volatile关键字剖析

package com.chunling.cloud.test;

public class Singleton {

    private static Singleton instance;

    private int value;

    private Singleton() {
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        value = 42; //
    }

    public int getValue() {
        return value;
    }

    public static Singleton getInstance() {
        if (instance == null) { // 第一次检查，避免不必要的同步
            synchronized (Singleton.class) {
                if (instance == null) { // 第二次检查，确保只有一个实例被创建
                    instance = new Singleton();
                }
            }
        }
        System.out.println(instance.getValue());
        return instance;
    }


    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            Thread thread = new Thread(() -> Singleton.getInstance());
            thread.start();
        }
    }

}