设计模式 单例模式（笔记）

懒汉式：

//饿汉模式
public class EagerSington {
    //方式一：创建静态变量
    private static EagerSington eagerSington = new EagerSington();
    //方式二：创建静态代码块
    private static EagerSington eagerSington1;
    static {
         eagerSington1 = new EagerSington();
    }
    private EagerSington() {
    }
    public static EagerSington getInstance(){
        return  eagerSington;
    }
    public static EagerSington getInstancePiece(){
        return  eagerSington1;
    }
}

//方式三：枚举
public enum EagerSingtonEnum {
    FIRST,SECOND;

    EagerSingtonEnum() {
    }
}

//懒汉模式
public class IdlerFirst {
    private static IdlerFirst idlerFirst;
    private IdlerFirst() {
    }
    public static IdlerFirst getInstance(){
        //判断对象是否为空，但是线程不安全
        if (idlerFirst == null){
            idlerFirst = new IdlerFirst();
        }
        return idlerFirst;
    }
}

public class IdlerSecond {
    private static IdlerSecond idlerSecond;
    private IdlerSecond() {
    }
    public static IdlerSecond getInstance(){
        //容易进入堵塞
        synchronized (IdlerSecond.class){
            if (idlerSecond == null){
                idlerSecond = new IdlerSecond();
            }
            return  idlerSecond;
        }
    }
}

public class IdlerThird {
    private static volatile IdlerThird idlerThird;
    private IdlerThird() {
    }
    public static IdlerThird getInstance(){
        //双重检查锁
        if (idlerThird == null){
            synchronized (IdlerThird.class){
                if (idlerThird == null){
                    idlerThird = new IdlerThird();
                }
            }
        }
        return idlerThird;
    }
}

public class IdlerFourth {
    private static IdlerFourth idlerFourth;
    private IdlerFourth() {
    }
    //通过静态内部类实现单例
    private static class IdlerInner{
        static {
            idlerFourth = new IdlerFourth();
        }
        private static IdlerFourth getInstance(){
            return  idlerFourth;
        }
    }
    public static IdlerFourth getInstance(){
        return IdlerInner.getInstance();
    }
}