Java如何为每个线程提供一个单例？

什么样的场景会要求每个线程都要单例？

class ThreadLocalSingleton {

    private ThreadLocalSingleton(){}

    public final static ThreadLocal<ThreadLocalSingleton> INSTANCE = 
            new ThreadLocal<ThreadLocalSingleton>(){
        @Override 
        protected ThreadLocalSingleton initialValue(){
            return new ThreadLocalSingleton();
        }
    };

}

public class ThreadLocalSingletonTest{

    @Test
    public void testThreadLocalSingleton() throws InterruptedException{
        for(int i=0; i<5; i++){
            new Thread(new Runnable(){
                @Override
                public void run() {
                    System.out.println(ThreadLocalSingleton.INSTANCE.get());
                }
            }).start();
        }
        Thread.sleep(3000);
    }

}

结果, 每个thread都对应唯一的不可变的 ThreadLocalSingleton实例:
`
com.test.loader.ThreadLocalSingleton@7d7c214d
com.test.loader.ThreadLocalSingleton@606e8697
com.test.loader.ThreadLocalSingleton@784654f6
com.test.loader.ThreadLocalSingleton@5174202a
com.test.loader.ThreadLocalSingleton@1eebcf2d`