[jdom-interest] Serialization Trouble through Inheritance

Thu Aug 9 09:23:03 PDT 2012

Hmmm... in a moment of daftness, I attached the wrong file (more haste...
less speed).

here is the working CloneBase file.

Rolf

On Thu, 09 Aug 2012 11:51:38 -0400, Rolf Lear <jdom at tuis.net> wrote:
> 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 {

	/**
	 * Make the default no-arg constructor available to all sub-classes, not just subclasses in the
	 * org.jdom2 package.
	 * 
	 */
	protected CloneBase() {
		super();
	}

	/**
	 * 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);
		}
	}

}