单例模式

p确保创建全局唯一对象


ackage com.zou.single;
//饿汉式单例
public class Hungry {
    //可能会浪费空间
    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];
    //构造器私有
    private Hungry() {
    }
    //保证new出来的对象是唯一的,一上来就把对象加载
    private final static Hungry HUNGRY=new Hungry();
    //给他一个抛出对外的方法
    public static Hungry getInstance(){
        return HUNGRY;
    }
}

package com.zou.single;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;

//懒汉式
public class LazyMan {
    private static boolean deng=false;
    //构造器私有
    private LazyMan() {//防止反射利用无参构造器
        synchronized (LazyMan.class){
            if (deng==false){
                deng=true;
            }else {
                throw new RuntimeException("不要试图用反射");
            }

        }
        System.out.println(Thread.currentThread().getName()+"ok");//确保线程启动
    }
    //不是一上来就加载对象
    private volatile static LazyMan lazyMan;//volatile避免指令重排
    public static LazyMan getInstance() {
        //需要双层检测锁，DCL懒汉式单例
        if (lazyMan == null) {
            synchronized (LazyMan.class) {//确保只有一个
                if (lazyMan == null) {//如果为空再去创建
                    lazyMan = new LazyMan();//不是原子性操作，指令重排
                }
            }

        }
        return lazyMan;
    }
//反射
    public static void main(String[] args) throws Exception {
        //LazyMan instance=LazyMan.getInstance();
        Field deng =LazyMan.class.getDeclaredField("deng");
        deng.setAccessible(true);
        Constructor<LazyMan> declaredConstructor = LazyMan.class.getDeclaredConstructor(null);//空参构造器
        declaredConstructor.setAccessible(true);//破坏私有
        LazyMan instance = declaredConstructor.newInstance();//若两个都是反射创建--红路灯标志
        deng.set(instance,false);
        LazyMan instance2 = declaredConstructor.newInstance();
        System.out.println(instance2);
        System.out.println(instance);
    }
 }

package com.zou.single;
//静态内部类
public class Holder {
    //构造器私有

    private Holder() {

    }
    public static Holder getInstance() {
        return InnerClass.HOLDER;//不安全，反射破解
    }
    public static class InnerClass{
        private static final Holder HOLDER =new Holder();//在内部类调外部类
    }
}

package com.zou.single;

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

//枚举 --他也是一个类,反射不能破坏枚举
public enum EnumSingle {
INSTANCE;
public EnumSingle getInstance(){
    return  INSTANCE;
}
}
class test{
    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
        EnumSingle instance=EnumSingle.INSTANCE;
        Constructor<EnumSingle> declaredConstructor = EnumSingle.class.getDeclaredConstructor(String.class,int.class);//空参构造器
        EnumSingle enumSingle = declaredConstructor.newInstance();
        System.out.println(instance);
        System.out.println(enumSingle);
    }
}