单例的实现

很多java面试喜欢考单例模式，网上也一大堆文章。在这里也自己实现一次，作个纪念。

import java.io.*;

/**
 * User: sunlong
 * Date: 13-9-29
 * Time: 下午3:55
 */
public class Singleton {
    private static Singleton singleton = new Singleton();
    private Singleton(){}

    public static Singleton getInstance(){
        return singleton;
    }
}

/**
 * 饿汉式
 * 非线程安全实现
 */
class Singleton2{
    private Singleton2(){}

    private static Singleton2 singleton;

    public static Singleton2 getInstance(){
        if(singleton == null){
            singleton = new Singleton2();
        }
        return singleton;
    }
}

/**
 * 线程安全，但性能很糟糕
 */
class Singleton3{
    private Singleton3(){}

    private static Singleton3 singleton;

    public static Singleton3 getInstance(){
        synchronized (Singleton3.class){
            if(singleton == null){
                singleton = new Singleton3();
            }
        }
        return singleton;
    }
}

/**
 * 双检查锁
 * 可能会失效，因为同步块外面的if (singleton == null)可能看到已存在，但不完整的实例。
 * 一篇参考文章
 * http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
 */
class Singleton4{
    private Singleton4(){}

    private static Singleton4 singleton; //JDK5.0以后版本若singleton为volatile则可行

    public static Singleton4 getInstance(){
        if(singleton == null){
            synchronized (Singleton4.class){
                if(singleton == null){
                    singleton = new Singleton4();
                }
            }
        }
        return singleton;
    }
}

/**
 * 使用内部类实现延迟加载
 * 线程安全
 */
class Singleton5 {
    private static class Singleton5Holder {
        final static Singleton5 INSTANCE = new Singleton5();
    }

    private Singleton5 (){
    }

    public static Singleton5 getInstance() {
        return Singleton5Holder.INSTANCE;
    }
}

/**
 * 为了使Singleton类变成可序列化的(serializable)，
 * 仅仅实现Serializable接口是不够的。为了维护 Singleton的单例性，你必须给Singleton类提供一个readResolve方法，
 * 否则的话，一个序列化的实例，每次反序列化的时候都会产 生一个新的实例。Singleton 也不会例外。
 */
class Singleton6 implements Serializable{
    private static Singleton6 singleton = new Singleton6();
    private Singleton6(){}

    public static Singleton6 getInstance(){
        return singleton;
    }

//    private Object readResolve() throws ObjectStreamException {
//        return singleton;
//    }

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        ObjectOutputStream objectOutputStream = new ObjectOutputStream(new FileOutputStream("D:\\Singleton.obj"));
        Singleton6 singleton = Singleton6.getInstance();
        objectOutputStream.writeObject(singleton);
        objectOutputStream.close();
        //反序列化
        ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("D:\\Singleton.obj"));
        Singleton6 singleton2 = (Singleton6)objectInputStream.readObject();
        objectInputStream.close();
        //比较是否原来的实例
        System.out.println(singleton==singleton2);
    }
}