Java 单例模式几种实现方式

单例模式（singleton pattern ）是一种在开发中经常用到的设计模式。

模式定义

确保一个类仅有一个实例，并且提供一个访问它的全局访问点。

实现

1、单例模式-饿汉式

public class Singleton {
    private static final Singleton INSTANCE = new Singleton();

    private Singleton(){
        System.out.println("new Singleton instance");
    }

    public static Singleton getInstance(){
        return INSTANCE;
    }
}

2、单例模式-懒汉式(非线程安全)

public class Singleton {
    private static Singleton instance;
    private Singleton() {
        System.out.println("new Singleton instance");
    }

    //非线程安全
    public static Singleton getInstance() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

3、单例模式-懒汉式(线程安全)

public class Singleton {
    private volatile static Singleton instance;

    private Singleton(){
        System.out.println("new Singleton instance");
    }

    //double-check
    public static Singleton getInstance(){
        if(instance==null){
            synchronized (Singleton.class){
                if(instance==null){
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

4、单例模式-懒汉式

public class Singleton {

    private Singleton(){
        System.out.println("new Singleton instance");
    }

    public static Singleton getInstance(){
        return SingletonHolder.INSTANCE;
    }

    private static class SingletonHolder{
        public static final Singleton INSTANCE = new Singleton();
    }
}

5、单例模式-枚举

public enum Singleton {
    INSTANCE("singleton");

    private String value;
    Singleton(String value){
        this.value = value;
    }

    public void doSomeAction(){
        System.out.println("doSomeAction");
    }
}

源代码

https://github.com/TiFG/design-patterns/tree/master/singleton