系列文章目录

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一、方法

懒汉式：延迟加载，第一次调用getInstance()才能创建对象
饿汉式：立即加载，类加载时就创建对象

1、懒汉式+双重检查锁

package com.example.demo;public class SingleTest {private static volatile SingleTest instance;private SingleTest(){}private static SingleTest getInstance() {if (instance == null) {synchronized (SingleTest.class) {if (instance == null) {instance = new SingleTest();}}}return instance;}public static void main(String[] args) {new Thread(() -> {SingleTest singleTest1 = SingleTest.getInstance();System.out.println("singleTest1: " + singleTest1);}).start();new Thread(() -> {SingleTest singleTest2 = SingleTest.getInstance();System.out.println("singleTest2: " + singleTest2);}).start();new Thread(() -> {SingleTest singleTest3 = SingleTest.getInstance();System.out.println("singleTest3: " + singleTest3);}).start();}
}

2、饿汉式静态初始化

public class Singleton {private static final Singleton instance = new Singleton();private Singleton() {}public static Singleton getInstance() {return instance;}
}

3、使用静态内部类（懒汉式的一种变体）

public class Singleton {private Singleton() {}private static class SingletonHolder {private static final Singleton instance = new Singleton();}public static Singleton getInstance() {return SingletonHolder.instance;}
}

4、使用 AtomicReference

import java.util.concurrent.atomic.AtomicReference;public class Singleton {private static final AtomicReference<Singleton> INSTANCE = new AtomicReference<>();private Singleton() {}public static Singleton getInstance() {for (;;) {Singleton current = INSTANCE.get();if (current != null) {return current;}Singleton newInstance = new Singleton();if (INSTANCE.compareAndSet(null, newInstance)) {return newInstance;}}}
}

5、使用依赖注入框架，Spring

@Component
public class MyService {// ...
}