Java并发编程ThreadLocal

ThreadLocal理解:

This class provides thread-local variables.  These variables differ from
* their normal counterparts in that each thread that accesses one (via its
* <tt>get</tt> or <tt>set</tt> method) has its own, independently initialized
* copy of the variable.  <tt>ThreadLocal</tt> instances are typically private
* static fields in classes that wish to associate state with a thread (e.g.,
* a user ID or Transaction ID).

* <p>Each thread holds an implicit reference to its copy of a thread-local
* variable as long as the thread is alive and the <tt>ThreadLocal</tt>
* instance is accessible; after a thread goes away, all of its copies of
* thread-local instances are subject to garbage collection (unless other
* references to these copies exist).

就是说，对于threadlocal的变量,每一个线程都会拷贝一个该变量的副本,每个线程操作的都是这个副本,在一个线程销毁后，这些副本也会被垃圾回收.

如下例:

public class ThreadLocalTest extends Thread {

    private int  i = 0;
    @Override
    public void run() {
	     i=3;
        System.out.println("次线程"+i);
    }
    public static void main(String args[]) throws Exception{
        ThreadLocalTest threadLocalTest = new ThreadLocalTest();
        threadLocalTest.start();
        Thread.sleep(2000);
        System.out.println("主线程"+threadLocalTest.i);
    }
}

结果:

可以看到，主线程和次线程输出i的值都为3

加上threadlocal后:

public class ThreadLocalTest extends Thread {

    private ThreadLocal<Integer>  local = new ThreadLocal<Integer>();
    @Override
    public void run() {
        local.set(3);
        System.out.println("次线程"+local.get());
    }
    public static void main(String args[]) throws Exception{
        ThreadLocalTest threadLocalTest = new ThreadLocalTest();
        threadLocalTest.local.set(1);
        threadLocalTest.start();
        Thread.sleep(2000);
        System.out.println("主线程"+threadLocalTest.local.get());
    }
}

结果:

可以看到主线程和次线程拿到的值都是自己设置的值，所以threadlocal的变量再运行时，会进行拷贝.

从上面可以看到，使用threadlocal不存在共享变量的，也就是说它不是作为同步机制