单例模式 | 易学教程

懒汉式（双重锁定）

public class Singleton {

    private volatile static Singleton singleton;//第一步检查

    public static Singleton getInstance() {
        if (singleton == null) {//第二步检查
            synchronized (Singleton.class) {
                if (singleton == null) {
                    singleton = new Singleton();
                }
            }
        }
        return singleton;
    }
}

饿汉式（先创建出实例）

class Singleton{
    //静态实例
    private static Singleton singleton = new Singleton();

    //构造函数
    private Singleton(){};

    public Singleton getInstance(){
        return singleton;
    }
}

枚举

enum Singleton{
    INSTANCE;  //枚举成员

    private Singleton singleton;

    private Singleton{
        singleton = new Singleton();
    }

    public Singleton getInstance(){
        return singleton;
    }
}

//获得实例：Singleton.INSTANCE.getInstance();

静态内部类

class Singleton{
    //静态内部类
    private static calss SingletonHolder{
        private static final Singleton singleton = new Singleton();
    }

    //构造函数
    private Singleton(){};

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

静态代码块

class Singleton{
    //静态变量
    private static Singleton singleton = null;

    //构造函数
    private Singleton(){};

    //静态代码块
    static{
        singleton = new Singleton();
    }

    public Singleton getInstance(){
        return singleton;
    }
}

来源：CSDN

作者：Q___0

链接：https://blog.csdn.net/Q___0/article/details/103460232

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单例模式