Java Language Basic Serialization in Java


Example

What is Serialization

Serialization is the process of converting an object's state (including its references) to a sequence of bytes, as well as the process of rebuilding those bytes into a live object at some future time. Serialization is used when you want to persist the object. It is also used by Java RMI to pass objects between JVMs, either as arguments in a method invocation from a client to a server or as return values from a method invocation, or as exceptions thrown by remote methods. In general, serialization is used when we want the object to exist beyond the lifetime of the JVM.

java.io.Serializable is a marker interface (has no body). It is just used to "mark" Java classes as serializable.

The serialization runtime associates with each serializable class a version number, called a serialVersionUID, which is used during de-serialization to verify that the sender and receiver of a serialized object have loaded classes for that object that are compatible with respect to serialization. If the receiver has loaded a class for the object that has a different serialVersionUID than that of the corresponding sender's class, then deserialization will result in an InvalidClassException. A serializable class can declare its own serialVersionUID explicitly by declaring a field named serialVersionUID that must be static, final, and of type long:

ANY-ACCESS-MODIFIER static final long serialVersionUID = 1L;

How to make a class eligible for serialization

To persist an object the respective class must implement the java.io.Serializable interface.

import java.io.Serializable;

public class SerialClass implements Serializable {

    private static final long serialVersionUID = 1L;  
    private Date currentTime;
    
    public SerialClass() {
        currentTime = Calendar.getInstance().getTime();
    }

    public Date getCurrentTime() {
        return currentTime;
    }
}

How to write an object into a file

Now we need to write this object to a file system. We use java.io.ObjectOutputStream for this purpose.

import java.io.FileOutputStream;
import java.io.ObjectOutputStream;
import java.io.IOException;

public class PersistSerialClass {

    public static void main(String [] args) {
        String filename = "time.ser";            
        SerialClass time = new SerialClass(); //We will write this object to file system.
        try {
            ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(filename));
            out.writeObject(time); //Write byte stream to file system.
            out.close();
        } catch(IOException ex){
            ex.printStackTrace();
        }
     }
 }

How to recreate an object from its serialized state

The stored object can be read from file system at later time using java.io.ObjectInputStream as shown below:

import java.io.FileInputStream;
import java.io.ObjectInputStream;
import java.io.IOException;
import java.io.java.lang.ClassNotFoundException;

 public class ReadSerialClass {

    public static void main(String [] args) {
        String filename = "time.ser";    
        SerialClass time = null;

        try {
            ObjectInputStream in = new ObjectInputStream(new FileInputStream(filename));
            time = (SerialClass)in.readObject();
            in.close();
        } catch(IOException ex){
            ex.printStackTrace();
        } catch(ClassNotFoundException cnfe){
            cnfe.printStackTrace();
        }
        // print out restored time
        System.out.println("Restored time: " + time.getTime());
     }
 }

The serialized class is in binary form. The deserialization can be problematic if the class definition changes: see the Versioning of Serialized Objects chapter of the Java Serialization Specification for details.

Serializing an object serializes the entire object graph of which it is the root, and operates correctly in the presence of cyclic graphs. A reset() method is provided to force the ObjectOutputStream to forget about objects that have already been serialized.

Transient-fields - Serialization