线程安全单例设计模式+序列化

懒汉式单例模式会引来线程安全问题，即多线程时可能会创建多个实例，而且在反序列化时也可能会创建新的实例。看了大大们的实现方式，觉得以下一种比较简单方便理解。

一、下面是会造成线程安全的饿汉单例模式。用四打印下会发现hashcode不一致

public class Singleton {  
      
    private static Singleton instance = null;  
    //禁止new
    private Singleton(){}  
      
    public static Singleton getInstance() {  
        if(instance == null){//懒汉式   
            instance = new MySingleton();
        }
        return instance;  
    }  
}

二、静态代码块中的代码在使用类的时候就已经执行了，所以可以应用静态代码块的这个特性实现单例设计模式。

public class Singleton implements Serializable{
    private static final long serialVersionUID = 1L;
    private static Singleton instance = null;//饿汉式

    //静态代码块实现单例模式线程安全
    static {
        instance = new Singleton();
    }

    public static Singleton getSingleton() {
        if (instance == null) {
            instance = new Singleton();
        }
        return instance;
    }

    //该方法在反序列化时会被调用
    protected Object readResolve() throws ObjectStreamException {
        System.out.println("调用了readResolve方法！");
        return Singleton.instance;
    }
}

三、多线程调用打印实例的hashCode

public class MyThread extends Thread {
    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println(Singleton.getSingleton().hashCode());
        }
    }

    public static void main(String[] args) throws Exception {
        MyThread[] mts = new MyThread[3];
        for (int i = 0; i < mts.length; i++) {
            mts[i] = new MyThread();
        }

        for (int j = 0; j < mts.length; j++) {
            mts[j].start();
        }
    }
}

四、打印序列化和反序列后实例的hashcode

/**
 * java类简单作用描述
 *
 * @Author: Sosmmh
 * @CreateDate: 2018/2/27 10:56
 **/
public class SaveAndReadForSingleton {
    public static void main(String[] args) {
        Singleton singleton = Singleton.getSingleton();

        File file = new File("MySingleton.txt");

        try {
            FileOutputStream fos = new FileOutputStream(file);
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(singleton);
            fos.close();
            oos.close();
            System.out.println(singleton.hashCode());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            FileInputStream fis = new FileInputStream(file);
            ObjectInputStream ois = new ObjectInputStream(fis);
            Singleton rSingleton = (Singleton) ois.readObject();
            fis.close();
            ois.close();
            System.out.println(rSingleton.hashCode());
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

    }
}

详细介绍请看原文大大。详细介绍单例模式

线程安全单例设计模式+序列化

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