[Chapter 9] Object Serialization
Author: David Flanagan
Date: Thu Jul 31 15:56:42 1997

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| Chapter 9 |

# 9. Object Serialization

Contents:
Simple Serialization
Custom Serialization
Serialization and Class Versioning
Serialized Applets
Advanced Serialization

Object serialization is one of the important new features of Java 1.1. Despite its importance, however, serialization is done with a very simple API. This chapter demonstrates several uses of serialization.

## 9.1 Simple Serialization

Objects are serialized with the `ObjectOutputStream` and they are deserialized with the `ObjectInputStream`. Both of these classes are part of the `java.io` package, and they function, in many ways, like `DataOutputStream` and `DataInputStream` because they define the same methods for writing and reading binary representations of Java primitive types to and from streams. What `ObjectOutputStream` and `ObjectInputStream` add, however, is the ability to write and read non-primitive object and array values to and from a stream.

An object is serialized by passing it to the `writeObject()` method of an `ObjectOutputStream`. This writes out the values of all of its fields, including private fields and fields inherited from superclasses. The values of primitive fields are simply written to the stream as they would be with a `DataOutputStream`. When a field in an object refers to another object, an array, or a string, however, the `writeObject()` method is invoked recursively to serialize that object as well. If that object (or an array element) refers to another object, `writeObject()` is again invoked recursively. Thus, a single call to `writeObject()` may result in an entire "object graph" being serialized. When two or more objects each refer to the other, the serialization algorithm is careful to only output each object once--`writeObject()` cannot enter infinite recursion.

Deserializing an object simply follows the reverse of this process. An object is read from a stream of data by calling the `readObject()` method of an `ObjectInputStream`. This re-creates the object in the state it was in when serialized. If the object refers to other objects, they are recursively deserialized as well.

This ability to serialize an entire graph of objects and read those objects back in later is a very powerful feature that hides itself in two simple looking methods. We used object serialization in Example 8.1, but unless you were paying attention, you might have missed those crucial `writeObject()` and `readObject()` calls. Serialization is used in that `Scribble` example to give the program an automatic file format for saving the user's scribbles.

To refresh your memory, Example 9.1 shows the `save()` method of that application. Note the creation of the `ObjectOutputStream` and the use of the `writeObject()` method. The corresponding `load()` method simply reverses the streams to read the scribble back in. You may want to refer to the complete example in Chapter 8, New AWT Features to examine the `save()` and `load()` methods in context. Also note the use of a `GZIPOutputStream` (from `java.util.zip`) to compress the serialized object data before writing it to disk.

#### Example 9.1: Using Serialized Objects as an Application File Format

    /**
     * Prompt the user for a filename, and save the scribble in that file.
     * Serialize the vector of lines with an ObjectOutputStream.
     * Compress the serialized objects with a GZIPOutputStream.
     * Write the compressed, serialized data to a file with a FileOutputStream.
     * Don't forget to flush and close the stream.
     */
    public void save() {
      // Create a file dialog to query the user for a filename.
      FileDialog f = new FileDialog(frame, "Save Scribble", FileDialog.SAVE);
      f.show();                        // Display the dialog and block.
      String filename = f.getFile();   // Get the user's response
      if (filename != null) {          // If user didn't click "Cancel."
        try {
          // Create the necessary output streams to save the scribble.
          FileOutputStream fos = new FileOutputStream(filename);  // Save to file.
          GZIPOutputStream gzos = new GZIPOutputStream(fos);      // Compress.
          ObjectOutputStream out = new ObjectOutputStream(gzos);  // Save objects
          out.writeObject(lines);      // Write the entire Vector of scribbles.
          out.flush();                 // Always flush the output.
          out.close();                 // And close the stream.
        }
        // Print out exceptions.  We should really display them in a dialog...
        catch (IOException e) { System.out.println(e); }
      }
    }

New Feature Demo |  | Custom Serialization