[jdom-interest] Serialization Trouble through Inheritance

Thu Aug 9 08:51:38 PDT 2012

Hi Björn, Oliver

I have identified what the problem is...... this is not actually a Java
bug, but it describes what the JDOM bug is... I think....

http://bugs.sun.com/view_bug.do?bug_id=6522514

What this implies is that there is an issue in the permissions of the JDOM
element hierarchy.

in my understanding, it goes like this....

org.jdom2.Element extends org.jdom2.Content.
org.jdom2.Content extends org.jdom2.CloneBase.
org.jdom2.CloneBase is a package-private (not public) class, and CloneBase
has no declared constructor (it has the default no-arg constructor).

Your class 'MyElement' is not in the org.jdom2 package, so, it cannot
'see' the no-arg constructor for (package private) org.jdom2.CloneBase.

Thus the MyElement class cannot be de-serialized.

I have 'proven' that this is the logic fault by putting the MyElement
class in the org.jdom2 package, and then, miraculously, the code works.
When I get home I will try it again but instead with a 'protected' no-arg
constructor on org.jdom2.CloneBase, and the MyElement class in some other
package..... actually, I have just tried it now, and it works.....

Thus, there are two work-arounds:
 - putting your MyElement code in the org.jdom2 package....
 - using the attached org.jdom2.CloneBase class which has a protected
no-arg constructor.

I have created a new issue (#88), and I have attached a working CloneBase
class you can add to your project temporarily.....

I will push out JDOM 2.0.3

Rolf

On Thu, 9 Aug 2012 11:03:52 -0400, Oliver Ruebenacker <curoli at gmail.com>
wrote:
> Hello,
> 
>   Rolf, it shouldn't be necessary to add these methods.
> 
>   Björn, can you check the import statement for Element?
> 
>      Take care
>      Oliver
> 
> On Thu, Aug 9, 2012 at 10:37 AM, Rolf Lear <jdom at tuis.net> wrote:
>>
>> Hi Björn
>>
>> I don't have my 'JDOM Laptop' with me at the moment, so I can't easily
>> reproduce your problem, but, I suspect this is just an issue of
>> implementing your own read/write object methods.
>>
>> What happens if you add the following methods to your 'MyElement'
class?
>> I
>> believe this should fix things..... try it... :-)
>>
>>
>>
>>         /**
>>          * Serialize out the MyElement.
>>          *
>>          * @serialData
>>          * The Stream protocol is:
>>          * <ol>
>>          *   <li>Write the super Element class out...
>>          * </ol>
>>          *
>>          * @param out where to write the Element to.
>>          * @throws IOException if there is a writing problem.
>>          */
>>         private void writeObject(final ObjectOutputStream out) throws
>>         IOException
>> {
>>                 out.defaultWriteObject();
>>         }
>>
>>         /**
>>          * Read a MyElement off the ObjectInputStream.
>>          *
>>          * @see #writeObject(ObjectOutputStream)
>>          * @param in where to read the Element from.
>>          * @throws IOException if there is a reading problem.
>>          * @throws ClassNotFoundException when a class cannot be found
>>          */
>>         private void readObject(final ObjectInputStream in)
>>                         throws IOException, ClassNotFoundException {
>>                 in.defaultReadObject();
>>         }
>>
>>
>> Rolf
>>
>>
>>
>> On Thu, 09 Aug 2012 16:08:48 +0200, Björn Buchner
>> <b.buchner at isys-software.de> wrote:
>>> Hi Folks,
>>>
>>> I am using the JDOM 2.0.2 release and experienced some trouble when it
>>> comes to serialization.
>>>
>>> De-/serializing an Element object through the standard
>>> ObjectOutput/InputStream is no problem. Trouble starts when I try to
>>> deserialize an object of a class that inherits from Element.
>>>
>>> It always ends with a InvalidClassException: no valid constructor.
>>> Normally this indicates that JRE is missing a public default
>>> constructor, but as you can see in the example below the subclass
>>> has a default constructor.
>>>
>>> Thanks in advance for your help
>>>
>>> Example code:
>>>
>>> public class MyElement extends Element {
>>>
>>>      private static final long serialVersionUID =
-4220756491425652053L;
>>>
>>>      public MyElement() {
>>>          super();
>>>      }
>>>
>>>
>>>      public static void main(String ... args) throws IOException,
>>> ClassNotFoundException {
>>>         ByteArrayOutputStream buffer = new ByteArrayOutputStream();
>>>         ObjectOutputStream outStream = new ObjectOutputStream(buffer);
>>>
>>>         MyElement element = new MyElement();
>>>
>>>         outStream.writeObject(element);
>>>         outStream.flush();
>>>         ObjectInputStream inStream = new ObjectInputStream(new
>>> ByteArrayInputStream(buffer.toByteArray()));
>>>         element = (MyElement)inStream.readObject();
>>>      }
>>> }
>>>
>>> Result:
>>>
>>> Exception in thread "main" java.io.InvalidClassException:
>>> <packageremoved>; no valid constructor
>>>      at
>>>
>>
java.io.ObjectStreamClass$ExceptionInfo.newInvalidClassException(ObjectStreamClass.java:147)
>>>      at
>>> java.io.ObjectStreamClass.checkDeserialize(ObjectStreamClass.java:755)
>>>      at
>>>
>>
java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:1751)
>>>      at
>> java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1347)
>>>      at
java.io.ObjectInputStream.readObject(ObjectInputStream.java:369)
>>>      at <packageremoved>.main(MyElement.java:29)
>> _______________________________________________
>> To control your jdom-interest membership:
>> http://www.jdom.org/mailman/options/jdom-interest/youraddr@yourhost.com
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package org.jdom2;

/**
 * This simple class just tidies up any cloneable classes. This method deals
 * with any CloneNotSupported exceptions. THis class is package private only.
 * 
 * @author Rolf Lear
 */
class CloneBase implements Cloneable {

	/**
	 * Return a deep clone of this instance. Even if this instance has a parent,
	 * the returned clone will not.
	 * <p>
	 * All JDOM core classes are Cloneable, and never throw
	 * CloneNotSupportedException. Additionally all Cloneable JDOM classes
	 * return the correct type of instance from this method and there is no
	 * need to cast the result (co-variant return vaue).
	 * <p>
	 * Subclasses of this should still call super.clone() in their clone method.
	 */
	@Override
	protected CloneBase clone() {
		/*
		 * Additionally, when you use the concept of 'co-variant return values'
		 * you create 'bridge' methods. By way of example, because we change the
		 * return type of clone() from Object to CloneBase, Java is forced to
		 * put in a 'bridge' method that has an Object return type, even though
		 * we never actually call it. <p> This has an impact on the code
		 * coverage tool Cobertura, which reports that there is missed code (and
		 * there is, the bridge method). It reports it as being '0' calls to the
		 * 'class' line (the class line is marked red). By making this CloneBase
		 * code do the first level of co-variant return, it is this class which
		 * is victim of the Cobertura reporting, not the multiple subclasses
		 * (like Attribute, Document, Content, etc.).
		 */
		try {
			return (CloneBase) super.clone();
		} catch (CloneNotSupportedException e) {
			throw new IllegalStateException(String.format(
					"Unable to clone class %s which should always support it.",
					this.getClass().getName()), e);
		}
	}

}