FULL PRODUCT VERSION :
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)


ADDITIONAL OS VERSION INFORMATION :
Linux itis-asus-desktop 2.6.20-16-generic #2 SMP Thu Jun 7 20:19:32 UTC 2007 i686 GNU/Linux


A DESCRIPTION OF THE PROBLEM :
Incorrect XML is produced for a particulary set of beans:
ParentBean having one property "bean", an ImmutableBean
ImmutableBean having one property "a", an int. This property is immutable - it is set in the constructor, and has a getter only. A DefaultPersistenceDelegate is registered, and this bean will encode correctly when it is encoded directly rather than as a property of ParentBean.

When ParentBean has a default constructor giving "bean" property a default value, any instance of ParentBean will be encoded with default "bean" property in the XML. That is, the XML will just create a default ParentBean, regardless of what was encoded.

If ParentBean gives "bean" property the value null in default constructor, it will be encoded correctly

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Run the ParentTest class, check output to console and user.dir

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Should serialise with the ImmutableBean encoded, producing the following output:

ImmutableBean a value, should be 254: 254
Default parent.getBean().getA() value: 1
parent.getBean().getA() value after calling parent.setBean(): 47
parent.getBean().getA() value after XML encoding and decoding: 47

And the following XML in user directory:

ParentBean.xml:

<?xml version="1.0" encoding="UTF-8"?>
<java version="1.6.0" class="java.beans.XMLDecoder">
 <object class="com.itis.proto.ParentBean">
  <void property="bean">
   <object class="com.itis.proto.ImmutableBean">
    <int>47</int>
   </object>
  </void>
 </object>
</java>


ImmutableBean.xml:

<?xml version="1.0" encoding="UTF-8"?>
<java version="1.6.0" class="java.beans.XMLDecoder">
 <object class="com.itis.proto.ImmutableBean">
  <int>254</int>
 </object>
</java>

ACTUAL -
Actually serialises with ImmutableBean missing (assumed to be default), producing the following output:

ImmutableBean a value, should be 254: 254
Default parent.getBean().getA() value: 1
parent.getBean().getA() value after calling parent.setBean(): 47
parent.getBean().getA() value after XML encoding and decoding: 1

And the following XML in user directory:

ParentBean.xml:

<?xml version="1.0" encoding="UTF-8"?>
<java version="1.6.0" class="java.beans.XMLDecoder">
 <object class="com.itis.proto.ParentBean"/>
</java>

ImmutableBean a value, should be 254: 254
Default parent.getBean().getA() value: 1
parent.getBean().getA() value after calling parent.setBean(): 47
parent.getBean().getA() value after XML encoding and decoding: 47

And the following XML in user directory:

ParentBean.xml:

<?xml version="1.0" encoding="UTF-8"?>
<java version="1.6.0" class="java.beans.XMLDecoder">
 <object class="com.itis.proto.ParentBean">
  <void property="bean">
   <object class="com.itis.proto.ImmutableBean">
    <int>47</int>
   </object>
  </void>
 </object>
</java>

ImmutableBean.xml:


<?xml version="1.0" encoding="UTF-8"?>
<java version="1.6.0" class="java.beans.XMLDecoder">
 <object class="com.itis.proto.ImmutableBean">
  <int>254</int>
 </object>
</java>


ERROR MESSAGES/STACK TRACES THAT OCCUR :
No exceptions - listeners were registered on the encoder and decoder to print stack traces of any exceptions.

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
//(please note there are 3 classes, for the two beans and the test).


import java.beans.BeanInfo;
import java.beans.DefaultPersistenceDelegate;
import java.beans.IntrospectionException;
import java.beans.Introspector;
  
public class ImmutableBean {
  
    //Set up XML persistence delegate
    static {
        try {
            BeanInfo info = Introspector.getBeanInfo(ImmutableBean.class);
            info.getBeanDescriptor().setValue("persistenceDelegate",
                    new DefaultPersistenceDelegate(
                            new String[]{
                                    "a"
                                    })
                    );
            } catch (IntrospectionException exception) {
            throw new RuntimeException("Unable to persist ImmutableBean", exception);
        }
    }
      
    int a;
      
    public ImmutableBean(int a) {
        super();
        this.a = a;
    }
    public int getA() {
        return a;
    }
}
  
  
  
  
public class ParentBean {
  
    ImmutableBean bean;
      
    public ParentBean() {
        super();
        //Change this line to the commented version to make
        //the encoding work. If default value of "bean" property
        //in this bean is null, then XMLEncoder will bother to encode
        //the actual property, otherwise it will always consider any
        //value of the "bean" property to be the same as the default,
        //and not encode it
        this.bean = new ImmutableBean(1);
        //this.bean = null;
    }
      
    public ImmutableBean getBean() {
        return bean;
    }
    public void setBean(ImmutableBean bean) {
        this.bean = bean;
    }
}
  
  
  
  
import java.beans.ExceptionListener;
import java.beans.XMLDecoder;
import java.beans.XMLEncoder;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
  
public class ParentBeanTest {
  
    public final static void main(String[] args) {
  
        try {
              
            //First encode an ImmutableBean, just to show it works on its own
            encodeAndDecodeImmutableBean();
  
            //Now encode/decode the parent bean
            encodeAndDecodeParentBean();
              
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
  
    public static void encodeAndDecodeParentBean() throws FileNotFoundException {
        ParentBean parent = new ParentBean();
          
        if (parent.getBean() != null) System.out.println("Default parent.getBean().getA() value: " + parent.getBean().getA());
          
        ImmutableBean newBigBean = new ImmutableBean(47);
          
        parent.setBean(newBigBean);
  
        System.out.println("parent.getBean().getA() value after calling parent.setBean(): " + parent.getBean().getA());
          
        File xmlFile = new File(System.getProperty("user.home"), "ParentBean.xml");
        ParentBean clone = (ParentBean)encodeAndDecode(parent, xmlFile);
          
        System.out.println("parent.getBean().getA() value after XML encoding and decoding: " + clone.getBean().getA());
    }
      
    public static void encodeAndDecodeImmutableBean() throws FileNotFoundException {
        ImmutableBean immutable = new ImmutableBean(254);
        File xmlFile = new File(System.getProperty("user.home"), "ImmutableBean.xml");
        ImmutableBean clone = (ImmutableBean)encodeAndDecode(immutable, xmlFile);
        System.out.println("ImmutableBean a value, should be 254: " + clone.getA());
    }
      
    public static Object encodeAndDecode(Object bean, File file) throws FileNotFoundException {
        XMLEncoder encoder = new XMLEncoder(new FileOutputStream(file));
          
        //Make sure we see any encoder exceptions
        encoder.setExceptionListener(new ExceptionListener() {
            public void exceptionThrown(Exception e) {
                e.printStackTrace();
            }
        });
        encoder.writeObject(bean);
        encoder.close();
          
        //Make sure we see any decoder exceptions
        XMLDecoder decoder = new XMLDecoder(new FileInputStream(file), null,
                new ExceptionListener() {
                    public void exceptionThrown(Exception e) {
                        e.printStackTrace();
                    }
                });
          
        return decoder.readObject();
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Setting the default value of the "bean" property in ParentBean default constructor to null causes XMLEncoder to notice the "bean" property has changed. Presumably a custom persistence delegate for ImmutableBean and/or ParentBean could also work.