java-ThreadLocal简介

android中的Looper类，使用了ThreadLocal保存每个线程的Looper

static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();

简单了解下ThreadLocal的概念和使用。

（一）概念

ThreadLocal为每个线程提供变量的副本，该变量线程间独立不影响。

（二）使用

/**
 * ThreadLocal用法测试
 * 
 * @author peter_wang
 * @create-time 2014-12-24 下午8:15:47
 */
public class ThreadLocalDemo {
    private static ThreadLocal<Integer> num = new ThreadLocal<Integer>(){
        //提供初始化，不实现初始化方法首次使用get()的时候也需要初始化对象
        protected Integer initialValue() {
            return 0;
        };
    };


    private static class ThreadLocalThread
        extends Thread {


        @Override
        public void run() {
            for (int i = 0; i < 3; i++) {
                num.set(num.get() + 1);
                System.out.println("Thread name is " + getName() + ",num is " + num.get());
            }
        }
    }


    /**
     * @param args
     */
    public static void main(String[] args) {
        ThreadLocalThread thread1 = new ThreadLocalThread();
        thread1.start();
        ThreadLocalThread thread2 = new ThreadLocalThread();
        thread2.start();
    }


}

运行结果：

Thread name is Thread-0,num is 1
Thread name is Thread-1,num is 1
Thread name is Thread-1,num is 2
Thread name is Thread-1,num is 3
Thread name is Thread-0,num is 2
Thread name is Thread-0,num is 3

ThreadLocal和线程同步机制都是为了解决多线程中相同变量的访问冲突问题。ThreadLocal提供变量拷贝，线程间独立，访问效率高，用“空间换时间”；线程同步机制对变量进行线程间访问互斥锁，变量多线程共享，访问效率低，处理负责，用“时间换空间”。

ThreadLocal不是用来解决变量线程共享安全的，而是提供了保存对象的方法。

（四）源码分析

查看源码，每个Thread中都带有ThreadLocal.ThreadLocalMap变量

在ThreadLocal中将变量和Thread通过map绑定

public void set(T value) {
        Thread t = Thread.currentThread();
        //获取ThreadLocalMap绑定该线程和对象变量
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }