readResolve

readResolve is a special method in Java that allows a class to control its deserialization. When an object of a class that implements the readResolve method is deserialized, the Java deserialization mechanism will call this method. The method can then return a different object than the one that was originally created by the deserialization process. This is useful for several reasons, such as ensuring that only one instance of a singleton class is created, or returning a pre-existing object from a cache.

To implement readResolve, a class must define a method with the signature Object readResolve() throws ObjectStreamException. This method should return an Object. If the method returns an object that is not of the same class as the one being deserialized, that returned object will be used instead. If the method returns null, the object that was originally created by the deserialization process will be used.

A common use case for readResolve is to maintain singleton pattern during deserialization. If a class is designed as a singleton, without readResolve, deserializing a serialized instance would create a new, distinct object, violating the singleton principle. By implementing readResolve to always return the single instance of the class, this issue is prevented. Another application is to substitute serialized objects with objects from a cache, which can improve performance by avoiding redundant object creation.

The readResolve method is part of Java's serialization API and is invoked after the default deserialization process has occurred, but before the fully deserialized object is returned to the caller. It provides a hook for customizing the deserialization outcome.