java - 需要解释，为什么向 Singleton 类添加 implements Serializable 是不够的？

Question

谁能给我解释一下这是什么意思？特别是粗线。

To make a singleton class serializable, it is not sufficient merely to add implements Serializable to its declaration. To maintain the singleton guarantee, you have to declare all instance fields transient and provide a readResolve() method. Otherwise, each time a serialized instance is deserialized, a new instance will be created, leading, in the case of our example, to spurious Elvis sightings. To prevent this, add this readResolve() method to the Elvis class:

// Singleton with static factory
public class Elvis {
    private static final Elvis INSTANCE = new Elvis();
    private Elvis() { ... }
    public static Elvis getInstance() { return INSTANCE; }
    public void leaveTheBuilding() { ... }
}

// readResolve method to preserve singleton property
private Object readResolve() {
    // Return the one true Elvis and let the garbage collector
    // take care of the Elvis impersonator.
    return INSTANCE;
}

仅供引用:这些行来自 Effective Java Book，第 3 项

Answer 1

readResolve 方法在 ObjectInputStream 从流中读取一个对象并准备将其返回给调用者时被调用。 ObjectInputStream 检查对象的类是否定义了 readResolve 方法。如果定义了该方法，则调用readResolve方法让流中的对象指定要返回的对象。

在单例情况下，我们将返回在类加载时创建的相同实例，并且不会返回任何新实例。所以保持单例。