ThreadLocal不能保证对象全局唯一，但是能保证在单个线程中是唯一的，天生线程安全；

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

    /**
     * 构造器私有
     */
    private ThreadLocalSingleton(){
    }

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

测试代码：

    public static void main(String[] args) throws Exception {
        ThreadLocalSingleton.getInstance();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                System.out.println(Thread.currentThread().getName()+"  "+ThreadLocalSingleton.getInstance());
            }
        },"线程1").start();
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                System.out.println(Thread.currentThread().getName()+"  "+ThreadLocalSingleton.getInstance());
            }
        },"线程2").start();
        TimeUnit.SECONDS.sleep(5);
    }

结论：
在同一个线程里，对象是单例的。
原因：
单例一般为了线程安全要给方法上锁，用时间换空间，而ThreadLocal会把所有的对象放到ThreadLocalMap中，每个线程都提供一个对象，实际上是空间换时间实现线程隔离的。