Tuesday May 15, 2007

JavaOne 2007

All of last week I was at JavaOne. It was an exhausting but very interesting week. Like last year, there were many interesting sessions, too many to list them here. Let me just mention the one I enjoyed most was the one by Neal Gafter on Closures for the Java Programming Language (BOF-2358). I can't wait until they're in the Java language!

Not only did I attend sessions and BOFs, I also presented BOFs. Three of them to be precise. I recorded the audio on my MP3 player. Unfortunately the quality of the audio is pretty bad. I'm posting the audio recordings below. I'm also posting the slides. Here they are:

BOF8847: Developing Components for Java Business Integration: Binding Components and Service Engines

Presented by Frank Kieviet, Alex Fung, Sherry Weng, and Srinivasan Chikkala
Attendance: about 100

You cannot cover how to write JBI components in just 45 minutes. We were also not sure about what the audience was interested in. That's why we assumed that the audience would consist mostly of people who have never written a JBI component before, and are relatively new to JBI. That's why we decided to talk mostly about general information on JBI and JBI components, and highlight the power of JBI and discuss how to go about developing one.

As an experiment I wanted to try a new format (at least new for me): rather than slicing up the session into four parts of 10 minutes, we cast the session into a "discussion forum". Of course the questions and answers (and even the jokes) were well rehearsed.

Unfortunately, the audio/visual people that control the meeting rooms, had forgotten to start the session timer. As a result the audio was cut unexpectedly just a minute before we could finish up.

Nevertheless, I think it was an interesting session.

Presentation JavaOne07-BOF8847 (pdf)

Audio JavaOne07-BOF8847 (mp3)


BOF8745: Leveraging Java EE in JBI and vice versa

Presented by Frank Kieviet and Bhavanishankara Sapaliga
Attendance: about 60

This BOF was originally to be presented by Vikas Awasthi and Bhavanishankara Sapaliga, but Vikas couldn't make it, so I replaced him. We focused the session on how JBI and EE can play together, trying to make it interesting for both JBI application developers as well as for EE developers. At the end I ran a demo with NetBeans showing three different scenarios. The demo-gods were with me: the demo went very smoothly. Unfortunately I forgot to demo how to add an EJB to a composite application. Another valuable lesson learned.

Presentation JavaOne07-BOF8745 (pdf)

Audio JavaOne07-BOF8745 (mp3)


BOF9982: The java.lang.OutOfMemoryError: PermGen Space error demystified

Presented by Edward Chou and Frank Kieviet
Attendance: about 116

This session was on Thursday night at 10pm. That night was the JavaOne After dark bash. Free beers, music and snacks for everyone. Therefore we didn't expect much of an attendance: memory leaks are a rather dry subject, and why leave the party early to go to this session? Also, some of our thunder had been stolen by SAP who demo-ed a tool to track memory leaks in a morning-session earlier that week. So we were quite surprised when about 116 people turned up for our session. Most stayed until the very end, and there were also quite a few interesting questions. Apparently a lot of people struggle with memory leaks in permgen space -- in my presentation I mention that I get about a hundred hits on my blog every day from people who search for this memory exception in Google.

Presentation JavaOne07-BOF9982 (pdf)

Audio JavaOne07-BOF9982 (mp3)

Wednesday Nov 15, 2006

More on... How to fix the dreaded "java.lang.OutOfMemoryError: PermGen space" exception (classloader leaks)

I got quite a few comments on my last blog (How to fix the dreaded "java.lang.OutOfMemoryError: PermGen space" exception (classloader leaks)). Apparently more people have been struggling with this problem.


Why bring this up? What's the news? Edward Chou continued to explore options to diagnose classloader leaks. First of all, he explored how to generate a list of orphaned classloaders with jhat. An orphaned classloader is a classloader that is not referenced by any object directly but cannot be garbage collected. The thinking behind this is that programs that create classloaders (e.g. application servers) do maintain references to them. So if there's a classloader that is no longer directly referenced, this classloader is probably a leak. Read about it on his blog (Find Orphaned Classloaders).

Still we were not satisfied: when examining some memory dumps from code that we were not familiar with, we explored yet some other options to diagnose classloader leaks: duplicate classes and duplicate classloaders. Let me explain.

Let's say that your application has a com.xyz.Controller class. If you find many instances of this class object, you likely have a classloader leak. Note the phrase "instances of this class object". What I mean by this: the class com.xyz.Controller is loaded multiple times, i.e. multiple instances of the com.xyz.Controller.class are present.  You can use jhat to run this query: simply list all instances of java.lang.Class.

Edward modified jhat to generate a list of all classloader instances that have an identical set of classes that it loaded. Typically there's no reason why someone would create two classloader instances and load exactly the same set of classes into them. If you find any in your memory dump, you should get suspicious and take a closer look. Monitor Edward's blog for more details on this.

One more thing: Edward found out that the method java.lang.String.intern() allocates memory in PermGen space. So if your application frequently uses this method with different strings, watch out. Fortunately these strings are subject to garbage collection. But if your application holds references to these strings, thereby making garbage collection impossible, your application may cause the dreaded "java.lang.OutOfMemoryError: PermGen space" exception. No classloaders involved this time.

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