ConcurrentHashMap

java.util.concurrent.ConcurrentHashMap支持检索的完全并发和更新的所期望可调整并发的哈希表。此类遵守与Hashtable相同的功能规范，并且包括对应于Hashtable的每个方法的方法版本。不过，尽管所有操作都是线程安全的，但检索操作不必锁定，并且不支持以某种防止所有访问的方式锁定整个表。此类可以通过程序完全与Hashtable进行互操作，这取决于其线程安全，而与其同步细节无关。
检索操作（包括get）通常不会受阻塞，因此，可能与更新操作交迭（包括put和remove）。
检索会影响最近完成的更新操作的结果。对于一些聚合操作，比如putAll和clear，并发检索可能只影响某些条目的插入和移除。
类似地，在创建迭代器/枚举时或自此之后，Iterators和Enumerations返回在某一时间点上影响哈希表状态的元素。
它们不会抛出ConcurrentModificationException。不过，迭代器被设计成每次仅由一个线程使用。
这允许通过可选的concurrencyLevel构造方法参数（默认值为16）来引导更新操作之间的并发，
该参数用作内部调整大小的一个提示。表示在内部进行分区的，试图允许指示无争用并发更新的数量。
因为哈希表中的位置基本上是随意的，所以实际的并发将各不相同。理想情况下，应该选择一个尽可能多地容纳并发修改该表的线程的值。
使用一个比所需要的值高很多的值可能会浪费空间和时间，而使用一个显然低很多的值可能导致线程争用。
对数量级估计过高或估计过低通常都会带来非常显著的影响。当仅有一个线程将执行修改操作，而其他所有线程都只是执行读取操作时，
才认为某个值是合适的。此外，重新调整此类或其他任何种类哈希表的大小都是一个相对较慢的操作，
因此，在可能的时候，提供构造方法中期望表大小的估计值是一个好主意。
此类及其视图和迭代器实现了Map和Iterator接口的所有可选方法。
此类与Hashtable相似，但与HashMap不同，它“不”允许将null用作键或值。
<wbr style="line-height:25px">注意1</wbr><wbr style="line-height:25px">：它“不”允许将null用作键或值。<br style="line-height:25px"><span style="line-height:25px"><wbr style="line-height:25px">注意2</wbr></span><wbr style="line-height:25px">:尽管所有操作都是线程安全的，但检索操作不必锁定，不会受阻塞，因此，可能与更新操作交迭（包括put和remove）。<br style="line-height:25px"><span style="line-height:25px"><wbr style="line-height:25px">注意3</wbr></span><wbr style="line-height:25px">：<span style="color:#993300; line-height:25px">ConcurrentHashMa</span>p允许多个修改操作并发进行，其关键在于使用了锁分离技术。它使用了多个锁来控制对hash表的不同部分进行的修改。<br style="line-height:25px"> 通过可选的<span style="color:#993300; line-height:25px">concurrencyLevel</span>构造方法参数（默认值为16）来设置更新操作之间的并发数。<br style="line-height:25px"><span style="line-height:25px"><wbr style="line-height:25px">注意4</wbr></span><wbr style="line-height:25px">：关于ConcurrentHashMap的原理及实现细节可以参考《<strong><a title="阅读全文" target="_blank" href="http://hubingforever.blog.163.com/blog/static/17104057920107104948143/" style="color:rgb(207,121,28); line-height:25px; text-decoration:none">ConcurrentHashMap实现细节</a></strong>》<br style="line-height:25px"><div style="line-height:25px"><span style="line-height:25px"><span style="line-height:25px">构造函数</span></span></div> <div style="line-height:25px"> <span style="line-height:25px"><span style="line-height:normal; color:rgb(51,51,51); font-family:arial,sans-serif; font-size:13px"></span></span> <table id="pubctors" style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:1em; margin-left:1em; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; font-size:0.9em; border-collapse:collapse; empty-cells:show; width:933px"><tbody style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial"> <tr style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial"> <th colspan="12" style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:left; vertical-align:top; background-color:rgb(222,232,241)"> Public Constructors</th> </tr> <tr style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; background-color:rgb(246,246,246)"> <td style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:right; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"></nobr> </td> <td width="100%" style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:left; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"><span style="line-height:21px; margin-right:2px"><a rel="nofollow" href="http://developer.android.com/reference/java/util/concurrent/ConcurrentHashMap.html#ConcurrentHashMap(int,%20float,%20int)" style="color:rgb(0,102,153); line-height:21px; text-decoration:none">ConcurrentHashMap</a></span>(int initialCapacity, float loadFactor, int concurrencyLevel)</nobr><div style="line-height:25px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:3px; padding-right:1em; padding-bottom:0px; padding-left:1em; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; border-style:initial; border-color:initial"> Creates a new, empty map with the specified initial capacity, load factor and concurrency level.</div> </td> </tr> <tr style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial"> <td style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:right; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"></nobr> </td> <td width="100%" style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:left; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"><span style="line-height:21px; margin-right:2px"><a rel="nofollow" href="http://developer.android.com/reference/java/util/concurrent/ConcurrentHashMap.html#ConcurrentHashMap(int,%20float)" style="color:rgb(0,102,153); line-height:21px; text-decoration:none">ConcurrentHashMap</a></span>(int initialCapacity, float loadFactor)</nobr><div style="line-height:25px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:3px; padding-right:1em; padding-bottom:0px; padding-left:1em; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; border-style:initial; border-color:initial"> Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).</div> </td> </tr> <tr style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; background-color:rgb(246,246,246)"> <td style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:right; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"></nobr> </td> <td width="100%" style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:left; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"><span style="line-height:21px; margin-right:2px"><a rel="nofollow" href="http://developer.android.com/reference/java/util/concurrent/ConcurrentHashMap.html#ConcurrentHashMap(int)" style="color:rgb(0,102,153); line-height:21px; text-decoration:none">ConcurrentHashMap</a></span>(int initialCapacity)</nobr><div style="line-height:25px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:3px; padding-right:1em; padding-bottom:0px; padding-left:1em; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; border-style:initial; border-color:initial"> Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).</div> </td> </tr> <tr style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial"> <td style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:right; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"></nobr> </td> <td width="100%" style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:left; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"><span style="line-height:21px; margin-right:2px"><a rel="nofollow" href="http://developer.android.com/reference/java/util/concurrent/ConcurrentHashMap.html#ConcurrentHashMap()" style="color:rgb(0,102,153); line-height:21px; text-decoration:none">ConcurrentHashMap</a></span>()</nobr><div style="line-height:25px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:3px; padding-right:1em; padding-bottom:0px; padding-left:1em; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; border-style:initial; border-color:initial"> Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).</div> </td> </tr> <tr style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:0px; padding-right:0px; padding-bottom:0px; padding-left:0px; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; background-color:rgb(246,246,246)"> <td style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:right; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"></nobr> </td> <td width="100%" style="line-height:21px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:6px; padding-right:12px; padding-bottom:6px; padding-left:12px; border-top-width:1px; border-right-width:1px; border-bottom-width:1px; border-left-width:1px; border-style:initial; border-color:initial; border-top-style:solid; border-right-style:solid; border-bottom-style:solid; border-left-style:solid; border-top-color:rgb(204,204,204); border-right-color:rgb(204,204,204); border-bottom-color:rgb(204,204,204); border-left-color:rgb(204,204,204); text-align:left; vertical-align:top; background-color:inherit"> <nobr style="line-height:21px"><span style="line-height:21px; margin-right:2px"><a rel="nofollow" href="http://developer.android.com/reference/java/util/concurrent/ConcurrentHashMap.html#ConcurrentHashMap(java.util.Map&lt;?%20extends%20K,%20?%20extends%20V&gt;)" style="color:rgb(0,102,153); line-height:21px; text-decoration:none">ConcurrentHashMap</a></span>(<a rel="nofollow" href="http://developer.android.com/reference/java/util/Map.html" style="color:rgb(0,102,153); line-height:21px; text-decoration:none">Map</a>&lt;?extendsK,?extendsV&gt; m)</nobr><div style="line-height:25px; margin-top:0px; margin-right:0px; margin-bottom:0px; margin-left:0px; padding-top:3px; padding-right:1em; padding-bottom:0px; padding-left:1em; border-top-width:0px; border-right-width:0px; border-bottom-width:0px; border-left-width:0px; border-style:initial; border-color:initial; border-style:initial; border-color:initial"> Creates a new map with the same mappings as the given map.</div> </td> </tr> </tbody></table> </div> <div style="line-height:25px"><span style="line-height:25px">结束！</span></div> </wbr></wbr></wbr></wbr>