大话设计模式之单例模式

单例模式

一、饿汉式

在类初始化时，已经自行实例化

public class Singleton {
    private static Singleton instance = new Singleton();

    private Singleton() {

    }

    public static Singleton getInstance() {
        
        return instance;
    }

}

二、懒汉式

在首次调用的时候实例化

public class Singleton {
    private static Singleton instance = null;

    private Singleton() {

    }

    public static Singleton getInstance() {
        if (null == instance) {
            instance = new Singleton();
        }
        return instance;
    }

}

三、双重检测

public class Singleton {
    private static Singleton instance = null;

    private Singleton() {

    }

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

}

四、initialization on demand holder 

public class Singleton {

    private Singleton() {

    }
    
    public static class Holder{
        static Singleton instance = new Singleton();
    }
    
    public static Singleton getInstance() {
        return Holder.instance;
    }

}