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t=document.getElementsByTagName("script")[0];t.parentNode.insertBefore(e,t)}()</script></head><body itemscope itemtype="http://schema.org/WebPage" lang="zh-Hans"><div class="container sidebar-position-left page-post-detail"><div class="headband"></div><header id="header" class="header" itemscope itemtype="http://schema.org/WPHeader"><div class="header-inner"><div class="site-brand-wrapper"><div class="site-meta"><div class="custom-logo-site-title"><a href="/" class="brand" rel="start"><span class="logo-line-before"><i></i></span> <span class="site-title">从入门到放弃</span><span class="logo-line-after"><i></i></span></a></div><h1 class="site-subtitle" itemprop="description">从入门到放弃-程序员进阶之路</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"><ul id="menu" class="menu"><li class="menu-item menu-item-home"><a href="/" rel="section"><i class="menu-item-icon fa fa-fw 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fa-calendar-o"></i></span> <span class="post-meta-item-text">发表于</span> <time title="创建于" itemprop="dateCreated datePublished" datetime="2019-07-23T22:05:48+08:00">2019-07-23</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="/304643e5.html#comments" itemprop="discussionUrl"><span class="post-comments-count valine-comment-count" data-xid="/304643e5.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">3.3k</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">16</span></div></div></header><div class="post-body" itemprop="articleBody"><h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>上文<a href="https://nc2era.com/3b701acb.html">【从入门到放弃-Java】并发编程-锁-synchronized</a>中，我们介绍了可以使用内置锁synchronized同步类或代码块儿，到达线程安全的目的。</p><p>jdk帮我们把常用的一些模块封装成同步容器，如Vector、Hashtable、Collections.synchronizedXxx等。实现方式主要是将常用的容器类加了Synchronized同步。但我们知道，synchronized的频繁使用及竞争较为激烈时，对性能的影响比较大。</p><p>jdk1.5之后为我们提供了多种并发容器类，来提升同步容器的性能，这些类主要在java.util.concurrent包（简称juc，包内还有很多其它的并发工具类）中。我们本文先来学习下最常用的并发容器-ConcurrentHashMap。</p><h2 id="ConcurrentHashMap"><a href="#ConcurrentHashMap" class="headerlink" title="ConcurrentHashMap"></a>ConcurrentHashMap</h2><h3 id="put"><a href="#put" class="headerlink" title="put"></a>put</h3><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><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br><span class="line">101</span><br><span class="line">102</span><br><span class="line">103</span><br><span class="line">104</span><br><span class="line">105</span><br><span class="line">106</span><br><span class="line">107</span><br><span class="line">108</span><br><span class="line">109</span><br><span class="line">110</span><br><span class="line">111</span><br><span class="line">112</span><br><span class="line">113</span><br><span class="line">114</span><br><span class="line">115</span><br><span class="line">116</span><br><span class="line">117</span><br><span class="line">118</span><br><span class="line">119</span><br><span class="line">120</span><br><span class="line">121</span><br><span class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * Maps the specified key to the specified value in this table.</span></span><br><span class="line"><span class="comment"> * Neither the key nor the value can be null.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <p>The value can be retrieved by calling the {<span class="doctag">@code</span> get} method</span></span><br><span class="line"><span class="comment"> * with a key that is equal to the original key.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@param</span> key key with which the specified value is to be associated</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@param</span> value value to be associated with the specified key</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@return</span> the previous value associated with {<span class="doctag">@code</span> key}, or</span></span><br><span class="line"><span class="comment"> * {<span class="doctag">@code</span> null} if there was no mapping for {<span class="doctag">@code</span> key}</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@throws</span> NullPointerException if the specified key or value is null</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> </span><br><span class="line"><span class="comment">// key和value都不能是null</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> V <span class="title">put</span><span class="params">(K key, V value)</span> </span>{</span><br><span class="line"> <span class="keyword">return</span> putVal(key, value, <span class="keyword">false</span>);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">/** Implementation for put and putIfAbsent */</span></span><br><span class="line"><span class="function"><span class="keyword">final</span> V <span class="title">putVal</span><span class="params">(K key, V value, <span class="keyword">boolean</span> onlyIfAbsent)</span> </span>{</span><br><span class="line"> <span class="comment">//如果key或者value是null则立即抛出空指针异常</span></span><br><span class="line"> <span class="keyword">if</span> (key == <span class="keyword">null</span> || value == <span class="keyword">null</span>) <span class="keyword">throw</span> <span class="keyword">new</span> NullPointerException();</span><br><span class="line"> </span><br><span class="line"> <span class="comment">//求hash值，将哈希的高位扩展到低位，并将高位强制为0。主要是为了减少hash冲突。</span></span><br><span class="line"> <span class="keyword">int</span> hash = spread(key.hashCode());</span><br><span class="line"> <span class="keyword">int</span> binCount = <span class="number">0</span>;</span><br><span class="line"> </span><br><span class="line"> <span class="comment">//Node是Map.Entry的实现类，存放key、value。但key、value都不能是null。table的个数是2的n次方</span></span><br><span class="line"> <span class="keyword">for</span> (Node<K,V>[] tab = table;;) {</span><br><span class="line"> Node<K,V> f; <span class="keyword">int</span> n, i, fh; K fk; V fv;</span><br><span class="line"> <span class="comment">//Node会延迟初始化、即在第一次插入数据的时候进行初始化</span></span><br><span class="line"> <span class="keyword">if</span> (tab == <span class="keyword">null</span> || (n = tab.length) == <span class="number">0</span>)</span><br><span class="line"> tab = initTable();</span><br><span class="line"> <span class="comment">//以原子的方式获取Node数组n-1位置的node，如果未null，尝试插入新值</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> ((f = tabAt(tab, i = (n - <span class="number">1</span>) & hash)) == <span class="keyword">null</span>) {</span><br><span class="line"> <span class="comment">//使用cas的方式设置新node的key、value值</span></span><br><span class="line"> <span class="keyword">if</span> (casTabAt(tab, i, <span class="keyword">null</span>, <span class="keyword">new</span> Node<K,V>(hash, key, value)))</span><br><span class="line"> <span class="keyword">break</span>; <span class="comment">// no lock when adding to empty bin</span></span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//如果Node是一个ForwardingNode，即有其它线程在扩容，则一起进行扩容操作</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> ((fh = f.hash) == MOVED)</span><br><span class="line"> <span class="comment">//如果当前正在扩容，则当前线程加入一起帮助扩容。</span></span><br><span class="line"> tab = helpTransfer(tab, f);</span><br><span class="line"> <span class="comment">//当使用putIfAbsent时，如果map中存在key，则返回对应的value</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (onlyIfAbsent <span class="comment">// check first node without acquiring lock</span></span><br><span class="line"> && fh == hash</span><br><span class="line"> && ((fk = f.key) == key || (fk != <span class="keyword">null</span> && key.equals(fk)))</span><br><span class="line"> && (fv = f.val) != <span class="keyword">null</span>)</span><br><span class="line"> <span class="keyword">return</span> fv;</span><br><span class="line"> <span class="keyword">else</span> {</span><br><span class="line"> V oldVal = <span class="keyword">null</span>;</span><br><span class="line"> <span class="comment">/** </span></span><br><span class="line"><span class="comment"> * currentHashMap在JDK1.8中使用synchronized对需要修改的Node加锁同步，替代了JDK1.7及之前版本采用分段锁的方式。两种方式对比：</span></span><br><span class="line"><span class="comment"> * 1、1.7采用数组+Segment+分段锁的方式实现，分段锁及将几个map分为多个类似hashmap的结构，内部是多个Entry链表数组。加锁时，使用ReentrantLock对访问的Segment加锁，其它Segment可以正常操作。缺点是寻找节点需要两次hash，一次找到Segment，一次找到Entry链表的头部。</span></span><br><span class="line"><span class="comment"> * 2、1.8采用数组+链表或红黑树的方式实现。使用Node替代了Segment，采用了CAS及synchronized进行同步。当Node链表的长度大于阙值（默认为8）时，会将链表转化为红黑树，提升查找性能。</span></span><br><span class="line"><span class="comment"> * </span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"> <span class="comment">//通过synchronized的方式，对当前Node进行加锁操作。</span></span><br><span class="line"> <span class="keyword">synchronized</span> (f) {</span><br><span class="line"> <span class="comment">//判断f节点是否已被其它线程修改</span></span><br><span class="line"> <span class="keyword">if</span> (tabAt(tab, i) == f) {</span><br><span class="line"> <span class="comment">//如果当前Node还是链表结构时</span></span><br><span class="line"> <span class="keyword">if</span> (fh >= <span class="number">0</span>) {</span><br><span class="line"> binCount = <span class="number">1</span>;</span><br><span class="line"> <span class="comment">//遍历Node链表，设置value</span></span><br><span class="line"> <span class="keyword">for</span> (Node<K,V> e = f;; ++binCount) {</span><br><span class="line"> K ek;</span><br><span class="line"> <span class="comment">//如果当前节点的key与我们要设置的key相等时，则将值设置为value。</span></span><br><span class="line"> <span class="keyword">if</span> (e.hash == hash &&</span><br><span class="line"> ((ek = e.key) == key ||</span><br><span class="line"> (ek != <span class="keyword">null</span> && key.equals(ek)))) {</span><br><span class="line"> oldVal = e.val;</span><br><span class="line"> <span class="keyword">if</span> (!onlyIfAbsent)</span><br><span class="line"> e.val = value;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> }</span><br><span class="line"> Node<K,V> pred = e;</span><br><span class="line"> <span class="comment">//设置e为Node链表中的下一个元素，继续判断key是否相等，直到找到相等的key设置值。但如果链表中没有相等的key时，则在链表尾部新增一个元素，并设置值。</span></span><br><span class="line"> <span class="keyword">if</span> ((e = e.next) == <span class="keyword">null</span>) {</span><br><span class="line"> pred.next = <span class="keyword">new</span> Node<K,V>(hash, key, value);</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//如果当前Node为红黑树结构时</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (f <span class="keyword">instanceof</span> TreeBin) {</span><br><span class="line"> Node<K,V> p;</span><br><span class="line"> binCount = <span class="number">2</span>;</span><br><span class="line"> <span class="comment">//设置值</span></span><br><span class="line"> <span class="keyword">if</span> ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,</span><br><span class="line"> value)) != <span class="keyword">null</span>) {</span><br><span class="line"> oldVal = p.val;</span><br><span class="line"> <span class="keyword">if</span> (!onlyIfAbsent)</span><br><span class="line"> p.val = value;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (f <span class="keyword">instanceof</span> ReservationNode)</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> IllegalStateException(<span class="string">"Recursive update"</span>);</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//如果Node链表的长度大于8时，判断是链表结构扩容，或者转为红黑树结构</span></span><br><span class="line"> <span class="keyword">if</span> (binCount != <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">if</span> (binCount >= TREEIFY_THRESHOLD)</span><br><span class="line"> treeifyBin(tab, i);</span><br><span class="line"> <span class="keyword">if</span> (oldVal != <span class="keyword">null</span>)</span><br><span class="line"> <span class="keyword">return</span> oldVal;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> addCount(<span class="number">1L</span>, binCount);</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * Copies all of the mappings from the specified map to this one.</span></span><br><span class="line"><span class="comment"> * These mappings replace any mappings that this map had for any of the</span></span><br><span class="line"><span class="comment"> * keys currently in the specified map.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@param</span> m mappings to be stored in this map</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> <span class="keyword">void</span> <span class="title">putAll</span><span class="params">(Map<? extends K, ? extends V> m)</span> </span>{</span><br><span class="line"> tryPresize(m.size());</span><br><span class="line"> <span class="keyword">for</span> (Map.Entry<? extends K, ? extends V> e : m.entrySet())</span><br><span class="line"> putVal(e.getKey(), e.getValue(), <span class="keyword">false</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure><h3 id="get"><a href="#get" class="headerlink" title="get"></a>get</h3><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="comment">/**</span></span><br><span class="line"><span class="comment"> * Returns the value to which the specified key is mapped,</span></span><br><span class="line"><span class="comment"> * or {<span class="doctag">@code</span> null} if this map contains no mapping for the key.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <p>More formally, if this map contains a mapping from a key</span></span><br><span class="line"><span class="comment"> * {<span class="doctag">@code</span> k} to a value {<span class="doctag">@code</span> v} such that {<span class="doctag">@code</span> key.equals(k)},</span></span><br><span class="line"><span class="comment"> * then this method returns {<span class="doctag">@code</span> v}; otherwise it returns</span></span><br><span class="line"><span class="comment"> * {<span class="doctag">@code</span> null}. (There can be at most one such mapping.)</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@throws</span> NullPointerException if the specified key is null</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="keyword">public</span> V <span class="title">get</span><span class="params">(Object key)</span> </span>{</span><br><span class="line"> Node<K,V>[] tab; Node<K,V> e, p; <span class="keyword">int</span> n, eh; K ek;</span><br><span class="line"> <span class="comment">//获取hash值</span></span><br><span class="line"> <span class="keyword">int</span> h = spread(key.hashCode());</span><br><span class="line"> <span class="keyword">if</span> ((tab = table) != <span class="keyword">null</span> && (n = tab.length) > <span class="number">0</span> &&</span><br><span class="line"> (e = tabAt(tab, (n - <span class="number">1</span>) & h)) != <span class="keyword">null</span>) {</span><br><span class="line"> <span class="comment">//如果获取到的Node的hash值和key的相等，则说明是链表。</span></span><br><span class="line"> <span class="keyword">if</span> ((eh = e.hash) == h) {</span><br><span class="line"> <span class="keyword">if</span> ((ek = e.key) == key || (ek != <span class="keyword">null</span> && key.equals(ek)))</span><br><span class="line"> <span class="keyword">return</span> e.val;</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//如果获取Node的hash值小于0则说明是非链式结构</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (eh < <span class="number">0</span>)</span><br><span class="line"> <span class="comment">//不断查找Node的下一个节点，知道找到为止</span></span><br><span class="line"> <span class="keyword">return</span> (p = e.find(h, key)) != <span class="keyword">null</span> ? p.val : <span class="keyword">null</span>;</span><br><span class="line"> <span class="comment">//不断查找Node的下一个节点，直到找到为止（感觉和find重复了。最外层的if中只需要一个Node::find方法就能搞定。知道原因的大神请指正）</span></span><br><span class="line"> <span class="keyword">while</span> ((e = e.next) != <span class="keyword">null</span>) {</span><br><span class="line"> <span class="keyword">if</span> (e.hash == h &&</span><br><span class="line"> ((ek = e.key) == key || (ek != <span class="keyword">null</span> && key.equals(ek))))</span><br><span class="line"> <span class="keyword">return</span> e.val;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> <span class="keyword">null</span>;</span><br><span class="line">}</span><br></pre></td></tr></table></figure><h3 id="treeifyBin"><a href="#treeifyBin" class="headerlink" title="treeifyBin"></a>treeifyBin</h3><p>扩容或将结构转为红黑树<br></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="function"><span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">treeifyBin</span><span class="params">(Node<K,V>[] tab, <span class="keyword">int</span> index)</span> </span>{</span><br><span class="line"> Node<K,V> b; <span class="keyword">int</span> n;</span><br><span class="line"> <span class="keyword">if</span> (tab != <span class="keyword">null</span>) {</span><br><span class="line"> <span class="comment">//如果当前Node数组小于64则扩容，大于64时则转换为红黑树结构</span></span><br><span class="line"> <span class="keyword">if</span> ((n = tab.length) < MIN_TREEIFY_CAPACITY)</span><br><span class="line"> <span class="comment">//rehash：resize</span></span><br><span class="line"> tryPresize(n << <span class="number">1</span>);</span><br><span class="line"> <span class="comment">//如果是链表结构则转换为红黑树结构</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> ((b = tabAt(tab, index)) != <span class="keyword">null</span> && b.hash >= <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">synchronized</span> (b) {</span><br><span class="line"> <span class="keyword">if</span> (tabAt(tab, index) == b) {</span><br><span class="line"> TreeNode<K,V> hd = <span class="keyword">null</span>, tl = <span class="keyword">null</span>;</span><br><span class="line"> <span class="keyword">for</span> (Node<K,V> e = b; e != <span class="keyword">null</span>; e = e.next) {</span><br><span class="line"> <span class="comment">//创建树节点，加入红黑树中</span></span><br><span class="line"> TreeNode<K,V> p =</span><br><span class="line"> <span class="keyword">new</span> TreeNode<K,V>(e.hash, e.key, e.val,</span><br><span class="line"> <span class="keyword">null</span>, <span class="keyword">null</span>);</span><br><span class="line"> <span class="keyword">if</span> ((p.prev = tl) == <span class="keyword">null</span>)</span><br><span class="line"> hd = p;</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> tl.next = p;</span><br><span class="line"> tl = p;</span><br><span class="line"> }</span><br><span class="line"> setTabAt(tab, index, <span class="keyword">new</span> TreeBin<K,V>(hd));</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">}</span><br></pre></td></tr></table></figure><p></p><h3 id="tryPresize"><a href="#tryPresize" class="headerlink" title="tryPresize"></a>tryPresize</h3><p>扩容操作<br></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></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * Tries to presize table to accommodate the given number of elements.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * <span class="doctag">@param</span> size number of elements (doesn't need to be perfectly accurate)</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">tryPresize</span><span class="params">(<span class="keyword">int</span> size)</span> </span>{</span><br><span class="line"> <span class="comment">//size在传入前已经翻倍，这里会再次调整，变为为：大于(1.5 * oldSize + 1)的2的幂，且小于MAXIMUM_CAPACITY的大小</span></span><br><span class="line"> <span class="keyword">int</span> c = (size >= (MAXIMUM_CAPACITY >>> <span class="number">1</span>)) ? MAXIMUM_CAPACITY :</span><br><span class="line"> tableSizeFor(size + (size >>> <span class="number">1</span>) + <span class="number">1</span>);</span><br><span class="line"> <span class="keyword">int</span> sc;</span><br><span class="line"> <span class="comment">//当sizeCtl小于等于0时。说明已有线程在初始化或者rehash了</span></span><br><span class="line"> <span class="keyword">while</span> ((sc = sizeCtl) >= <span class="number">0</span>) {</span><br><span class="line"> Node<K,V>[] tab = table; <span class="keyword">int</span> n;</span><br><span class="line"> 如果table是空，即未初始化的话，进行初始化。</span><br><span class="line"> <span class="keyword">if</span> (tab == <span class="keyword">null</span> || (n = tab.length) == <span class="number">0</span>) {</span><br><span class="line"> n = (sc > c) ? sc : c;</span><br><span class="line"> <span class="keyword">if</span> (U.compareAndSetInt(<span class="keyword">this</span>, SIZECTL, sc, -<span class="number">1</span>)) {</span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="keyword">if</span> (table == tab) {</span><br><span class="line"> <span class="meta">@SuppressWarnings</span>(<span class="string">"unchecked"</span>)</span><br><span class="line"> Node<K,V>[] nt = (Node<K,V>[])<span class="keyword">new</span> Node<?,?>[n];</span><br><span class="line"> table = nt;</span><br><span class="line"> <span class="comment">//sc = n - n / 4 = 0.75，在final中，将sizeCtl设置为当前大小的0.75倍。大于这个阙值时，会再次进行扩容。</span></span><br><span class="line"> sc = n - (n >>> <span class="number">2</span>);</span><br><span class="line"> }</span><br><span class="line"> } <span class="keyword">finally</span> {</span><br><span class="line"> sizeCtl = sc;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (c <= sc || n >= MAXIMUM_CAPACITY)</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> <span class="comment">//如果还未开始迁移</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (tab == table) {</span><br><span class="line"> <span class="keyword">int</span> rs = resizeStamp(n);</span><br><span class="line"> <span class="keyword">if</span> (U.compareAndSetInt(<span class="keyword">this</span>, SIZECTL, sc,</span><br><span class="line"> (rs << RESIZE_STAMP_SHIFT) + <span class="number">2</span>))</span><br><span class="line"> <span class="comment">// 开始迁移</span></span><br><span class="line"> transfer(tab, <span class="keyword">null</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><h3 id="transfer"><a href="#transfer" class="headerlink" title="transfer"></a>transfer</h3><p>将Node迁移至新的table中<br></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><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br><span class="line">93</span><br><span class="line">94</span><br><span class="line">95</span><br><span class="line">96</span><br><span class="line">97</span><br><span class="line">98</span><br><span class="line">99</span><br><span class="line">100</span><br><span class="line">101</span><br><span class="line">102</span><br><span class="line">103</span><br><span class="line">104</span><br><span class="line">105</span><br><span class="line">106</span><br><span class="line">107</span><br><span class="line">108</span><br><span class="line">109</span><br><span class="line">110</span><br><span class="line">111</span><br><span class="line">112</span><br><span class="line">113</span><br><span class="line">114</span><br><span class="line">115</span><br><span class="line">116</span><br><span class="line">117</span><br><span class="line">118</span><br><span class="line">119</span><br><span class="line">120</span><br><span class="line">121</span><br><span class="line">122</span><br><span class="line">123</span><br><span class="line">124</span><br><span class="line">125</span><br><span class="line">126</span><br><span class="line">127</span><br><span class="line">128</span><br><span class="line">129</span><br><span class="line">130</span><br><span class="line">131</span><br><span class="line">132</span><br><span class="line">133</span><br><span class="line">134</span><br><span class="line">135</span><br><span class="line">136</span><br><span class="line">137</span><br><span class="line">138</span><br><span class="line">139</span><br><span class="line">140</span><br><span class="line">141</span><br><span class="line">142</span><br><span class="line">143</span><br><span class="line">144</span><br><span class="line">145</span><br><span class="line">146</span><br><span class="line">147</span><br><span class="line">148</span><br><span class="line">149</span><br><span class="line">150</span><br><span class="line">151</span><br><span class="line">152</span><br><span class="line">153</span><br><span class="line">154</span><br><span class="line">155</span><br><span class="line">156</span><br><span class="line">157</span><br><span class="line">158</span><br><span class="line">159</span><br><span class="line">160</span><br><span class="line">161</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/**</span></span><br><span class="line"><span class="comment"> * Moves and/or copies the nodes in each bin to new table. See</span></span><br><span class="line"><span class="comment"> * above for explanation.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">final</span> <span class="keyword">void</span> <span class="title">transfer</span><span class="params">(Node<K,V>[] tab, Node<K,V>[] nextTab)</span> </span>{</span><br><span class="line"> <span class="keyword">int</span> n = tab.length, stride;</span><br><span class="line"> <span class="comment">//设置线程迁移数据的步长，单核步长为n，多核为(n >>> 3) / NCPU， 最小为16</span></span><br><span class="line"> <span class="keyword">if</span> ((stride = (NCPU > <span class="number">1</span>) ? (n >>> <span class="number">3</span>) / NCPU : n) < MIN_TRANSFER_STRIDE)</span><br><span class="line"> stride = MIN_TRANSFER_STRIDE; <span class="comment">// subdivide range</span></span><br><span class="line"> <span class="comment">//如果要迁移的table还未初始化，则进行初始化动作</span></span><br><span class="line"> <span class="keyword">if</span> (nextTab == <span class="keyword">null</span>) { <span class="comment">// initiating</span></span><br><span class="line"> <span class="keyword">try</span> {</span><br><span class="line"> <span class="meta">@SuppressWarnings</span>(<span class="string">"unchecked"</span>)</span><br><span class="line"> Node<K,V>[] nt = (Node<K,V>[])<span class="keyword">new</span> Node<?,?>[n << <span class="number">1</span>];</span><br><span class="line"> nextTab = nt;</span><br><span class="line"> } <span class="keyword">catch</span> (Throwable ex) { <span class="comment">// try to cope with OOME</span></span><br><span class="line"> sizeCtl = Integer.MAX_VALUE;</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> nextTable = nextTab;</span><br><span class="line"> transferIndex = n;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">int</span> nextn = nextTab.length;</span><br><span class="line"> <span class="comment">//开始迁移，为要迁移的Node创建一个ForwardingNode节点。key和value都是null，hashcode为MOVED，nextTable指向新的table</span></span><br><span class="line"> ForwardingNode<K,V> fwd = <span class="keyword">new</span> ForwardingNode<K,V>(nextTab);</span><br><span class="line"> <span class="comment">//表示一个节点已被迁移完毕，可以迁移下一个了。</span></span><br><span class="line"> <span class="keyword">boolean</span> advance = <span class="keyword">true</span>;</span><br><span class="line"> <span class="comment">//迁移过程是否完毕。</span></span><br><span class="line"> <span class="keyword">boolean</span> finishing = <span class="keyword">false</span>; <span class="comment">// to ensure sweep before committing nextTab</span></span><br><span class="line"> <span class="comment">//i是迁移的起始位置，bound是迁移的末尾。</span></span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> i = <span class="number">0</span>, bound = <span class="number">0</span>;;) {</span><br><span class="line"> Node<K,V> f; <span class="keyword">int</span> fh;</span><br><span class="line"> <span class="keyword">while</span> (advance) {</span><br><span class="line"> <span class="keyword">int</span> nextIndex, nextBound;</span><br><span class="line"> <span class="comment">//其实位置大于结束位置，说明已经迁移完毕</span></span><br><span class="line"> <span class="keyword">if</span> (--i >= bound || finishing)</span><br><span class="line"> advance = <span class="keyword">false</span>;</span><br><span class="line"> <span class="comment">//如果transferIndex小于等于0，则说明节点都已有线程在迁移了</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> ((nextIndex = transferIndex) <= <span class="number">0</span>) {</span><br><span class="line"> i = -<span class="number">1</span>;</span><br><span class="line"> advance = <span class="keyword">false</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (U.compareAndSetInt</span><br><span class="line"> (<span class="keyword">this</span>, TRANSFERINDEX, nextIndex,</span><br><span class="line"> nextBound = (nextIndex > stride ?</span><br><span class="line"> nextIndex - stride : <span class="number">0</span>))) {</span><br><span class="line"> bound = nextBound;</span><br><span class="line"> i = nextIndex - <span class="number">1</span>;</span><br><span class="line"> advance = <span class="keyword">false</span>;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//迁移结束</span></span><br><span class="line"> <span class="keyword">if</span> (i < <span class="number">0</span> || i >= n || i + n >= nextn) {</span><br><span class="line"> <span class="keyword">int</span> sc;</span><br><span class="line"> <span class="comment">//迁移完毕后，将新的table赋值给table成员变量，修改sizeCtl完成迁移</span></span><br><span class="line"> <span class="keyword">if</span> (finishing) {</span><br><span class="line"> nextTable = <span class="keyword">null</span>;</span><br><span class="line"> table = nextTab;</span><br><span class="line"> sizeCtl = (n << <span class="number">1</span>) - (n >>> <span class="number">1</span>);</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">if</span> (U.compareAndSetInt(<span class="keyword">this</span>, SIZECTL, sc = sizeCtl, sc - <span class="number">1</span>)) {</span><br><span class="line"> <span class="comment">//不相等说明还有线程没迁移完毕</span></span><br><span class="line"> <span class="keyword">if</span> ((sc - <span class="number">2</span>) != resizeStamp(n) << RESIZE_STAMP_SHIFT)</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"> <span class="comment">//所有线程迁移完毕后，设置finishing为完成。</span></span><br><span class="line"> finishing = advance = <span class="keyword">true</span>;</span><br><span class="line"> i = n; <span class="comment">// recheck before commit</span></span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//如果tab[i] = null，设置为fwd</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> ((f = tabAt(tab, i)) == <span class="keyword">null</span>)</span><br><span class="line"> advance = casTabAt(tab, i, <span class="keyword">null</span>, fwd);</span><br><span class="line"> <span class="comment">//如果当前节点已经迁移，则处理下一个节点</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> ((fh = f.hash) == MOVED)</span><br><span class="line"> advance = <span class="keyword">true</span>; <span class="comment">// already processed</span></span><br><span class="line"> <span class="keyword">else</span> {</span><br><span class="line"> <span class="comment">//加锁同步处理</span></span><br><span class="line"> <span class="keyword">synchronized</span> (f) {</span><br><span class="line"> <span class="comment">//验证下是否已经被其它线程处理</span></span><br><span class="line"> <span class="keyword">if</span> (tabAt(tab, i) == f) {</span><br><span class="line"> Node<K,V> ln, hn;</span><br><span class="line"> <span class="comment">//如果是链表结构</span></span><br><span class="line"> <span class="keyword">if</span> (fh >= <span class="number">0</span>) {</span><br><span class="line"> <span class="comment">//按照Node中元素hash值的第log(2)(n)位，记为runBit，是0或1将Node链表分为两个新的链表。</span></span><br><span class="line"> <span class="keyword">int</span> runBit = fh & n;</span><br><span class="line"> Node<K,V> lastRun = f;</span><br><span class="line"> <span class="keyword">for</span> (Node<K,V> p = f.next; p != <span class="keyword">null</span>; p = p.next) {</span><br><span class="line"> <span class="keyword">int</span> b = p.hash & n;</span><br><span class="line"> <span class="keyword">if</span> (b != runBit) {</span><br><span class="line"> runBit = b;</span><br><span class="line"> lastRun = p;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//将runBit位为0的链表记为ln，为1的设为hn。这里是标记最后一个不一致的节点，lastRun后节点的runBit都一样，因此不用新修改节点，减少消耗</span></span><br><span class="line"> <span class="keyword">if</span> (runBit == <span class="number">0</span>) {</span><br><span class="line"> ln = lastRun;</span><br><span class="line"> hn = <span class="keyword">null</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> {</span><br><span class="line"> hn = lastRun;</span><br><span class="line"> ln = <span class="keyword">null</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">for</span> (Node<K,V> p = f; p != lastRun; p = p.next) {</span><br><span class="line"> <span class="keyword">int</span> ph = p.hash; K pk = p.key; V pv = p.val;</span><br><span class="line"> <span class="comment">//将runBit位为0的链表记为ln，为1的设为hn。 </span></span><br><span class="line"> <span class="keyword">if</span> ((ph & n) == <span class="number">0</span>)</span><br><span class="line"> ln = <span class="keyword">new</span> Node<K,V>(ph, pk, pv, ln);</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> hn = <span class="keyword">new</span> Node<K,V>(ph, pk, pv, hn);</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//将分开的两个节点设置为table的i和i+n位。</span></span><br><span class="line"> setTabAt(nextTab, i, ln);</span><br><span class="line"> setTabAt(nextTab, i + n, hn);</span><br><span class="line"> setTabAt(tab, i, fwd);</span><br><span class="line"> advance = <span class="keyword">true</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="comment">//如果是红黑树结构</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (f <span class="keyword">instanceof</span> TreeBin) {</span><br><span class="line"> TreeBin<K,V> t = (TreeBin<K,V>)f;</span><br><span class="line"> <span class="comment">//按照Node中元素hash值的第log(2)(n)位，记为runBit，是0或1将Node红黑树分为两颗树。</span></span><br><span class="line"> TreeNode<K,V> lo = <span class="keyword">null</span>, loTail = <span class="keyword">null</span>;</span><br><span class="line"> TreeNode<K,V> hi = <span class="keyword">null</span>, hiTail = <span class="keyword">null</span>;</span><br><span class="line"> <span class="keyword">int</span> lc = <span class="number">0</span>, hc = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">for</span> (Node<K,V> e = t.first; e != <span class="keyword">null</span>; e = e.next) {</span><br><span class="line"> <span class="keyword">int</span> h = e.hash;</span><br><span class="line"> TreeNode<K,V> p = <span class="keyword">new</span> TreeNode<K,V></span><br><span class="line"> (h, e.key, e.val, <span class="keyword">null</span>, <span class="keyword">null</span>);</span><br><span class="line"> <span class="keyword">if</span> ((h & n) == <span class="number">0</span>) {</span><br><span class="line"> <span class="keyword">if</span> ((p.prev = loTail) == <span class="keyword">null</span>)</span><br><span class="line"> lo = p;</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> loTail.next = p;</span><br><span class="line"> loTail = p;</span><br><span class="line"> ++lc;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> {</span><br><span class="line"> <span class="keyword">if</span> ((p.prev = hiTail) == <span class="keyword">null</span>)</span><br><span class="line"> hi = p;</span><br><span class="line"> <span class="keyword">else</span></span><br><span class="line"> hiTail.next = p;</span><br><span class="line"> hiTail = p;</span><br><span class="line"> ++hc;</span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :</span><br><span class="line"> (hc != <span class="number">0</span>) ? <span class="keyword">new</span> TreeBin<K,V>(lo) : t;</span><br><span class="line"> hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :</span><br><span class="line"> (lc != <span class="number">0</span>) ? <span class="keyword">new</span> TreeBin<K,V>(hi) : t;</span><br><span class="line"> setTabAt(nextTab, i, ln);</span><br><span class="line"> setTabAt(nextTab, i + n, hn);</span><br><span class="line"> setTabAt(tab, i, fwd);</span><br><span class="line"> advance = <span class="keyword">true</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (f <span class="keyword">instanceof</span> ReservationNode)</span><br><span class="line"> <span class="keyword">throw</span> <span class="keyword">new</span> IllegalStateException(<span class="string">"Recursive update"</span>);</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">}</span><br></pre></td></tr></table></figure><p></p><h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>本文分析了currentHashMap是如何实现线程安全并提升性能的、如何扩容、JDK1.7和1.8实现方式的区别等</p><ul><li>分Node加synchronize锁，不影响其它node的读写</li><li>Node节点hash冲突的元素数量少于8时，使用链表结构，大于等于8时，转换为红黑树结构提升查找性能</li><li>扩容时，会将table的长度扩大为大于(1.5 * oldSize + 1)的2的幂大小，并将每个Node根据log(2)(n)位是0或1，分为两个Node，放在新table的i和i+n的位置</li><li>JDK1.8将原currentHashMap使用数组+segment+ReentrantLock的方式改为数组+Node+CAS+synchronized的方式。减少了hash次数并采用cas和红黑树等多种优化提升性能</li></ul></div><footer class="post-footer"><div 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