Providing Object Serialization for Your Classes

An object is serializable only if its class implements the Serializable interface. Thus, if you want to serialize the instances of one of your classes, the class must implement the Serializable interface. The good news is that Serializable is an empty interface. That is, it doesn't contain any method declarations; it's purpose is simply to identify classes whose objects are serializable.
Implementing the Serializable Interface
Here's the complete definition of the Serializable interface:
package java.io;
public interface Serializable {
    // there's nothing in here!
};
Making instances of your classes serializable is easy. You just add the implements Serializable clause to your class declaration like this:
public class MySerializableClass implements Serializable {
    ...
}
You don't have to write any methods. The serialization of instances of this class are handled by the defaultWriteObject method of ObjectOutputStream. This method automatically writes out everything required to reconstruct an instance of the class, including the following:

Class of the object
Class signature
Values of all non-transient and non-static members, including members that refer to other objects
For many classes, this default behavior is good enough. However, default serialization can be slow, and a class might want more explicit control over the serialization.
Customizing Serialization
You can customize serialization for your classes by providing two methods for it: writeObject and readObject. The writeObject method controls what information is saved. It is typically used to append additional information to the stream. The readObject method either reads the information written by the corresponding writeObject method or can be used to update the state of the object after it has been restored.
The writeObject method must be declared exactly as shown in the following example. Also, it should call the stream's defaultWriteObject as the first thing it does to perform default serialization. Any special arrangements can be handled afterwards:
private void writeObject(ObjectOutputStream s)
                         throws IOException {
    s.defaultWriteObject();
    // customized serialization code
}
The readObject method must read in everything written by writeObject in the same order in which it was written. Also, the readObject method can perform calculations or update the state of the object in some way. Here's the readObject method that corresponds to the writeObject method just shown:
private void readObject(ObjectInputStream s)
                        throws IOException  {
    s.defaultReadObject();
    // customized deserialization code
    ...
    // followed by code to update the object, if necessary
}
The readObject method must be declared exactly as shown.
The writeObject and readObject methods are responsible for serializing only the immediate class. Any serialization required by the superclasses is handle automatically. However, a class that needs to explicitly coordinate with its superclasses to serialize itself can do so by implementing the Externalizable interface.
Implementing the Externalizable Interface
For complete, explicit control of the serialization process, a class must implement the Externalizable interface. For Externalizable objects, only the identity of the object's class is automatically saved by the stream. The class is responsible for writing and reading its contents, and it must coordinate with its superclasses to do so.
Here's the complete definition of the Externalizable interface that extends Serializable:
package java.io;
public interface Externalizable extends Serializable
{
    public void writeExternal(ObjectOutput out)
                              throws IOException;
    public void readExternal(ObjectInput in)
                             throws IOException, 
                             java.lang.ClassNotFoundException;
}
The following holds for an Externalizable class:

It must implement the java.io.Externalizable interface.
It must implement a writeExternal method to save the state of the object. Also, it must explicitly coordinate with its supertype to save its state.
It must implement a readExternal method to read the data written by the writeExternal method from the stream and restore the state of the object. It must explicitly coordinate with the supertype to restore its state.
If externally defined format is being written, the writeExternal and readExternal methods are solely responsible for that format.
The writeExternal and readExternal methods are public and carry the risk that a client may be able to write or read information in the object other than by using its methods and fields. These methods must be used only when the information held by the object is not sensitive or when exposing that information would not present a security risk.
Protecting Sensitive Information

When developing a class that provides controlled access to resources, you must take care to protect sensitive information and functions. During deserialization, the private state of the object is restored. For example, a file descriptor contains a handle that provides access to an operating system resource. Being able to forge a file descriptor would allow some forms of illegal access, since restoring state is done from a stream. Therefore the serializing runtime must take the conservative approach and not trust the stream to contain only valid representations of objects. To avoid compromising a class, you must provide either that the sensitive state of an object must not be restored from the stream or that it must be reverified by the class.
Several techniques are available to protect sensitive data in classes. The easiest is to mark fields that contain sensitive data as private transient. transient and static fields are not serialized or deserialized. Marking the field will prevent the state from appearing in the stream and from being restored during deserialization. Since writing and reading (of private fields) cannot be superseded outside of the class, the class's transient fields are safe.
Particularly sensitive classes should not be serialized. To accomplish this, the object should not implement either the Serializable or Externalizable interface.
Some classes may find it beneficial to allow writing and reading but to specifically handle and revalidate the state as it is deserialized. The class should implement writeObject and readObject methods to save and restore only the appropriate state. If access should be denied, throwing a NotSerializableException will prevent further access.

Reading and Writing