https://www.jianshu.com/p/d6fef5b9fbe5

package com.ma.DesignPatterns;

import com.ma.Utils.IOUtil;

import java.io.Serializable;
import java.lang.reflect.Constructor;
import java.util.ArrayList;
import java.util.HashMap;

/**
 * @Classname Singleton
 * @Description 單例模式：指一個類只有一個例項，且該類能自行建立這個例項的一種模式。構造器私有，
 *              提供公共的方法getInstance()來建立物件，可能存在反射和反序列化不安全的問題。
 * @Date 2020/6/25 21:00
 * @Created by Fearless
 
 
*/
public class Singleton implements Serializable {
    //懶漢式：某些編譯器可能發生指令重排，先instance指向記憶體空間，後初始化物件，導致後來的執行緒會獲得未初始化的物件
    /*private static volatile Singleton instance;    //避免指令重排
    private Singleton(){
        //利用異常保證反射安全
        if (Singleton.instance != null){
            throw new IllegalStateException("該物件已建立！");
        }
    }
    public static Singleton getInstance(){
        //同步程式碼塊，doubleChecking保證執行緒安全
        if (instance == null){
            synchronized (Singleton.class){
                if (instance == null){
                    instance = new Singleton();
                    System.out.println("已建立！");
                }
            }
        }
        return instance;
    }
 
*/

    //餓漢式
    /*private static Singleton instance = new Singleton();
    private Singleton(){
        //利用異常保證反射安全
        if (Singleton.instance != null){
            throw new IllegalStateException("該物件已建立！");
        }
    }
    public static Singleton getInstance(){
        return instance;
    }
    private Object readResolve(){
        return SingletonHolder.instance;
    }
 
*/

    //靜態內部類(餓漢式的增強版)
    /*private static class SingletonHolder{
        private static Singleton instance = new Singleton();
    }
    private Singleton(){
        //利用異常保證反射安全
        if (SingletonHolder.instance != null){
            throw new IllegalStateException("該物件已建立！");
        }
    }
    public static Singleton getInstance(){
        return SingletonHolder.instance;
    }
    private Object readResolve(){
        return SingletonHolder.instance;
    }*/

    //列舉：反射和反序列化均是安全的
    /*enum Singleton4{
        INSTANCE{
            @Override
            public void doSomething() {
                System.out.println("建立成功！");
            }
        };
        public abstract void doSomething();
    }*/
    //ThreadLocal：多執行緒中可以保證當前執行緒內的物件是單例模式的，但是不同執行緒中無法保證
    /*private static ThreadLocal<Singleton> threadLocal = new ThreadLocal<>();
    private static Singleton instance = null;
    private Singleton(){}
    public static Singleton getInstance(){
        //第一次訪問則進入if語句塊，否則直接返回threadLocal中的物件
        if (instance == null){
            synchronized (Singleton.class){
                if (instance == null){//判斷單例是否建立
                    instance = new Singleton();
                    threadLocal.set(instance);
                }
            }
        }
        return threadLocal.get();
    }*/
    public static void main(String[] args) throws Exception {
        /*Singleton s1 = Singleton.getInstance();
        Class aClass = s1.getClass();
        Constructor constructor = aClass.getDeclaredConstructor();
        //設定私有方法訪問許可權
        constructor.setAccessible(true);
        Singleton s2 = (Singleton) constructor.newInstance();
        System.out.println(s1 == s2);*/
//        IOUtil.serialize(s1);
        /*Singleton s2 = (Singleton)IOUtil.unserialize();
        Singleton s3 = (Singleton)IOUtil.unserialize();
        System.out.println(s1 == s2);
        System.out.println(s3 == s2);*/
        //測試列舉
        /*Singleton4 s1 = Singleton4.INSTANCE;
        Singleton4 s2 = Singleton4.INSTANCE;
        System.out.println(s2 == s1);
        */
        /*Singleton4 s1 = Singleton4.INSTANCE;
        IOUtil.serialize(s1);
        Singleton4 s3 = (Singleton4) IOUtil.unserialize();
        Singleton4 s4 = (Singleton4) IOUtil.unserialize();
        System.out.println(s1 == s3);
        System.out.println(s3 == s4);*/
        //驗證靜態內部類反射是否安全
        /*Class<?> aClass = Class.forName("com.ma.DesignPatterns.Singleton");
        Constructor<?> constructor = aClass.getDeclaredConstructor();
        constructor.setAccessible(true);
        Singleton s1 = (Singleton)constructor.newInstance();
        Singleton s2 = Singleton.getInstance();
        System.out.println(s1 == s2);*/
        //ThreadLocal測試
        /*Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();
        System.out.println(s1 == s2);
        System.out.println(s1);*/
        /*for (int i=0;i<10;i++){
            new Thread(()->{
                Singleton s3 = Singleton.getInstance();
                Singleton s4 = Singleton.getInstance();
                System.out.println(Thread.currentThread()+"s:"+s3+(s3==s4));
            }).start();
        }*/
        //阻塞主執行緒
        System.in.read();
    }
}