单例模式

import java.util.*;

public class Main{
    public static void main(String[] args){
        System.out.println("饿汉式");
        System.out.println(Singleton1.getInstance().toString());
        System.out.println(Singleton1.getInstance().toString());
        System.out.println("懒汉式");
        System.out.println(Singleton2.getInstance().toString());
        System.out.println(Singleton2.getInstance().toString());
        System.out.println("静态内部类");
        System.out.println(Singleton3.getInstance().toString());
        System.out.println(Singleton3.getInstance().toString());
        System.out.println("双检锁");
        System.out.println(Singleton5.getInstance().toString());
        System.out.println(Singleton5.getInstance().toString());
        System.out.println(Singleton4.INSTANCE);
        Singleton4.INSTANCE.anyMethod();
        System.out.println(Singleton4.INSTANCE);
    }
}

class Singleton1{
    private static Singleton1 instance = new Singleton1(); 
    // static类变量, 通过类的初始化<clinit>()的自动加锁同步，确保线程安全

    private Singleton1(){};
    // 饿汉式
    public static Singleton1 getInstance(){
        return instance;
    }
}


class Singleton2{
    private static Singleton2 instance;

    private Singleton2(){};
    // 懒汉式
    public synchronized static Singleton2 getInstance(){
        if (instance==null)
            instance = new Singleton2();
        return instance;
    }
}

class Singleton3{

    private Singleton3(){};
    // 静态内部类
    private static class Helper{
        private static Singleton3 instance = new Singleton3();
    }
    // 缺点：无法带参数
    public static Singleton3 getInstance(){
        return Helper.instance;
    }
}
// 枚举
enum Singleton4{
    INSTANCE;
    public void anyMethod(){
        System.out.println("This is enum.");
    }
}

class Singleton5{
    private volatile static Singleton5 instance; // 定义volatile，避免线程不安全

    private Singleton5(){};
    // 懒汉式 双检锁
    public static Singleton5 getInstance(){
        if (instance==null){
            synchronized(Singleton5.class){
                if (instance==null) // 由于new方法包括三条字节码指令，JVM的乱序执行有可能导致某时刻可能存在未初始化但是非null的实例，造成双检锁线程不安全。
                    instance = new Singleton5();
            }
        }
        return instance;
    }
}

可参考：https://blog.csdn.net/mnb65482/article/details/80458571