There are multiple reasons one might want to use custom serialization instead of relying on Java's default serialization. One of the most common reasons is for performance improvements, but another reason for writing custom serialization is when the default serialization mechanism is unsupported. Specifically, as will be demonstrated in this post, custom serialization can be used to allow a larger object to be serialized even when attributes of that object are not themselves directly serializable.

The next code listing shows a simple class for serializing a given class to a file of the provided name and for deserializing an object from that same file. I will be using it in this post to demonstrate serialization.

SerializationDemonstrator.java

The next code listing illustrates use of the SerializationDemonstrator class to serialize and deserialize a standard Java string (which is Serializable). A screen snapshot follows the code listing and shows the output (in NetBeans) of running that String through the serialize and deserialize methods of theSerializationDemonstrator class.

Running SerializationDemonstrator Methods on String

SerializationDemonstrator.serialize("Inspired by Actual Events", "string.dat");
final String stringOut = SerializationDemonstrator.deserialize("string.dat", String.class);

The next two code listings are for the class Person.java and a class that it has as an attribute type (CityState.java). Although Person implements Serializable, the CityAndState class does not.

Person.java

package dustin.examples.serialization;

import java.io.Serializable;

/**
 * Person class.
 * 
 * @author Dustin
 */
public class Person implements Serializable
{
   private String lastName;
   private String firstName;
   private CityState cityAndState;

   public Person(
      final String newLastName, final String newFirstName,
      final CityState newCityAndState)
   {
      this.lastName = newLastName;
      this.firstName = newFirstName;
      this.cityAndState = newCityAndState;
   }

   public String getFirstName()
   {
      return this.firstName;
   }

   public String getLastName()
   {
      return this.lastName;
   }

   @Override
   public String toString()
   {
      return this.firstName + " " + this.lastName + " of " + this.cityAndState;
   }
}

CityAndState.java

package dustin.examples.serialization;

/**
 * Simple class storing city and state names that is NOT Serializable.
 * 
 * @author Dustin
 */
public class CityState
{
   private final String cityName;
   private final String stateName;

   public CityState(final String newCityName, final String newStateName)
   {
      this.cityName = newCityName;
      this.stateName = newStateName;
   }

   public String getCityName()
   {
      return this.cityName;
   }

   public String getStateName()
   {
      return this.stateName;
   }

   @Override
   public String toString()
   {
      return this.cityName + ", " + this.stateName;
   }
}

The next code listing demonstrates running SerializationDemonstrator on the serializable Person class with a non-serializable CityState. The code listing is followed by a screen snapshot of the output in NetBeans.

Running SerializationDemonstrator Methods on Serializable Person with Non-Serializable CityState

final Person personIn = new Person("Flintstone", "Fred", new CityState("Bedrock", "Cobblestone"));
SerializationDemonstrator.serialize(personIn, "person.dat");

final Person personOut = SerializationDemonstrator.deserialize("person.dat", Person.class);

In this case, the CityState class is my own class and I could make it Serializable. However, supposing that this class was part of a third-party framework or library and I was not able to change the class itself, I can change Person to use custom serialization and deserialization and work with CityState properly. This is shown in the next code listing for the class SerializablePerson that is adapted from Person.

SerializablePerson.java

package dustin.examples.serialization;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;

/**
 * Person class.
 * 
 * @author Dustin
 */
public class SerializablePerson implements Serializable
{
   private String lastName;
   private String firstName;
   private CityState cityAndState;

   public SerializablePerson(
      final String newLastName, final String newFirstName,
      final CityState newCityAndState)
   {
      this.lastName = newLastName;
      this.firstName = newFirstName;
      this.cityAndState = newCityAndState;
   }

   public String getFirstName()
   {
      return this.firstName;
   }

   public String getLastName()
   {
      return this.lastName;
   }

   @Override
   public String toString()
   {
      return this.firstName + " " + this.lastName + " of " + this.cityAndState;
   }

   /**
    * Serialize this instance.
    * 
    * @param out Target to which this instance is written.
    * @throws IOException Thrown if exception occurs during serialization.
    */
   private void writeObject(final ObjectOutputStream out) throws IOException
   {
      out.writeUTF(this.lastName);
      out.writeUTF(this.firstName);
      out.writeUTF(this.cityAndState.getCityName());
      out.writeUTF(this.cityAndState.getStateName());
   }
 
   /**
    * Deserialize this instance from input stream.
    * 
    * @param in Input Stream from which this instance is to be deserialized.
    * @throws IOException Thrown if error occurs in deserialization.
    * @throws ClassNotFoundException Thrown if expected class is not found.
    */
   private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException
   {
      this.lastName = in.readUTF();
      this.firstName = in.readUTF();
      this.cityAndState = new CityState(in.readUTF(), in.readUTF());
   }

   private void readObjectNoData() throws ObjectStreamException
   {
      throw new InvalidObjectException("Stream data required");
   }
}

The above code listing shows that SerializablePerson has custom writeObject and readObject methods to support custom serialization/deserialization that handle its attribute of unserializable type CityStateappropriately. A snippet of code for running this class through the SerializationDemonstrator and the successful output of doing so are shown next.

Running SerializationDemonstrator on SerializablePerson

final SerializablePerson personIn = new SerializablePerson("Flintstone", "Fred", new CityState("Bedrock", "Cobblestone"));
SerializationDemonstrator.serialize(personIn, "person.dat");

final SerializablePerson personOut = SerializationDemonstrator.deserialize("person.dat", SerializablePerson.class);

The approach depicted above will allow non-serializable types to be used as attributes of serializable classes without the need to make those fields transient. However, if the CityState instance shown earlier needs to be used in multiple serializable classes, it might be better to decorate  the CityState class with a serializable decorator  and then used that serialized decorator class in classes needing to be serialized. The next code listing shows SerializableCityState which decorates  CityState with a serialized version.

SerializableCityState

package dustin.examples.serialization;

import java.io.IOException;
import java.io.InvalidObjectException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.ObjectStreamException;
import java.io.Serializable;

/**
 * Simple class storing city and state names that IS Serializable. This class
 * decorates the non-Serializable CityState class and adds Serializability.
 * 
 * @author Dustin
 */
public class SerializableCityState implements Serializable
{
   private CityState cityState;

   public SerializableCityState(final String newCityName, final String newStateName)
   {
      this.cityState = new CityState(newCityName, newStateName);
   }

   public String getCityName()
   {
      return this.cityState.getCityName();
   }

   public String getStateName()
   {
      return this.cityState.getStateName();
   }

   @Override
   public String toString()
   {
      return this.cityState.toString();
   }

   /**
    * Serialize this instance.
    * 
    * @param out Target to which this instance is written.
    * @throws IOException Thrown if exception occurs during serialization.
    */
   private void writeObject(final ObjectOutputStream out) throws IOException
   {
      out.writeUTF(this.cityState.getCityName());
      out.writeUTF(this.cityState.getStateName());
   }
 
   /**
    * Deserialize this instance from input stream.
    * 
    * @param in Input Stream from which this instance is to be deserialized.
    * @throws IOException Thrown if error occurs in deserialization.
    * @throws ClassNotFoundException Thrown if expected class is not found.
    */
   private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException
   {
      this.cityState = new CityState(in.readUTF(), in.readUTF());
   }

   private void readObjectNoData() throws ObjectStreamException
   {
      throw new InvalidObjectException("Stream data required");
   }
}

This serializable decorator can be used in the Person class directly and that enclosing Person can use default serialization because its fields are all serializable. This is shown in the next code listing for Person2 adapted from Person.

Person2.java

package dustin.examples.serialization;

import java.io.Serializable;

/**
 * Person class.
 * 
 * @author Dustin
 */
public class Person2 implements Serializable
{
   private final String lastName;
   private final String firstName;
   private final SerializableCityState cityAndState;

   public Person2(
      final String newLastName, final String newFirstName,
      final SerializableCityState newCityAndState)
   {
      this.lastName = newLastName;
      this.firstName = newFirstName;
      this.cityAndState = newCityAndState;
   }

   public String getFirstName()
   {
      return this.firstName;
   }

   public String getLastName()
   {
      return this.lastName;
   }

   @Override
   public String toString()
   {
      return this.firstName + " " + this.lastName + " of " + this.cityAndState;
   }
}

This code can be executed as shown in the next code listing, which is followed by its output as seen in the NetBeans output window.

Running SerializationDemonstrator Against Person2/SerializableCityState

final Person2 personIn = new Person2("Flintstone", "Fred", new SerializableCityState("Bedrock", "Cobblestone"));
SerializationDemonstrator.serialize(personIn, "person2.dat");

final Person2 personOut = SerializationDemonstrator.deserialize("person2.dat", Person2.class);

Custom serialization can be used to allow a class with attributes of nonserializable types to be serialized without making those attributes of nonserializable type transient. This is a useful technique when serializable classes need to use attributes of types that are not serializable and that cannot be changed.