Java单例类与对象序列化


为了保证全局只有一个实例,单例类往往将构造方法的访问权限设置为private:

public class Singleton implements Serializable{

    private static final Singleton SINGLETON = new Singleton(100);

    private int value;

    private Singleton(int value) {
        // private!
        this.value = value;
    }

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

对单例类进行序列化需要格外小心,下面看一段代码:

    public static void main(String[] args) throws IOException, ClassNotFoundException {
        Singleton singleton1 = Singleton.getInstance();

        ObjectOutputStream output = new ObjectOutputStream(new FileOutputStream("singleton.dat"));
        output.writeObject(singleton1);

        ObjectInputStream input = new ObjectInputStream(new FileInputStream("singleton.dat"));
        Singleton singleton2 = (Singleton) input.readObject();

        System.out.println("创建新实例:" + (singleton1 != singleton2));
    }

控制台输出:

创建新实例:true

上面代码的运行结果表明,对单例类进行序列化可能会创建新的实例,即使构造方法是私有的!

为了解决上述问题,需要定义一种被称为readResolve的特殊序列化方法。如果定义了readResolve方法,在对象序列化时就会调用它。

public class Singleton implements Serializable {
    private static final Singleton SINGLETON = new Singleton(100);

    private int value;

    private Singleton(int value) {
        // private!
        this.value = value;
    }

    public static Singleton getInstance() {
        return SINGLETON;
    }

    protected Object readResolve() throws ObjectStreamException {
        return SINGLETON;
    }
}

再次执行上面的测试代码,控制台输出如下:

创建新实例:false