Java XML serialization: XMLEncoder/XMLDecoder example

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04/09/2007: Posts:
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July 29, 2010 18:46:57 Last update: July 29, 2010 18:48:15

This is an example of using java.beans.XMLEncoder and java.beans.XMLDecoder to serialize/deserialize Java objects to XML.

Java code

TestXMLEncoder.java:

import java.io.*;
import java.beans.XMLEncoder;
import java.beans.XMLDecoder;

public class TestXMLEncoder {
    public static void main(String[] args) throws Exception {
	TestXMLEncoder t = new TestXMLEncoder();
	t.testSimpleBean();
	t.testCompositeBean();
	t.testNoDefaultConstructor();
    }

    private void testSimpleBean() throws Exception {
	System.out.println("Testing simple bean");
	File simpleBeanXml = new File("simplebean.xml");
	SimpleBean b = new SimpleBean();
	b.setName("Java");
	encodeObject(b, simpleBeanXml);
	SimpleBean b2 = (SimpleBean) decodeObject(simpleBeanXml);

	System.out.println("Retrieved: " + b2);
	System.out.println();
    }

    private void testCompositeBean() throws Exception {
	System.out.println("Testing composite bean");
	File compositeBeanXml = new File("compositebean.xml");
	CompositeBean b = new CompositeBean();
	SimpleBean s = new SimpleBean();
	s.setName("Nested");
	b.setS(s);
	encodeObject(b, compositeBeanXml);
	CompositeBean b2 = (CompositeBean) decodeObject(compositeBeanXml);

	System.out.println("Retrieved: " + b2);
	System.out.println();
    }

    private void testNoDefaultConstructor() throws Exception {
	System.out.println("Testing noDefaultConstructor");
	File noDefaultXml = new File("nodefault.xml");
	NotABean b = new NotABean("NotBean");
	encodeObject(b, noDefaultXml);
	NotABean b2 = (NotABean) decodeObject(noDefaultXml);

	System.out.println("Retrieved: " + b2);
	System.out.println();
    }

    void encodeObject(Object o, File xmlFile) throws IOException {
	XMLEncoder encoder = new XMLEncoder(
				new FileOutputStream(xmlFile)
			     );
	encoder.writeObject(o);
	encoder.close();
    }

    Object decodeObject(File xmlFile) throws IOException {
	XMLDecoder decoder = new XMLDecoder(
				new FileInputStream(xmlFile)
			     );
	Object o = decoder.readObject();
	decoder.close();
	return o;
    }
}

SimpleBean.java:

public class SimpleBean {
    private String name;

    public String getName() {
	return this.name;
    }

    public void setName(String name) {
	this.name = name;
    }

    public String toString() {
	return String.format("[%s] %s",
		    this.getClass().getName(),
		    getName()
	       );
    }
}

CompositeBean.java:

public class CompositeBean {
    private SimpleBean s;

    public SimpleBean getS() {
	return s;
    }

    public void setS(SimpleBean s) {
	this.s = s;
    }

    public String toString() {
	return String.format("[%s] %s",
		    this.getClass().getName(),
		    getS().getName()
	       );

    }
}

NotABean.java:

public class NotABean {
    private String name;

    public NotABean(String name) {
	this.name = name;
    }

    public String getName() {
	return this.name;
    }

    public void setName(String name) {
	this.name = name;
    }

    public String toString() {
	return String.format("[%s] %s", 
		    this.getClass().getName(),
		    getName()
	       );
    }
}

Test Output
By default, only Java beans can be serialized using XMLEncoder. Exception occurs when you try to deserialize an object which is not a Java bean. Errors actually occur when you try to serialize, but they are ignored. As a result, an XML file is generated by the serialization but the file is useless!

Console output:

C:\>java TestXMLEncoder
Testing simple bean
Retrieved: [SimpleBean] Java

Testing composite bean
Retrieved: [CompositeBean] Nested

Testing noDefaultConstructor
java.lang.InstantiationException: NotABean
Continuing ...
java.lang.Exception: XMLEncoder: discarding statement XMLEncoder.writeObject(NotABean);
Continuing ...
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
        at com.sun.beans.ObjectHandler.dequeueResult(ObjectHandler.java:139)
        at java.beans.XMLDecoder.readObject(XMLDecoder.java:201)
        at TestXMLEncoder.decodeObject(TestXMLEncoder.java:62)
        at TestXMLEncoder.testNoDefaultConstructor(TestXMLEncoder.java:44)
        at TestXMLEncoder.main(TestXMLEncoder.java:10)

simplebean.xml:

<?xml version="1.0" encoding="UTF-8"?> 
<java version="1.6.0_20" class="java.beans.XMLDecoder"> 
 <object class="SimpleBean"> 
  <void property="name"> 
   <string>Java</string> 
  </void> 
 </object> 
</java>

compositebean.xml:

<?xml version="1.0" encoding="UTF-8"?> 
<java version="1.6.0_20" class="java.beans.XMLDecoder"> 
 <object class="CompositeBean"> 
  <void property="s"> 
   <object class="SimpleBean"> 
    <void property="name"> 
     <string>Nested</string> 
    </void> 
   </object> 
  </void> 
 </object> 
</java>

nodefault.xml:

<?xml version="1.0" encoding="UTF-8"?> 
<java version="1.6.0_20" class="java.beans.XMLDecoder"> 
</java>

More advanced techniques are presented here: Using XMLEncoder

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Java XML serialization: XMLEncoder/XMLDecoder example