为什么ConcurrentHashMap是弱一致的

ゞ浴缸里的玫瑰 2022-05-23 01:15 125阅读 0赞

转载 :http://ifeve.com/concurrenthashmap-weakly-consistent/

本文将用到Java内存模型的happens-before偏序关系（下文将简称为hb）以及ConcurrentHashMap的底层模型相关的知识。happens-before相关内容参见：[JLS §17.4.5. Happens-before Order][JLS _17.4.5. Happens-before Order]、[深入理解Java内存模型][Java]以及[Happens before][]；ConcurrentHashMap的详细介绍以及底层原理见[深入分析ConcurrentHashMap][ConcurrentHashMap]。本文将从ConcurrentHashMap的get，clear，iterator（entrySet、keySet、values方法）三个方法来分析它们的弱一致问题。

#### ConcurrentHashMap\#get ####

get方法是弱一致的，是什么含义？可能你期望往ConcurrentHashMap底层数据结构中加入一个元素后，立马能对get可见，但ConcurrentHashMap并不能如你所愿。换句话说，put操作将一个元素加入到底层数据结构后，get可能在某段时间内还看不到这个元素，若不考虑内存模型，单从代码逻辑上来看，却是应该可以看得到的。

下面将结合代码和java内存模型相关内容来分析下put/get方法（*本文中所有ConcurrentHashMap相关的代码均来自hotspot1.6.0\_18*）。put方法我们只需关注Segment\#put，get方法只需关注Segment\#get，在继续之前，先要说明一下Segment里有两个volatile变量：**count**和**table**；HashEntry里有一个volatile变量：**value**。

Segment\#put

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Segment\#get

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   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">if</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">(count !=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);">0</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">) {&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(63,95,191);">// read-volatile</code></td> 
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我们如何确定线程1放入某个变量的值是否对线程2可见？文章开头提到的JLS链接中有说到，当a hb c时，a对c可见，那么我们接下来我们只要寻找put和get之间所有可能的执行轨迹上的hb关系。要找出hb关系，我们需要先找出与hb相关的Action。为方便，这里将两段代码放到了一张图片上。

[![concurrentHashMap_put_get-1024x512.png][]][concurrentHashMap_put_get-1024x512.png 1]

可以注意到，同一个Segment实例中的put操作是加了锁的，而对应的get却没有。根据hb关系中的线程间Action类别，可以从上图中找出这些Action，主要是volatile读写和加解锁，也就是图中画了横线的那些。

put操作可以分为两种情况，一是key已经存在，修改对应的value；二是key不存在，将一个新的Entry加入底层数据结构。

key已经存在的情况比较简单，即if (e != null)部分，前面已经说过HashEntry的value是个volatile变量，当线程1给value赋值后，会立马对执行get的线程2可见，而不用等到put方法结束。

key不存在的情况稍微复杂一些，新加一个Entry的逻辑在else中。那么将new HashEntry赋值给tab\[index\]是否能立刻对执行get的线程可见呢？我们只需分析写tab\[index\]与读取tab\[index\]之间是否有hb关系即可。

假设执行put的线程与执行get的线程的轨迹是这样的

<table style="margin:0px 0px 1em;padding:0px;width:579px;border:0px;color:rgb(102,102,102);font-family:Arial, Helvetica, sans-serif;font-size:14px;"> 
 <tbody> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">执行put的线程</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">执行get的线程</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">⑧tab[index] = new HashEntry&lt;K,V&gt;(key, hash, first, value)</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">②count = c</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">③if (count != 0)</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">⑨HashEntry e = getFirst(hash);</td> 
  </tr> 
 </tbody> 
</table>

tab变量是一个普通的变量，虽然给它赋值的是volatile的table。另外，虽然引用类型（数组类型）的变量table是volatile的，但table中的元素不是volatile的，因此⑧只是一个普通的写操作；count变量是volatile的，因此②是一个volatile写；③很显然是一个volatile读；⑨中getFirst方法中读取了table，因此包含一个volatile读。

根据Synchronization Order，对同一个volatile变量，有volatile写 hb volatile读。在这个执行轨迹中，时间上②在③之前发生，且②是写count，③是读count，都是针对同一个volatile变量count，因此有② hb ③；又因为⑧和②是同一个线程中的，③和⑨是同一个线程中的，根据Program Order，有⑧ hb ②，③ hb ⑨。目前我们有了三组关系了⑧ hb ②，② hb ③，③ hb ⑨，再根据hb关系是可传递的（即若有x hb y且y hb z，可得出x hb z），可以得出⑧ hb ⑨。因此，如果按照上述执行轨迹，⑧中写入的数组元素对⑨中的读取操作是可见的。

再考虑这样一个执行轨迹：

<table style="margin:0px 0px 1em;padding:0px;width:579px;border:0px;color:rgb(102,102,102);font-family:Arial, Helvetica, sans-serif;font-size:14px;"> 
 <tbody> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">执行put的线程</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">执行get的线程</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">⑧tab[index] = new HashEntry&lt;K,V&gt;(key, hash, first, value)</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">③if (count != 0)</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">②count = c</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
  </tr> 
  <tr> 
   <td style="border-color:rgb(204,204,204);padding:15px;">&nbsp;</td> 
   <td style="border-color:rgb(204,204,204);padding:15px;">⑨HashEntry e = getFirst(hash);</td> 
  </tr> 
 </tbody> 
</table>

这里只是变换了下执行顺序。每条语句的volatile读写含义同上，但它们之间的hb关系却改变了。Program Order是我们一直拥有的，即我们有⑧ hb ②，③ hb ⑨。但这次对volatile的count的读时间上发生在对count的写之前，我们无法得出② hb ⑨这层关系了。因此，通过count变量，在这个轨迹上是无法得出⑧ hb ⑨的。那么，存不存在其它可替换关系，让我们仍能得出⑧ hb ⑨呢？

我们要找的是，在⑧之后有一条语句或指令x，在⑨之前有一条语句或指令y，存在x hb y。这样我们可以有⑧ hb x，x hb y， y hb ⑨。就让我们来找一下是否存在这样的x和y。图中的⑤、⑥、⑦、①存在volatile读写，但是它们在⑧之前，因此对确立⑧ hb ⑨这个关系没有用处；同理，④在⑨之后，我们要找的是⑨之前的，因此也对这个问题无益。前面已经分析过了②，③之间没法确立hb关系。

在⑧之后，我们发现一个unlock操作，如果能在⑨之前找到一个lock操作，那么我们要找的x就是unlock，要找的y就是lock，因为Synchronization Order中有unlock hb lock的关系。但是，很不幸运，⑨之前没有lock操作。因此，对于这样的轨迹，是没有⑧ hb ⑨关系的，也就是说，如果某个Segment实例中的put将一个Entry加入到了table中，在未执行count赋值操作之前有另一个线程执行了同一个Segment实例中的get，来获取这个刚加入的Entry中的value，那么是有可能取不到的！

此外，如果getFirst(hash)先执行，tab\[index\] = new HashEntry<K,V>(key, hash, first, value)后执行，那么，这个get操作也是看不到put的结果的。

……

正是因为get操作几乎所有时候都是一个无锁操作（get中有一个readValueUnderLock调用，不过这句执行到的几率极小），使得同一个Segment实例上的put和get可以同时进行，这就是get操作是弱一致的根本原因。Java API中对此有一句简单的描述:

> Retrievals reflect the results of the most recently ***completed*** update operations holding upon their onset.

也就是说API上保证get操作一定能看到**已完成**的put操作。已完成的put操作肯定在get读取count之前对count做了写入操作。因此，也就是我们第一个轨迹分析的情况。

#### ConcurrentHashMap\#clear ####

clear方法很简单，看下代码即知。

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">2</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">for</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">(</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">int</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">i =&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);">0</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">; i &lt; segments.length; ++i)</code></td> 
  </tr> 
 </tbody> 
</table>

因为没有全局的锁，在清除完一个segments之后，正在清理下一个segments的时候，已经清理segments可能又被加入了数据，因此clear返回的时候，ConcurrentHashMap中是可能存在数据的。因此，clear方法是弱一致的。

#### ConcurrentHashMap中的迭代器 ####

ConcurrentHashMap中的迭代器主要包括entrySet、keySet、values方法。它们大同小异，这里选择entrySet解释。当我们调用entrySet返回值的iterator方法时，返回的是EntryIterator，在EntryIterator上调用next方法时，最终实际调用到了HashIterator.advance()方法，看下这个方法：

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">02</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">if</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">(nextEntry !=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">null</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">&amp;&amp; (nextEntry = nextEntry.next) !=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">null</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">)</code></td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">05</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">while</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">(nextTableIndex &gt;=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);">0</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">) { </code></td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">06</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">if</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">( (nextEntry = currentTable[nextTableIndex--]) !=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">null</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">)</code></td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">10</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">while</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">(nextSegmentIndex &gt;=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);">0</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">) { </code></td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">12</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">if</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">(seg.count !=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);">0</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">) { </code></td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">14</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">for</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">(</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">int</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">j = currentTable.length -&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);">1</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">; j &gt;=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,153,0);">0</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">; --j) { </code></td> 
  </tr> 
 </tbody> 
</table>

<table style="margin:0px;padding:0px;width:auto;border:0px;background:none;float:none;vertical-align:baseline;height:auto;line-height:1.1em;font-size:1em;"> 
 <tbody style="margin:0px;padding:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
  <tr style="margin:0px;padding:0px;border-top:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-size:1em;"> 
   <td style="border-width:0px;padding:0px;margin:0px;background-image:none;float:none;vertical-align:top;height:auto;width:3em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(120,120,120);"><code style="margin:0px;padding:0px .3em 0px 0px;border:0px;background:none;text-align:right;float:none;vertical-align:baseline;height:auto;width:2.7em;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">15</code></td> 
   <td style="border-top:0px;border-right:0px;border-bottom:0px;border-left-color:rgb(212,208,200);padding:0px 0px 0px .5em;margin:0px;background:none;float:none;vertical-align:top;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);"><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">if</code>&nbsp;<code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">( (nextEntry = currentTable[j]) !=&nbsp;</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-weight:bold;font-size:1em;color:rgb(127,0,85);">null</code><code style="margin:0px;padding:0px;border:0px;background:none;float:none;vertical-align:baseline;height:auto;width:auto;line-height:1.1em;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;font-size:1em;color:rgb(0,0,0);">) { </code></td> 
  </tr> 
 </tbody> 
</table>

这个方法在遍历底层数组。在遍历过程中，如果已经遍历的数组上的内容变化了，迭代器不会抛出ConcurrentModificationException异常。如果未遍历的数组上的内容发生了变化，则有可能反映到迭代过程中。这就是ConcurrentHashMap迭代器弱一致的表现。

#### 总结 ####

ConcurrentHashMap的弱一致性主要是为了提升效率，是一致性与效率之间的一种权衡。要成为强一致性，就得到处使用锁，甚至是全局锁，这就与Hashtable和同步的HashMap一样了。

**原创文章，转载请注明：** 转载自[并发编程网 – ifeve.com][_ ifeve.com]**本文链接地址:** [为什么ConcurrentHashMap是弱一致的][ConcurrentHashMap 1]

[JLS _17.4.5. Happens-before Order]: http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.5
[Java]: http://ifeve.com/java-memory-model-7/
[Happens before]: http://ifeve.com/happens-before/
[ConcurrentHashMap]: http://ifeve.com/concurrenthashmap/
[concurrentHashMap_put_get-1024x512.png]: /images/20220523/80823520138b4efba7aa764782977c96.png
[concurrentHashMap_put_get-1024x512.png 1]: http://ifeve.com/concurrenthashmap-weakly-consistent/concurrenthashmap_put_get/
[_ ifeve.com]: http://ifeve.com/
[ConcurrentHashMap 1]: http://ifeve.com/concurrenthashmap-weakly-consistent/