ThreadLocal实现线程单例

提示：其实是伪线程安全的。
使用场景：ORM框架中实现多数据源动态切换。

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

    private ThreadLocalSingleton(){}

    public static ThreadLocalSingleton getInstance(){
        return threadLocalInstance.get();
    }
}

public static void main(String[] args) { 
    System.out.println(ThreadLocalSingleton.getInstance()); 
    System.out.println(ThreadLocalSingleton.getInstance()); 
    System.out.println(ThreadLocalSingleton.getInstance()); 
    System.out.println(ThreadLocalSingleton.getInstance()); 
    System.out.println(ThreadLocalSingleton.getInstance());
    Thread t1 = new Thread(new ExectorThread()); 
    Thread t2 = new Thread(new ExectorThread()); 
    t1.start(); 
    t2.start();
    System.out.println("End"); 
}

结果：

mian线程，Thread-1线程，Thread-0线程，各自线程中是唯一的，但是各个线程之间是互不一样的。
通过查看ThreadLocal的源码

ThreadLocalMap的set方法key其实就是当前的线程本身，value就是我们放到map中的value的值，所以就保证了上面出现的结果。即线程内是安全的。

这里其实是注册式单利中的容器类

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来源：https://www.cnblogs.com/LeesinDong/p/12246633.html