Custom Tag in race condition with OC4J v9.0.2.0.0...

Question

Hello all,
(I tried deploying my application with OC4J 9.0.3 but none of my existing
tags worked)
I developed a OC4J web application implementing my own tag library extension
and found out that there was occurring a race condition to the fact that two
users (different sessions) where accessing the same
tag at the same time. In the offending tag implementation I only have
instance members and not static which could also cause a
concurrent access problem.
I found that OC4J was reusing concurrently the same
Tag instance when it should not do that!!!
public class pagesIteratorTag extends BodyTagSupport {
// only instance class members...
public void doInitBody() throws JspException {
System.out.println("pageSetIteratorTag::doInitBody - Tag instance value: " + this.toString() + " for user: " + this.pageContext.getSession().getAttribute("j_username"));
public int doAfterBody() throws JspException {           
System.out.println("pageSetIteratorTag::doAfterBody - Tag instance value: " + this.toString() + " for user: " + this.pageContext.getSession().getAttribute("j_username"));
Note the pagesIteratorTag@2e same instance is used when it should not
1 because the tag instance should be protected from multiple concurrent
access even though it can be pooled and reused if it is free.
pageSetIteratorTag::doInitBody - Tag instance value: com.kdlabs.fogal.tagext.pagesIteratorTag@2e for user: Frank
pageSetIteratorTag::doAfterBody - Tag instance value: com.kdlabs.fogal.tagext.pagesIteratorTag@2e for user: Frank
pageSetIteratorTag::doInitBody - Tag instance value: com.kdlabs.fogal.tagext.pagesIteratorTag@2e for user: Giovanni
pageSetIteratorTag::doAfterBody - Tag instance value: com.kdlabs.fogal.tagext.pagesIteratorTag@2e for user: Giovanni
pageSetIteratorTag::doInitBody - Tag instance value: com.kdlabs.fogal.tagext.pagesIteratorTag@2e for user: Giovanni
pageSetIteratorTag::doAfterBody - Tag instance value: com.kdlabs.fogal.tagext.pagesIteratorTag@2e for user: Giovanni
Session for user Frank throws a null pointer exception because Giovanni's session
started accessing the variable.
Can anyone advise please?
Best Regards,
Giovanni

Answer 1

First of all, you said none of your tags worked in 9.0.3. What happened? Did you do any debugging of the problem?
Comparing the "toString()" output of your tag does not guarantee they are the same instance. That is just the output of "hashcode()", not the "pointer" to the object. The hashcode is generated from the contents of the object, not its "identity".
I suggest you track the NPE in the debugger and get more information before you assume the container is at fault (which is still an outside possibility). SDK API Javadoc
Object class:
The general contract of hashCode is:
1-. Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
So, the toString() is properly returning the suffix of the object instance
number unique for that instance and is then being reused by OC4J even though
it is being accessed from another session thread, that must not happen and it
is not compliant with the JSP spec.
The other history is that when I implemented my custom tags OC4J was JSP 1.1 compliant
and not 1.2, so after moving to 1.2 the scripting variables can be defined in XML and
by subclassing TagInfo class, the last option doesn't work with OC4J 9.0.3, not good...
Thanks for your help,
I will keep trying to figure out what the problem migth be,
Best Regards,
Giovanni