單例模式之餓漢式模式

• 餓漢式模式來歷 直接例項化猶如餓漢子儘快想把飯吃完故稱為餓漢式

public class Hungry{
    public static final Hungry hungry = new Hungry();
    
    private Hungry(){}

    private static final Hungry getInstance{
      return hungry;
    }
}

單例模式之懶漢式模式

• 懶漢式模式來歷 延遲例項化，懶載入故稱為懶漢式

public class LayMan{
    public static volatile  LayMan layMan = null;
    
    private LayMan(){}

    //DCL雙重檢測 
    private static LayMan getInstance{
      if(layMan!=null){
          synchronized (layMan.class){
            if(layMan != null){
              layMan = new LayMan();  //不是原子的操作，需要在 private volatile  static LayMan1 layMan1 = null;
            }
          }
      }
      return layMan;
    }
}
 

單例模式之靜態類實現模式

• 靜態類單例模式

package com.gof23.single1;

import com.gof23.single.StaticClass;

public class StaticClass1 {
    private StaticClass1(){

    }
    private static class InnerClass{
        private static StaticClass1 staticClass1 = new StaticClass1();
    }
    private static StaticClass1 getInstance(){
        return InnerClass.staticClass1;
    }

    public static void main(String[] args) {
        StaticClass1 s1 = StaticClass1.getInstance();
        StaticClass1 s2 = StaticClass1.getInstance();

        System.out.println(s1 == s2);
    }
}
 

單例模式之列舉類實現模式

• 列舉單例模式

package com.gof23.single;
import sun.security.jca.GetInstance;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

public enum EnumInstance {
    INSTANCE;

    private static EnumInstance getInstance(){
        return INSTANCE;
    }

    public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
        EnumInstance instance1 =  EnumInstance.getInstance();
        EnumInstance instance2 =  EnumInstance.getInstance();
        System.out.println(instance1);
        System.out.println(instance2);

        Constructor constructor = instance1.getClass().getDeclaredConstructor();
        constructor.setAccessible(true);
        EnumInstance instance3 = (EnumInstance) constructor.newInstance();
        System.out.println(instance3);
    }
}
 

小結: 1. 單例模式都會反射安全性問題,通過反射構造器newInstance會破壞單例模式
2. enum列舉類單例模式不存在反射安全性問題