ThreadLocal线程安全

ThreadLocal的作用是提供线程内的局部变量，这种单例属于线程内部安全。
那假如使用ThreadLocal，那就是用空间换时间的方式。

public class ThreadLocalSingleton {
    private ThreadLocalSingleton() {
    }
    private static final ThreadLocal<ThreadLocalSingleton> singleton = new ThreadLocal<ThreadLocalSingleton>() {
        @Override
        protected ThreadLocalSingleton initialValue() {
            return new ThreadLocalSingleton();
        }
    };
    public static ThreadLocalSingleton getInstance() {
        return singleton.get();
    }
}

测试

public class ThreadLocalSingletonTest {
    public static void main(String[] args) {
        System.out.println(ThreadLocalSingleton.getInstance());
        System.out.println(ThreadLocalSingleton.getInstance());
        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                ThreadLocalSingleton singleton = ThreadLocalSingleton.getInstance();
                System.out.println(Thread.currentThread().getName() + ":" + singleton);
            }
        });
        t1.start();
    }
}