详解Java单例模式的实现与原理剖析

package com.wanshi.single;

//饿汉式单例
public class Hungry {


    //会造成资源浪费，占用CPU
    private byte[] data1 = new byte[1024*1024];
    private byte[] data2 = new byte[1024*1024];
    private byte[] data3 = new byte[1024*1024];
    private byte[] data4 = new byte[1024*1024];


    private Hungry() {
        System.out.println("Hungry init...");
    }


    private static Hungry hungry = new Hungry();

    public static Hungry getInstance() {
        return hungry;
    }
}

class Test {
	public static void main(String[] args) {
        Hungry hungry = Hungry.getInstance();
        Hungry hungry2 = Hungry.getInstance();
        System.out.println(hungry);
        System.out.println(hungry2);
    }
}

package com.wanshi.single;

import java.lang.reflect.Constructor;

// enum 是一个class类
public enum EnumSingle {

    INSTANCE;

    public static EnumSingle getInstance() {
        return INSTANCE;
    }
}

class Test {

    public static void main(String[] args) throws Exception{
        EnumSingle instance1 = EnumSingle.INSTANCE;
        Constructor<EnumSingle> declaredConstructor = EnumSingle.class.getDeclaredConstructor(String.class, int.class);
        declaredConstructor.setAccessible(true);

        EnumSingle instance2 = declaredConstructor.newInstance();

        System.out.println(instance1);
        System.out.println(instance2);
    }
}

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 
// Source File Name:   EnumSingle.java

package com.wanshi.single;


public final class EnumSingle extends Enum
{

    public static EnumSingle[] values()
    {
        return (EnumSingle[])$VALUES.clone();
    }

    public static EnumSingle valueOf(String name)
    {
        return (EnumSingle)Enum.valueOf(com/wanshi/single/EnumSingle, name);
    }

    private EnumSingle(String s, int i)
    {
        super(s, i);
    }

    public static EnumSingle getInstance()
    {
        return INSTANCE;
    }

    public static final EnumSingle INSTANCE;
    private static final EnumSingle $VALUES[];

    static 
    {
        INSTANCE = new EnumSingle("INSTANCE", 0);
        $VALUES = (new EnumSingle[] {
            INSTANCE
        });
    }
}

package com.wanshi.single;

public class Lazy {

    private static Lazy lazy;

    public static Lazy getInterface() {
        synchronized (Lazy.class) {
            if (lazy == null) {
                lazy = new Lazy();
            }
        }
        return lazy;
    }

    public static void main(String[] args) {
        Lazy lazy = Lazy.getInterface();
        Lazy lazy2 = Lazy.getInterface();
        System.out.println(lazy);
        System.out.println(lazy2);
    }
}

package com.wanshi.single;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;

public class LazyMan {


    private static boolean flag = false;

    private LazyMan() {
        synchronized (this) {
            if (!flag) {
                flag = true;
            } else {
                throw new RuntimeException("不要试图通过反射破坏对象");
            }
        }
    }

    private volatile static LazyMan lazyMan;

    //双重检查锁，懒汉式（DCL懒汉式）
    public static LazyMan getInstance() {
        if (lazyMan == null) {
            synchronized (LazyMan.class) {
                if (lazyMan == null) {
                    //不是原子性操作，1.分配内存空间，2.执行构造方法，3.把对象指向这个空间 指令重排可能会发生   加上volatile关闭指令重排
                    lazyMan = new LazyMan();
                }
            }
        }
        return lazyMan;
    }

    public static void main(String[] args) throws Exception {
//        LazyMan lazyMan1 = LazyMan.getInstance();

        Field flag = LazyMan.class.getDeclaredField("flag");
        flag.setAccessible(true);


        Constructor<LazyMan> declaredConstructor = LazyMan.class.getDeclaredConstructor(null);

        declaredConstructor.setAccessible(true);
        LazyMan lazyMan1 = declaredConstructor.newInstance();
        flag.set(lazyMan1, false);
        LazyMan lazyMan2 = declaredConstructor.newInstance();
        System.out.println(lazyMan1);
        System.out.println(lazyMan2);
    }
}

package com.wanshi.single;

public class Holder {

    private Holder() {

    }

    public static class InnerClass {
        private static final Holder HOLDER = new Holder();
    }
}