Java | Serialization

Serialization is the process of converting an object into a stream of bytes which can be deserialized back to a copy of the object. A serialized object can be stored in a file and deserialized later into an object.

Serializable Objects

• An object is serializable if it is a sub-type of java.io.Serializable interface or its sub-interface java.io.Externalizable.
• Serializable objects can be stored into e.g. a file and restored back into an object by deserializing it.
• Serializable objects can be serialized using default Java way or custom way.
• Serializable object includes sufficient information to restore the fields in the stream to a compatible version of the class.

public class Universe implements Serializable  {

	private long timestamp;
	
	public Universe() {
		timestamp =  System.currentTimeMillis();
	}
}

Serializing an Object

• ObjectOutputStream.writeObject(Object ob) method is used to write an object to OutputStream e.g. FileOutputStream.

public static void serialize(Universe universe) throws Exception {
	    FileOutputStream file = new FileOutputStream("output.txt");
	    ObjectOutputStream out = new ObjectOutputStream(file);
	    out.writeObject(universe);
	    out.close();
	    file.close();
	}

Deserializing an Object

• ObjectInputStream.readObject() method is used to read an object from InputStream e.g. FileInputStream.

public static Universe deserialize() throws Exception {
	    FileInputStream file = new FileInputStream("output.txt");
	    ObjectInputStream in = new ObjectInputStream(file);
	    Universe universe = (Universe) in.readObject();
	    in.close();
	    file.close();
	    return universe;
}

Non-serializable Fields

• Fields declared as transient or static will not be serialized.

public class Universe implements Serializable  {

	public static Universe INSTANCE; //Non-serializable
	
	private long timestamp;
	private transient long age;  // Non-serializable 
	
	public Universe() {
		timestamp =  System.currentTimeMillis();
	}
}

Optional Methods for Serializable Class

• Classes implementing Serializable interface can optionally define the following methods:
  • writeObject() to control what is saved and append additional information to the stream.
  • readObject() to update the state of the object after it has been restored.
  • writeReplace() to replace object to be written to stream with different one.
  • readResolve() to replace object read from stream with different object.