Understanding and Resolving Java PermGen Space Errors

Understanding and Resolving Java PermGen Space Errors

The java.lang.OutOfMemoryError: PermGen space error is a common issue encountered in Java applications, particularly those running on older JVMs (prior to Java 8). This tutorial will explain what causes this error, how to diagnose it, and the various strategies to resolve it.

What is PermGen Space?

PermGen, short for Permanent Generation, was a portion of the Java Virtual Machine (JVM) heap used to store metadata about classes and interfaces loaded during runtime. This included class definitions, method bytecode, and constant pools. Essentially, it held information about the classes, not the objects created from those classes.

Why did PermGen exist?

In older JVMs, PermGen was separated from the main heap space to manage class metadata more efficiently. This separation aimed to prevent frequent garbage collection cycles triggered by class loading and unloading.

Why does the error occur?

The OutOfMemoryError: PermGen space signifies that the JVM has run out of space in the PermGen area. This commonly happens in applications that:

• Dynamically load and unload classes: Applications like web servers (Tomcat, Jetty), application servers (JBoss, WebSphere), and those using dynamic code generation frequently load and unload classes. If class unloading doesn’t happen efficiently, or if references to older classes are held onto unnecessarily, PermGen space can fill up.
• Use classloader leaks: A classloader leak occurs when a classloader is never garbage collected, even after the classes it loaded are no longer needed. This can happen when static references to classes loaded by that classloader are held by other parts of the application, or even by the application server itself.
• Use extensive bytecode manipulation libraries: Libraries that generate or modify bytecode at runtime, such as those used for aspect-oriented programming or dynamic proxies, can significantly increase PermGen space usage.

Diagnosing the Issue

Before attempting a fix, it’s crucial to confirm that PermGen space is indeed the problem. Here are some approaches:

• Heap Dumps: Generate a heap dump when the error occurs. Tools like jmap (part of the JDK) can capture the heap state, which can then be analyzed using tools like jhat or memory analyzers. These tools can pinpoint the objects consuming the most space in PermGen.
• Monitoring Tools: Use JVM monitoring tools to observe PermGen space usage over time. This helps determine if the problem is a gradual increase in usage or a sudden spike.
• Logging: Enable detailed logging to track class loading and unloading events.

Resolving the Issue

There are several strategies to address the OutOfMemoryError: PermGen space error:

1. Increase PermGen Space Size (Temporary Fix):

   A quick and often-used solution is to increase the maximum PermGen space size using JVM arguments. For example:

   java -XX:MaxPermSize=256m -jar your_application.jar
   

   This instructs the JVM to allocate more space to PermGen. However, this is usually a temporary fix and does not address the root cause of the problem. It merely postpones the error if the underlying leak persists.

2. Identify and Fix Classloader Leaks:

   This is the most effective long-term solution. The goal is to ensure that classloaders are garbage collected when their loaded classes are no longer needed. Common causes of classloader leaks include:

   • Static References: Static variables holding references to classes loaded by a specific classloader prevent that classloader from being garbage collected.
   • ThreadLocal Variables: Using ThreadLocal variables to store objects loaded by a specific classloader can also cause leaks.
   • Application Server Issues: Some application servers have known issues with classloader management. Consult the server’s documentation for potential workarounds.

3. Enable Class Unloading (If Applicable):

   Some application servers support features to automatically unload classes when they are no longer needed. For example, in Tomcat, you can enable class unloading by adding the following JVM arguments:

   -XX:+CMSClassUnloadingEnabled -XX:+CMSPermGenSweepingEnabled
   

   These flags instruct the Concurrent Mark Sweep (CMS) garbage collector to aggressively unload classes from PermGen.

4. Modernize your Java Version (Best Long-Term Solution)

   Java 8 and later versions removed the PermGen space and replaced it with Metaspace. Metaspace is allocated from native memory and dynamically resizes, reducing the likelihood of running out of space. Upgrading to a more recent Java version is highly recommended, as it eliminates the PermGen issue entirely and offers numerous other performance and security improvements.

Example: Increasing PermGen Space in Tomcat

1. Shut down the Tomcat service.
2. Navigate to the Tomcat/bin directory.
3. Run tomcat6w.exe (or tomcat7w.exe, tomcat8w.exe, etc., depending on your Tomcat version).
4. Go to the "Java" tab.
5. In the "Java Options" box, add -XX:MaxPermSize=256m (or a different value as needed).
6. Click "OK" and restart the Tomcat service.

Conclusion

The java.lang.OutOfMemoryError: PermGen space error can be frustrating, but it’s often solvable. By understanding the causes, diagnosing the issue, and implementing the appropriate solutions, you can prevent this error and ensure the stability of your Java applications. Prioritize fixing classloader leaks and, most importantly, consider upgrading to a modern Java version to eliminate the PermGen space entirely.