设计模式之单例模式

package com.hml.singleton;

/**
 * 饿汉式
 */
public class Singleton {
    private static Singleton instance  = new Singleton();
    
    private Singleton(){}
    public static Singleton getInstance () {
        return instance;
    }
}

package com.hml.singleton;

/**
 * 懒汉式
 */
public class Singleton2 {

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

package com.hml.singleton;

/**
 * 双重锁定
 */
public class Singleton3 {

    private static Singleton3 instance;
    
    private Singleton3(){}
    
    public static Singleton3 getInstance() {
        if (instance == null) {
            synchronized (Singleton3.class) {
                if (instance == null) {
                    instance = new Singleton3();
                }
            }
        }
        return instance;
    }
}