单例模式 懒汉式 为什么要两次判空

class User {
	public static int i;
	public static int j;

	private static User user = null;

	public User() {}

	public static User getInstance() {
		if(user == null) {
			try {
				Thread.sleep(1);
			} catch (InterruptedException e) {
			}
			synchronized (User.class) {
				i++;
				if(user == null) {
					user = new User();
					j++;
					return user;
				}
			}

		}
		return user;
	}

	private String name;
	private int age;
}
　

public static void main(String[] args) {
		int i = 10;
		ExecutorService executorService = Executors.newFixedThreadPool(10);
		do {
			executorService.execute(()->
			{
				User u = User.getInstance();
				System.out.println(System.identityHashCode(u));
			});
			i--;
		} while (i >0);

		try {
			Thread.sleep(10000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		System.out.println(User.getInstance().i  + " " + User.getInstance().j);
	}

　如果不判空的话,会出现多个线程在等待获取当前的锁,当其中的一个线程释放掉锁后,又会重新new 一个对象.就会出现不同的对象

 

 但有了判空之后