Java多线程四

1. Java内存模型

  在java中，所有实例域、静态域和数组元素存储在堆内存中，堆内存在线程之间共享（本文使用“共享变量”这个术语代指实例域，静态域和数组元素）。局部变量（Local variables），方法定义参数（java语言规范称之为formal method parameters）和异常处理器参数（exception handler parameters）不会在线程之间共享，它们不会有内存可见性问题，也不受内存模型的影响。Java线程之间的通信由Java内存模型（JMM）控制，JMM决定一个线程对共享变量的写入何时对另一个线程可见。从抽象的角度来看，JMM定义了线程和主内存之间的抽象关系：线程之间的共享变量存储在主内存（main memory）中，每个线程都有一个私有的本地内存（local memory），本地内存中存储了该线程以读/写共享变量的副本。本地内存是JMM的一个抽象概念，并不真实存在。它涵盖了缓存，写缓冲区，寄存器以及其他的硬件和编译器优化。Java内存模型的抽象示意图如下： 
       
  下面通过简单的案例分析。需求：多个线程独立的操作变量sharedData，每个线程都可修改、获得该变量。现实现每个线程单独修改该变量，且更改的值只能在本线程内可见。

2. static方式实现

  我们知道static变量保存在堆内存中，可以被多个线程所共享。将所要共享的变量用static修饰：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 利用static实现线程间数据的共享</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> sharedData;

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">staticShareValuable</span>() {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>; i++) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Thread(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Runnable() {
            @Override
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 线程范围内修改共享变量</span>
                sharedData = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Random().nextInt(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ModelA().<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span>();
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ModelB().<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span>();
            }
        }).start();
    }
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 线程的A模块</span>
class ModelA {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">get</span>(){
        System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ModelA from "</span> + Thread.currentThread().getName() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 线程 "</span> + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" get data :"</span> + sharedData);
    }
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 线程的B模块</span>
class ModelB {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">get</span>(){
        System.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ModelB from "</span> + Thread.currentThread().getName() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 线程 "</span> + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" get data :"</span> + sharedData);
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li></ul>

  运行输出：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ModelA <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">37</span>
ModelA <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">37</span>
ModelB <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">37</span>
ModelB <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">37</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

  由结果可知，static这种方式并不能实现线程单独修改该变量，且更改的值只能在本线程内可见。其实这是显然的，只是为了说明：我们在多线程中使用static共享数据的时候一定要考虑这个问题！

3. Map方式模拟ThreadLocal基本原理

  将数据保存在Map集合当中，将当前的Thread实例作为key，当前线程操作的变量作value，这样就实现了每个线程独立的操作变量。即线程操作数据的隔离。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 模拟ThreadLocal的实现</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> Map<Thread, Integer> context = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<Thread, Integer>();
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 *  每个线程独立的实现更改sharedData，更改的值只能在本线程内可见
 *  利用传统的static不能实现！
 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">mapShareValuable</span>() {

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>; i++) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Thread(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Runnable() {
            <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 线程范围内修改共享变量</span>
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> copySharedData = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Random().nextInt(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">100</span>);
                <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// map集合中放入key-value</span>
                context.put(Thread.currentThread(), copySharedData);
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ModelA().get();
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ModelB().get();
            }
        }).start();
    }

}

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 线程的A模块</span>
class ModelA {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">get</span>(){
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// map集合中根据key(当前线程实例)，取出当前线程所要操作的值</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> sharedData = context.get(Thread.currentThread());
        System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ModelA from "</span> + Thread.currentThread().getName() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 线程 "</span> + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" get data :"</span> + sharedData);
    }
}
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 线程的B模块</span>
class ModelB {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">get</span>(){
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> sharedData = context.get(Thread.currentThread());
        System.out.println(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"ModelB from "</span> + Thread.currentThread().getName() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" 线程 "</span> + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">" get data :"</span> + sharedData);
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li></ul>

  运行输出：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">ModelA <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">61</span>
ModelA <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">33</span>
ModelB <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">61</span>
ModelB <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">from</span> Thread-<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> 线程  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">get</span> data :<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">33</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

  由运行结果可以看出：Thread-0线程的模块A和B获得的数据为Thread-0线程所修改的数据，两个线程单独修改变量，且更改的值只能在本线程内可见。其实这就是ThreadLocal简单原理（只是key值不同，ThreadLocal内部的ThreadLocalMap（a customized hash map）对象，key为ThreadLocal对象）。

4. ThreadLocal

  ThreadLocal为解决多线程程序的并发问题提供了一种新的思路。 
  当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。如果共享的变量有多个，可以将其封装成对象，放入ThreadLocal中。例如Struts2的ActionContext需要保存当前请求线程的request、response，param，application以及action等等。下面的案例简单模拟了Struts2的ActionContext功能。作为上下文封装当前线程的请求Request操作、响应Response，在Action中可通过ActionContext获取这些对象。

<code class="hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 * 模拟Struts2的ActionContext
 * Struts2的ActionContext需要保存当前请求线程的request、response，param，application以及action等等。
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @author</span> markliu
 *
 */</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ThreadLocalTest</span> {</span>

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">main</span>(String[] args) {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 模拟两次请求</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">int</span> i = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>; i < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>; i++) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Thread(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Runnable() {

                <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
                <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">run</span>() {
                    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 以下代码模拟Tomcat封装请求信息等并绑定到对应线程</span>
                    Request request = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Request();
                    request.setRequestMsg(Thread.currentThread().getName() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"线程请求HelloWorld.action资源"</span>);
                    Response response = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Response();
                    response.setResponseMsg(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"向"</span> + Thread.currentThread().getName()+<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"线程"</span> + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"返回资源"</span>);
                    ActionContext context = ActionContext.getContext();
                    context.setRequest(request);
                    context.setResponse(response);

                    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 调用Action的方法</span>
                    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HelloWorldAction().execute();
                }
            }).start();
        }
    }

}

class HelloWorldAction {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String <span class="hljs-title" style="box-sizing: border-box;">execute</span>() {
        ActionContext context = ActionContext.getContext();
        System.out.println(Thread.currentThread().getName()+<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"线程在"</span> + 
                <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Action中获取的请求信息："</span> + context.getRequest().getRequestMsg() + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"-->"</span> +
                <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Action中获取的响应信息信息："</span> + context.getResponse().getResponseMsg());
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"success"</span>;
    }
}
<span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
 * 由于保存的数据有多个，需要多个ThreadLocal，可以
 * 将需要保存的request、response等信息封装成一个上下文对象
 *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @author</span> markliu
 *
 */</span>
class ActionContext {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> ThreadLocal<ActionContext> actionContext = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ThreadLocal<ActionContext>(); 
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 封装实际的request，response等信息</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> Map<String, Object> context = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> HashMap<String, Object>();

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> String REQUEST = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"com.xxx.request"</span>;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> String RESPONSE = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"com.xxx.response"</span>;

    <span class="hljs-javadoc" style="color: rgb(136, 0, 0); box-sizing: border-box;">/**
     * 获取当前线程所对应的ActionContext
     *<span class="hljs-javadoctag" style="color: rgb(102, 0, 102); box-sizing: border-box;"> @return</span>
     */</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> ActionContext <span class="hljs-title" style="box-sizing: border-box;">getContext</span>() {
        <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 从ThreadLocal中获取当前线程对应的上下文对象</span>
        ActionContext instance = actionContext.get();
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span>(instance == <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>){ <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 如果首次请求，为null</span>
            instance = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ActionContext();
            actionContext.set(instance);
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> instance;
    }
    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 从ThreadLocal移除当前线程的相关联的上下文对象</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">remove</span>() {
        actionContext.remove();
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">setRequest</span>(Request request) {
        context.put(REQUEST, request);
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Request <span class="hljs-title" style="box-sizing: border-box;">getRequest</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> (Request) context.get(REQUEST);
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">setResponse</span>(Response response) {
        context.put(RESPONSE, response);
    }

    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Response <span class="hljs-title" style="box-sizing: border-box;">getResponse</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> (Response) context.get(RESPONSE);
    }
}

class Request {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String requestMsg;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String <span class="hljs-title" style="box-sizing: border-box;">getRequestMsg</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> requestMsg;
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">setRequestMsg</span>(String requestMsg) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.requestMsg = requestMsg;
    }
}

class Response {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> String responseMsg;
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> String <span class="hljs-title" style="box-sizing: border-box;">getResponseMsg</span>() {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> responseMsg;
    }
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">setResponseMsg</span>(String responseMsg) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.responseMsg = responseMsg;
    }
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; 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padding: 0px 5px;">80</li><li style="box-sizing: border-box; padding: 0px 5px;">81</li><li style="box-sizing: border-box; padding: 0px 5px;">82</li><li style="box-sizing: border-box; padding: 0px 5px;">83</li><li style="box-sizing: border-box; padding: 0px 5px;">84</li><li style="box-sizing: border-box; padding: 0px 5px;">85</li><li style="box-sizing: border-box; padding: 0px 5px;">86</li><li style="box-sizing: border-box; padding: 0px 5px;">87</li><li style="box-sizing: border-box; padding: 0px 5px;">88</li><li style="box-sizing: border-box; padding: 0px 5px;">89</li><li style="box-sizing: border-box; padding: 0px 5px;">90</li><li style="box-sizing: border-box; padding: 0px 5px;">91</li><li style="box-sizing: border-box; padding: 0px 5px;">92</li><li style="box-sizing: border-box; padding: 0px 5px;">93</li><li style="box-sizing: border-box; padding: 0px 5px;">94</li><li style="box-sizing: border-box; padding: 0px 5px;">95</li><li style="box-sizing: border-box; padding: 0px 5px;">96</li><li style="box-sizing: border-box; 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  运行结果如下： 
     
  由此可见，实现了不同的线程分别处理各自线程范围内的Request和Response。 
注意： 
1. 上面案例的ThreadLocal实例只有一个。 
2. 当线程结束时，要调用ThreadLocal的remove()方法，从thread-local variable中移除。