threadLocal

可以看成存放变量副本的一个容器，各线程之间放在threadLocal中的变量互不依赖。

使用完 ThreadLocal后 最好手动调用remove()方法，清理掉 key 为 null 的记录，否则在垃圾回收的时候，key (弱引用)会被清理掉，而 value 不会被清理掉，造成内存泄露。

原理：

每个线程实例都有一个ThreadLocalMap类型的成员threadLocals，这个成员存放着键值对，key为ThreadLocal，value为存放的值，若想让一个线程保存多个变量，就需要定义多个Threadlocal类的对象。

//Thread类
public class Thread implements Runnable {
    //......
    //与此线程有关的ThreadLocal值。由ThreadLocal类维护
    ThreadLocal.ThreadLocalMap threadLocals = null;

    //与此线程有关的InheritableThreadLocal值。由InheritableThreadLocal类维护
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
    //......
}
//ThreadLocal类
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
}
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

使用

public class ThreadLocalExample implements Runnable{

     // SimpleDateFormat 不是线程安全的，所以每个线程都要有自己独立的副本
    private static final ThreadLocal<SimpleDateFormat> formatter = ThreadLocal.withInitial(() -> new SimpleDateFormat("yyyyMMdd HHmm"));

    public static void main(String[] args) throws InterruptedException {
        ThreadLocalExample obj = new ThreadLocalExample();
        for(int i=0 ; i<10; i++){
            Thread t = new Thread(obj, ""+i);
            Thread.sleep(new Random().nextInt(1000));
            t.start();
        }
    }

    @Override
    public void run() {
        System.out.println("Thread Name= "+Thread.currentThread().getName()+" default Formatter = "+formatter.get().toPattern());
        try {
            Thread.sleep(new Random().nextInt(1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        //formatter pattern is changed here by thread, but it won't reflect to other threads
        formatter.set(new SimpleDateFormat());

        System.out.println("Thread Name= "+Thread.currentThread().getName()+" formatter = "+formatter.get().toPattern());
    }

}

ThreadLocalMap的hash采用开放地址法: