitorexit来实现锁机制的。同一时刻，只能有一个线程进入监视区。从而保证线程的同步。</strong></p><p>正常情况下在指令4进入监视区，指令14退出监视区然后指令15直接跳到指令23 return</p><p>但是在异常情况下异常都会跳转到指令18，依次执行到指令20monitorexit释放锁，防止出现异常时未释放的情况。<br>这其实也是synchronized的优点：无论代码执行情况如何，都不会忘记主动释放锁。</p><p>想了解Monitors更多的原理可以<a href="https://www.artima.com/insidejvm/ed2/threadsynch.html" rel="external nofollow noopener noreferrer" target="_blank">点击查看</a></p><h2 id="锁升级"><a href="#锁升级" class="headerlink" title="锁升级"></a>锁升级</h2><p>因为monitor依赖操作系统的Mutex lock实现，是一个比较重的操作，需要切换系统至内核态，开销非常大。因此在jdk1.6引入了偏向锁和轻量级锁。<br>synchronized有四种状态：无锁 -> 偏向锁 -> 轻量级锁 -> 重量级锁。</p><h3 id="无锁"><a href="#无锁" class="headerlink" title="无锁"></a>无锁</h3><p>没有对资源进行锁定，所有线程都能访问和修改。但同时只有一个线程能修改成功</p><h3 id="偏向锁"><a href="#偏向锁" class="headerlink" title="偏向锁"></a>偏向锁</h3><p>在锁竞争不强烈的情况下，通常一个线程会多次获取同一个锁，为了减少获取锁的代价 引入了偏向锁，会在java对象头中记录获取锁的线程的threadID。</p><ul><li>当线程发现对象头的threadID存在时。判断与当前线程是否是同一线程。</li><li>如果是则不需要再次加、解锁。</li><li>如果不是，则判断threadID是否存活。不存活：设置为无锁状态，其他线程竞争设置偏向锁。存活：查找threadID堆栈信息判断是否需要继续持有锁。需要持有则升级threadID线程的锁为轻量级锁。不需要持有则撤销锁，设置为无锁状态等待其它线程竞争。</li></ul><p>因为偏向锁的撤销操作还是比较重的，导致进入安全点，因此在竞争比较激烈时，会影响性能，可以使用-XX:-UseBiasedLocking=false禁用偏向锁。</p><h3 id="轻量级锁"><a href="#轻量级锁" class="headerlink" title="轻量级锁"></a>轻量级锁</h3><p>当偏向锁升级为轻量级锁时，其它线程尝试通过CAS方式设置对象头来获取锁。</p><ul><li>会先在当前线程的栈帧中设置Lock Record，用于存储当前对象头中的mark word的拷贝。</li><li>复制mark word的内容到lock record，并尝试使用cas将mark word的指针指向lock record</li><li>如果替换成功，则获取偏向锁</li><li>替换不成功，则会自旋重试一定次数。</li><li>自旋一定次数或有新的线程来竞争锁时，轻量级锁膨胀为重量级锁。</li></ul><h4 id="CAS"><a href="#CAS" class="headerlink" title="CAS"></a>CAS</h4><p>CAS即compare and swap（比较并替换）。是一种乐观锁机制。通常有三个值</p><ul><li>V：内存中的实际值</li><li>A：旧的预期值</li><li>B：要修改的新值<br>即V与A相等时，则替换V为B。即内存中的实际值与我们的预期值相等时，则替换为新值。</li></ul><p>CAS可能遇到ABA问题，即内存中的值为A，变为B后，又变为了A，此时A为新值，不应该替换。<br>可以采取：A-1，B-2，A-3的方式来避免这个问题</p><h3 id="重量级锁"><a href="#重量级锁" class="headerlink" title="重量级锁"></a>重量级锁</h3><p>自旋是消耗CPU的，因此在自旋一段时间，或者一个线程在自旋时，又有新的线程来竞争锁，则轻量级锁会膨胀为重量级锁。<br>重量级锁，通过monitor实现，monitor底层实际是依赖操作系统的mutex lock（互斥锁）实现。<br>需要从用户态，切换为内核态，成本比较高</p><h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>本文我们一起学习了</p><ul><li>synchronized的几种用法：同步方法、同步代码块。实际上是同步类或同步实例对象。</li><li>锁升级：无锁、偏向锁、轻量级锁、重量级锁以及其膨胀过程。</li></ul><p>synchronized作为内置锁，虽然帮我们解决了线程安全问题，但是带来了性能的损失，因此一定不能滥用。使用时请注意同步块的作用范围。通常，作用范围越小，对性能的影响也就越小（注意权衡获取、释放锁的成本，不能为了缩小作用范围，而频繁的获取、释放）。</p></div><footer class="post-footer"><div class="post-tags"> <a href="/tags/Java/" rel="tag"># Java</a> <a href="/tags/线程安全/" rel="tag"># 线程安全</a> <a href="/tags/并发编程/" rel="tag"># 并发编程</a> <a href="/tags/锁机制/" rel="tag"># 锁机制</a> <a href="/tags/同步/" 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class="nav-number">2.2.3.</span> <span class="nav-text">锁类对象</span></a></li></ol></li></ol></li><li class="nav-item nav-level-2"><a class="nav-link" href="#锁机制"><span class="nav-number">3.</span> <span class="nav-text">锁机制</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#锁升级"><span class="nav-number">4.</span> <span class="nav-text">锁升级</span></a><ol class="nav-child"><li class="nav-item nav-level-3"><a class="nav-link" href="#无锁"><span class="nav-number">4.1.</span> <span class="nav-text">无锁</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#偏向锁"><span class="nav-number">4.2.</span> <span class="nav-text">偏向锁</span></a></li><li class="nav-item nav-level-3"><a class="nav-link" href="#轻量级锁"><span class="nav-number">4.3.</span> <span class="nav-text">轻量级锁</span></a><ol class="nav-child"><li class="nav-item nav-level-4"><a class="nav-link" href="#CAS"><span class="nav-number">4.3.1.</span> <span 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} else if (resultLeft.hitCount !== resultRight.hitCount) {
return resultRight.hitCount - resultLeft.hitCount;
} else {
return resultRight.id - resultLeft.id;
}
});
var searchResultList = '<ul class=\"search-result-list\">';
resultItems.forEach(function (result) {
searchResultList += result.item;
})
searchResultList += "</ul>";
resultContent.innerHTML = searchResultList;
}
}
if ('auto' === 'auto') {
input.addEventListener('input', inputEventFunction);
} else {
$('.search-icon').click(inputEventFunction);
input.addEventListener('keypress', function (event) {
if (event.keyCode === 13) {
inputEventFunction();
}
});
}
// remove loading animation
$(".local-search-pop-overlay").remove();
$('body').css('overflow', '');
proceedsearch();
}
});
}
// handle and trigger popup window;
$('.popup-trigger').click(function(e) {
e.stopPropagation();
if (isfetched === false) {
searchFunc(path, 'local-search-input', 'local-search-result');
} else {
proceedsearch();
};
});
$('.popup-btn-close').click(onPopupClose);
$('.popup').click(function(e){
e.stopPropagation();
});
$(document).on('keyup', function (event) {
var shouldDismissSearchPopup = event.which === 27 &&
$('.search-popup').is(':visible');
if (shouldDismissSearchPopup) {
onPopupClose();
}
});
</script><script>!function(){var t=document.createElement("script"),e=window.location.protocol.split(":")[0];t.src="https"===e?"https://zz.bdstatic.com/linksubmit/push.js":"http://push.zhanzhang.baidu.com/push.js";var s=document.getElementsByTagName("script")[0];s.parentNode.insertBefore(t,s)}()</script></body></html>