一篇讲双锁检测的文章

Under the new memory model, it is still true that volatile variables cannot be reordered with each other. The difference is that it is now no longer so easy to reorder normal field accesses around them. Writing to a volatile field has the same memory effect as a monitor release, and reading from a volatile field has the same memory effect as a monitor acquire. In effect, because the new memory model places stricter constraints on reordering of volatile field accesses with other field accesses, volatile or not, anything that was visible to thread A when it writes to volatile field f becomes visible to thread B when it reads f.

故事永远不是这么简单的结束的，结束实际上来源于两段诡异的代码。大致意思是，一个线程不断的创建ArrayList赋值给一个全局变量，另外几个线程同时读取这个ArrayList并调用add方法，那么在－server的jit编译下，会导致nullPointexception，如果对这个全局数组加上volatile，那么就不会抛这个异常出来（参考代码片段2）。

  另外一段代码就是模拟，工作线程copy 主内存的Integer，然后另一个线程不断给Integer赋值null，同样不会报错。这里有个技巧｜｜和&&的区别。（参考代码片段1）

这里插上两段代码－－

1.不会报错，因为max被copy到工作线程，当max加上volatile时，可以看到直接出错，因为max为null

public class Main {public static Integer max = null;public static void main(String[] args){Thread[] threads = new Thread[20];for (int i = 0; i < threads.length; i++) {threads[i] = new Thread(new Runnable() {@Overridepublic void run() {while (true) {if(max == null || max > 10){max = 10;}}}});threads[i].start();}Thread t1 = new Thread(){public void run(){while(true){max = null;}}};t1.start();}
}

</pre><span style="background-color: rgb(0, 0, 0);"></span><pre name="code" class="java">public class Test {public static List<String> l = new ArrayList<String>(1024); public static void main(String[] args) {
Thread[] threads = new Thread[20];for (int i = 0; i < threads.length; i++) {threads[i] = new Thread(new Runnable() {@Overridepublic void run() {int j = 0;while (true) {synchronized (HmacSHA1.class) {if(l.size() < 1024)l.add("ttt");}}}});threads[i].start();}Thread t1 = new Thread(){public void run(){while(true){List<String> local = l;l = new ArrayList<String>(1024);}}};t1.start();}}