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Trail: JavaBeans(TM)

Lesson: Bean Persistence

A Bean persists by having its properties, fields, and state information saved and restored to and from storage. The mechanism that makes persistence possible is called serialization. When a Bean instance is serialized, it is converted into a data stream and written to storage. Any applet, application, or tool that uses that Bean can then "reconstitute" it by deserialization. JavaBeans uses the JDK's object serialization API for its serialization needs.

Version note: As of v 1.4, you can save beans in XML format using the long-term persistence mechanism. The API for this support is summarized in API Enhancements to the JavaBeans Component API in v1.4 (outside of the tutorial). Information on the XML format and on how to implement long-term persistence for non-beans is in two articles in The Swing ConnectionXML Schema (outside of the tutorial) and Using XMLEncoder (outside of the tutorial), respectively.

All Beans must persist. To persist, your Beans must support serialization by implementing either the (in the API reference documentation) interface, or the (in the API reference documentation) interface. These interfaces offer you the choice between automatic serialization, and "roll your own". As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable.

Note:  For complete information on serialization, see the Java Object Serialization (outside of the tutorial) web site and the Object Serialization (in the JavaBeans(TM) trail) trail.

Controlling Serialization

You can control the level of serialization that your Beans undergo:

Default Serialization: The Serializable Interface

The Serializable interface provides automatic serialization by using the Java Object Serialization tools. Serializable declares no methods; it acts as a marker, telling the Object Serialization tools that your Bean class is serializable. Marking your class with Serializable means you are telling the Java Virtual Machine (JVM) that you have made sure your class will work with default serialization. Here are some important points about working with the Serializable interface:

The BeanBox writes serialized Beans to a file with a .ser extension.

The OurButton demo Bean uses default serialization to make its properties persist. OurButton only added Serializable to its class definition to make use of default serialization:

public class OurButton extends Component implements Serializable,...

If you drop an OurButton instance into the BeanBox, the properties sheet displays OurButton's properties. To ascertain that serialization is working

  1. Change some OurButton properties. For example change the font size and colors.
  2. Serialize the changed OurButton instance by selecting the File|SerializeComponent... BeanBox menu item. A file browser will pop up.
  3. Put the .ser file in a JAR file with a suitable manifest.
  4. Clear the BeanBox form by selecting the File|Clear menu item.
  5. Reload the serialized instance by selecting the File|LoadJar menu item.
The OurButton instance will appear in the BeanBox with your property changes intact. By implementing Serializable in your class, simple, primitive properties and fields can be serialized. For more complex class members, different techniques must be used, as described in the following sections.

Selective Serialization Using the transient Keyword

To exclude fields from serialization in a Serializable object from serialization, mark the fields with the transient modifier.

transient int Status;
Default serialization will not serialize transient and static fields.

Selective Serialization: writeObject and readObject

If your serializable class contains either of the following two methods (the signatures must be exact), then the default serialization will not take place.

private void writeObject( out)
    throws IOException;
private void readObject( in)
    throws IOException, ClassNotFoundException;

You can control how more complex objects are serialized, by writing your own implementations of the writeObject and readObject methods. Implement writeObject when you need to exercise greater control over what gets serialized, when you need to serialize objects that default serialization cannot handle, or when you need to add data to the serialization stream that is not an object data member. Implement readObject to reconstruct the data stream you wrote with writeObject.

Example: The Molecule Demo Bean

The Molecule demo keeps a version number in a static field. Since static fields are not serialized by default, writeObject and readObject are implemented to serialize this field. Here is the writeObject and readObject implementations in

private void writeObject( s)
                         throws {

private void readObject( s)
                        throws java.lang.ClassNotFoundException,
    // Compensate for missing constructor.
    if (s.readInt() != ourVersion) {
        throw new IOException("Molecule.readObject: version mismatch");
    moleculeName = (String) s.readObject();

These implementations limit the fields serialized to ourVersion and moleculeName. Any other data in the class will not be serialized.

It is best to use the ObjectInputStream's defaultWriteObject and defaultReadObject before doing your own specific stream writing. For example:

private void writeObject( s)
                         throws {
    //First write out defaults

private void readObject( s)
                        throws java.lang.ClassNotFoundException,
    //First read in defaults

The Externalizable Interface

Use the Externalizable interface when you need complete control over your Bean's serialization (for example, when writing and reading a specific file format). You need to implement two methods: readExternal and writeExternal. Externalizable classes must also have a no-argument constructor.

Example: The BlueButton and OrangeButton Demo Beans

When you run the BeanBox, you will see two Beans named BlueButton and OrangeButton in the ToolBox. These two Beans are actually serialized instances of the ExternalizableButton class.

ExternalizableButton implements the Externalizable interface. This means it does all its own serialization, by implementing Externalizable.readExternal and Externalizable.writeExternal. The BlueButtonWriter program is used by the buttons makefile to create an ExternalizableButton instance, change its background property to blue, and write the Bean out to the file BlueButton.ser. Another button, OrangeButton, is created the same way using OrangeButtonWriter. The button makefile then puts these .ser files in buttons.jar, where the ToolBox can find and reconstitute them.

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