Java object serialization is used to persist Java objects to a file, database, network, process or any other system. Serialization flattens objects into an ordered, or serialized stream of bytes. The ordered stream of bytes can then be read at a later time, or in another environment, to recreate the original objects.

Java serialization cannot occur for transient or static fields. Marking the field transient prevents the state from being written to the stream and from being restored during deserialization. Java provides classes to support writing objects to streams and restoring objects from streams. Only objects that support the java.io.Serializable interface or the java.io.Externalizable interface can be written to streams.

public interface Serializable

The Serializable interface has no methods or fields. (Marker Interface) Only objects of classes that implement java.io.Serializable interface can be serialized or deserialized.

Transient Fields and Java Serialization

The transient keyword is a modifier applied to instance variables in a class. It specifies that the variable is not part of the persistent state of the object and thus never saved during serialization.
You can use the transient keyword to describe temporary variables, or variables that contain local information, such as a process ID or a time lapse.

Input and Output Object Streams

ObjectOutputStream is the primary output stream class that implements the ObjectOutput interface for serializing objects. ObjectInputStream is the primary input stream class that implements the ObjectInput interface for deserializing objects.
These high-level streams are each chained to a low-level stream, such as FileInputStream or FileOutputStream. The low-level streams handle the bytes of data. The writeObject method saves the state of the class by writing the individual fields to the ObjectOutputStream. The readObject method is used to deserialize the object from the object input stream.