Lesson: Long Term Persistence

Long-term persistence is a model that enables beans to be saved in XML format.

Information on the XML format and on how to implement long-term persistence for non-beans can be found in XML Schema and Using XMLEncoder .

Encoder and Decoder

The XMLEncoder class is assigned to write output files for textual representation of Serializable objects. The following code fragment is an example of writing a Java bean and its properties in XML format:

     XMLEncoder encoder = new XMLEncoder(
                new BufferedOutputStream(
                        new FileOutputStream( "Beanarchive.xml" ) ) );

        encoder.writeObject( object );
        encoder.close(); 

The XMLDecoder class reads an XML document that was created with XMLEncoder:

      XMLDecoder decoder = new XMLDecoder(
                new BufferedInputStream(
                        new FileInputStream( "Beanarchive.xml" ) ) );

        Object object = decoder.readObject();
        decoder.close();

What's in XML?

An XML bean archive has its own specific syntax, which includes the following tags to represent each bean element:

• an XML preamble to describe a version of XML and type of encoding
• a <java> tag to embody all object elements of the bean
• an <object> tag to represent a set of method calls needed to reconstruct an object from its serialized form

  	<object class="javax.swing.JButton" method="new">
               <string>Ok</string>
          </object>
  	

  or statements

  	<object class="javax.swing.JButton">
               <void method="setText">
               <string>Cancel</string>
               </void>
          </object>
  	

• tags to define appropriate primitive types:
  • <boolean>
  • <byte>
  • <char>
  • <short>
  • <int>
  • <long>
  • <float>
  • <double>

  		<int>5555</int>
  		

• a <class> tag to represent an instance of Class.

  	<class>java.swing.JFrame</class>
  	

• an <array> tag to define an array

  	<array class="java.lang.String" length="5">
          </array>
  	

The following code represents an XML archive that will be generated for the SimpleBean component:

<?xml version="1.0" encoding="UTF-8" ?>
<java>
  <object class="javax.swing.JFrame">
    <void method="add">
      <object class="java.awt.BorderLayout" field="CENTER"/>
      <object class="SimpleBean"/>
    </void>
    <void property="defaultCloseOperation">
      <object class="javax.swing.WindowConstants" field="DISPOSE_ON_CLOSE"/>
    </void>
    <void method="pack"/>
    <void property="visible">
      <boolean>true</boolean>
    </void>
  </object>
</java>