单例模式

饿汉式


package com.data.sheji;

public class SingleInstance {
    private static SingleInstance singleInstance = new SingleInstance();
    private SingleInstance(){}

    private static SingleInstance getSingleInstance(){
        return singleInstance;
    }
}

懒汉式

package com.data.sheji;

//懒汉式
public class SingleInstance2 {
    private static SingleInstance2 singleInstance = null;
    private SingleInstance2(){}

    private static SingleInstance2 getInstance(){
        if (singleInstance == null) {
            return new SingleInstance2();
        }
        return singleInstance;
    }
}

双重验证


package com.data.sheji;

//双重验证
public class SingleInstance3 {
    private static SingleInstance3 singleInstance3 = null;

    public static void SingleInstance3(){}

    public static SingleInstance3 getInstance3(){
        if (singleInstance3 == null) {
            synchronized (SingleInstance3.class) {
                if (singleInstance3 == null) {
                    return new SingleInstance3();
                }
            }
        }
        return singleInstance3;
    }
}

class SingleInastance4{
    private static void SingleInastance4(){}
    private static class SingleHolder{
        private static SingleInastance4 singleInastance = new SingleInastance4();
    }

    public static SingleInastance4 getInstance(){
        return SingleHolder.singleInastance;
    }

}