/ February 2013 ~ Java EE Support Patterns


ClassCastException and ClassLoader Puzzle

The following question and puzzle will test your knowledge on Java class loaders and more precisely on one of the Java language specifications. It will also help you better troubleshoot problems such as java.lang.NoClassDefFoundError.

I highly suggest that you do not look at the explanation and solution until you review the code and come up with your own explanation.

You can download the Java program source code and binaries (compiled with JDK 1.7) here. In order to run the program, simply use the following command:

<JDK 1.7 HOME>\bin\java -classpath MainProgram.jar org.ph.javaee.training9.ChildFirstCLPuzzle

** Make sure that you also download the 3 JAR files below before you run the program.

  • MainProgram.jar contains the main program along with super class ProgrammingLanguage.
  • ProgrammingLanguage.jar contains the super class ProgrammingLanguage.
  • JavaLanguage.jar contains the implementation class JavaLanguage, which extends ProgrammingLanguage.

Question (puzzle)

Review closely the program source and packaging along with the diagram below reflecting the class loader delegation model used for this program.

Why can’t we cast (ChildFirstCLPuzzle.java @line 53) the Object javaLanguageInstance of type JavaLanguage, into ProgrammingLanguage?

// Finally, cast the object instance into ProgrammingLanguage
/** Question: why is the following code failing with ClassCastException given the fact JavaLanguage is indeed a ProgrammingLanguage?? **/
ProgrammingLanguage programmingLanguage = (ProgrammingLanguage)javaLanguageInstance;


Propose a solution to allow the above cast to succeed without changing the original source code. Hint: look again at the class loader delegation model, packaging and diagram.

Answer & solution

The Java program is attempting to demonstrate a Java language specification rule related to class loaders: two classes loaded by different class loaders are considered to be distinct and hence incompatible.

If you review closely the program source, packaging and diagram, you will realize the following facts:

  • Our main program is loaded to the parent class loader e.g. $AppClassLoader.
  • The super class ProgrammingLanguage is also loaded to the parent class loader since it is referenced by our main program at line 53.
  • The implementation class JavaLanguage is loaded to our child class loader e.g. ChildFirstClassLoader which is following a “child first” delegation model.
  • Finally, the super class ProgrammingLanguage is also loaded to our child class loader.

The key point to understand is that the super class is loaded by 2 different class loaders. As per Java language specification, two classes loaded by different class loaders are considered to be distinct and hence incompatible. This means that ProgrammingLanguage loaded from the “child firs” class loader is different and not compatible with ProgrammingLanguage loaded from the parent classloader. This is why the cast attempted at line 53 failed with the error below:

ChildFirstCLPuzzle execution failed with ERROR: java.lang.ClassCastException: org.ph.javaee.training9.JavaLanguage cannot be cast to org.ph.javaee.training9.ProgrammingLanguage

Now how can we fix this problem without actually changing the source code? Well please keep in mind that we are using a “child first” delegation model here. This is why we end up with 2 versions of the same ProgrammingLanguage class. The solution can be visualized as per below.

In order to fix this problem, we can simply ensure that we have only one version of ProgrammingLanguage loaded. Since our main program is referencing ProgrammingLanguage directly, the solution is to remove the ProgrammingLanguage.jar file from the child class loader. This will force the child class loader to look for the class from the parent class loader, problem solved! In order to test the solution, simply remove the ProgrammingLanguage.jar from your testing folder and re-run the program.

I hope you appreciated this puzzle related to “child first” class loader delegation model and class loader rules. This understanding is especially important when you are dealing with complex Java EE deployments involving many class loaders, exposing you to this type of problems at runtime.

Please do not hesitate to post any comment or question on this puzzle.


Java 8: From PermGen to Metaspace

As you may be aware, the JDK 8 Early Access is now available for download. This allows Java developers to experiment with some of the new language and runtime features of Java 8.

One of these features is the complete removal of the Permanent Generation (PermGen) space which has been announced by Oracle since the release of JDK 7. Interned strings, for example, have already been removed from the PermGen space since JDK 7. The JDK 8 release finalizes its decommissioning.

This article will share the information that we found so far on the PermGen successor: Metaspace. We will also compare the runtime behavior of the HotSpot 1.7 vs. HotSpot 1.8 (b75) when executing a Java program “leaking” class metadata objects.

The final specifications, tuning flags and documentation around Metaspace should be available once Java 8 is officially released.

Metaspace: A new memory space is born

The JDK 8 HotSpot JVM is now using native memory for the representation of class metadata and is called Metaspace; similar to the Oracle JRockit and IBM JVM's.

The good news is that it means no more java.lang.OutOfMemoryError: PermGen space problems and no need for you to tune and monitor this memory space anymore…not so fast. While this change is invisible by default, we will show you next that you will still need to worry about the class metadata memory footprint. Please also keep in mind that this new feature does not magically eliminate class and classloader memory leaks. You will need to track down these problems using a different approach and by learning the new naming convention.

I recommend that you read the PermGen removal summary and comments from Jon on this subject. 

In summary:

PermGen space situation

  • This memory space is completely removed.
  • The PermSize and MaxPermSize JVM arguments are ignored and a warning is issued if present at start-up.

Metaspace memory allocation model

  • Most allocations for the class metadata are now allocated out of native memory.
  • The klasses that were used to describe class metadata have been removed.

Metaspace capacity

  • By default class metadata allocation is limited by the amount of available native memory (capacity will of course depend if you use a 32-bit JVM vs. 64-bit along with OS virtual memory availability).
  • A new flag is available (MaxMetaspaceSize), allowing you to limit the amount of native memory used for class metadata. If you don’t specify this flag, the Metaspace will dynamically re-size depending of the application demand at runtime.

Metaspace garbage collection

  • Garbage collection of the dead classes and classloaders is triggered once the class metadata usage reaches the “MaxMetaspaceSize”.
  • Proper monitoring & tuning of the Metaspace will obviously be required in order to limit the frequency or delay of such garbage collections. Excessive Metaspace garbage collections may be a symptom of classes, classloaders memory leak or inadequate sizing for your application.

Java heap space impact

  • Some miscellaneous data has been moved to the Java heap space. This means you may observe an increase of the Java heap space following a future JDK 8 upgrade.

Metaspace monitoring

  • Metaspace usage is available from the HotSpot 1.8 verbose GC log output.
  • Jstat & JVisualVM have not been updated at this point based on our testing with b75 and the old PermGen space references are still present.

Enough theory now, let’s see this new memory space in action via our leaking Java program…

PermGen vs. Metaspace runtime comparison

In order to better understand the runtime behavior of the new Metaspace memory space, we created a class metadata leaking Java program. You can download the source here.

The following scenarios will be tested:

  • Run the Java program using JDK 1.7 in order to monitor & deplete the PermGen memory space set at 128 MB.
  • Run the Java program using JDK 1.8 (b75) in order to monitor the dynamic increase and garbage collection of the new Metaspace memory space.
  • Run the Java program using JDK 1.8 (b75) in order to simulate the depletion of the Metaspace by setting the MaxMetaspaceSize value at 128 MB.

JDK 1.7 @64-bit – PermGen depletion

  • Java program with 50K configured iterations
  • Java heap space of 1024 MB
  • Java PermGen space of 128 MB (-XX:MaxPermSize=128m)

As you can see form JVisualVM, the PermGen depletion was reached after loading about 30K+ classes. We can also see this depletion from the program and GC output.

Class metadata leak simulator
Author: Pierre-Hugues Charbonneau
ERROR: java.lang.OutOfMemoryError: PermGen space

Now let’s execute the program using the HotSpot JDK 1.8 JRE.

JDK 1.8 @64-bit – Metaspace dynamic re-size

  • Java program with 50K configured iterations
  • Java heap space of 1024 MB
  • Java Metaspace space: unbounded (default)

As you can see from the verbose GC output, the JVM Metaspace did expand dynamically from 20 MB up to 328 MB of reserved native memory in order to honor the increased class metadata memory footprint from our Java program. We could also observe garbage collection events in the attempt by the JVM to destroy any dead class or classloader object. Since our Java program is leaking, the JVM had no choice but to dynamically expand the Metaspace memory space.

The program was able to run its 50K of iterations with no OOM event and loaded 50K+ Classes.

Let's move to our last testing scenario.

JDK 1.8 @64-bit – Metaspace depletion

  • Java program with 50K configured iterations
  • Java heap space of 1024 MB
  • Java Metaspace space: 128 MB (-XX:MaxMetaspaceSize=128m)

As you can see form JVisualVM, the Metaspace depletion was reached after loading about 30K+ classes; very similar to the run with the JDK 1.7. We can also see this from the program and GC output. Another interesting observation is that the native memory footprint reserved was twice as much as the maximum size specified. This may indicate some opportunities to fine tune the Metaspace re-size policy, if possible, in order to avoid native memory waste.

Now find below the Exception we got from the Java program output.

Class metadata leak simulator
Author: Pierre-Hugues Charbonneau
ERROR: java.lang.OutOfMemoryError: Metadata space

As expected, capping the Metaspace at 128 MB like we did for the baseline run with JDK 1.7 did not allow us to complete the 50K iterations of our program. A new OOM error was thrown by the JVM. The above OOM event was thrown by the JVM from the Metaspace following a memory allocation failure.


Final words

I hope you appreciated this early analysis and experiment with the new Java 8 Metaspace. The current observations definitely indicate that proper monitoring & tuning will be required in order to stay away from problems such as excessive Metaspace GC or OOM conditions triggered from our last testing scenario. Future articles may include performance comparisons in order to identify potential performance improvements associated with this new feature.

Please feel free to provide any comment.