10.3 Serialization

Using streams and files, you can write an application that saves and loads its data to a disk drive. Java 1.1 provides an even more powerful mechanism called object serialization that does a lot of the work for you. In its simplest form, object serialization is an automatic way to save and load the state of an object. However, object serialization has depths that we cannot plumb within the scope of this book, including complete control over the serialization process and interesting conundrums like class versioning.

Basically, any class that implements the Serializable interface can be saved and restored from a stream. Special stream subclasses, ObjectInputStream and ObjectOutputStream, are used to serialize primitive types and objects. Subclasses of Serializable classes are also serializable. The default serialization mechanism saves the value of an object's nonstatic and nontransient member variables.

One of the tricky things about serialization is that when an object is serialized, any object references it contains are also serialized. Serialization can capture entire "graphs" of interconnected objects and put them back together on the receiving end. We'll see this in an upcoming example. The implication is that any object we serialize must only contain references to Serializable objects. There are ways around this problem, like marking nonserializable members as transient or overriding the default serialization mechanisms. The transient modifier can be applied to any instance variable to indicate that its contents are not useful outside of the current context and should never be saved.

In the following example, we create a Hashtable and write it to a disk file called h.ser:

import java.io.*;
import java.util.*;
public class Save {
  public static void main(String[] args) {
    Hashtable h = new Hashtable();
    h.put("string", "Gabriel Garcia Marquez");
    h.put("int", new Integer(26));
    h.put("double", new Double(Math.PI));
    
    try {
      FileOutputStream fileOut = new FileOutputStream("h.ser");
      ObjectOutputStream out = new ObjectOutputStream(fileOut);
      out.writeObject(h);
    }
    catch (Exception e) {
      System.out.println(e);
    }
  }
}

First we construct a Hashtable with a few elements in it. Then, in the three lines of code inside the try block, we write the Hashtable to a file called h.ser, using the writeObject() method of ObjectOutputStream. The ObjectOutputStream class is a lot like the DataOutputStream class, except that it includes the powerful writeObject() method. The Hashtable object is serializable because it implements the Serializable interface.

The Hashtable we created has internal references to the items it contains. Thus, these components are automatically serialized along with the Hashtable. We'll see this in the next example when we deserialize the Hashtable:

import java.io.*;
import java.util.*;
public class Load {
  public static void main(String[] args) {
    try {
      FileInputStream fileIn = new FileInputStream("h.ser");
      ObjectInputStream in = new ObjectInputStream(fileIn);
      Hashtable h = (Hashtable)in.readObject();
      System.out.println(h.toString());
    }
    catch (Exception e) {
      System.out.println(e);
    }
  }
}

In this example, we read the Hashtable from the h.ser file, using the readObject() method of ObjectInputStream. The ObjectInputStream class is a lot like DataInputStream, except it includes the readObject() method. The return type of readObject() is Object, so we need to cast it to a Hashtable. Finally, we print out the contents of the Hashtable using its toString() method.

We'll see more examples of serialization at work in Chapter 18 when we discuss JavaBeans. There we'll see that it is even possible to serialize graphical AWT components in mid-use and bring them back to life later.