How to Identify and Fix Class Loader Leaks in Java Web Applications: A Step-by-Step Guide

How to Identify and Fix Class Loader Leaks in Java Web Applications: A Step-by-Step Guide

The Scenario

In modern Java web applications, dynamic class loading using custom class loaders is a common practice, especially when dealing with plugins or modular components. However, improper handling of these class loaders can lead to memory leaks, causing severe performance issues over time. In this article, we will walk through a practical approach to identify and resolve class loader leaks, using real-world examples and diagrams to illustrate each step.

Step-by-Step Analysis

1. Generate a Heap Dump

Triggering a Heap Dump: When your application is experiencing high memory usage or you suspect a memory leak, the first step is to generate a heap dump. This can be done using tools like jmap or jconsole.

jmap -dump:live,format=b,file=heapdump.hprof <pid>


2. Load the Heap Dump

Opening the Heap Dump: Use tools like Eclipse MAT (Memory Analyzer Tool) to load the heap dump. Open Eclipse MAT, and load the .hprof file you generated.


3. Initial Analysis

Overview and Histogram: Start with the Overview page to get a snapshot of memory usage. Then, navigate to the Histogram view to identify the classes and objects consuming the most memory.

In MAT's overview, we notice a large number of class loaders:

Class Name | Instances | Shallow Heap | Retained Heap -----------------------------|-----------|--------------|--------------- com.example.PluginClassLoader| 1,500 | 120,000 B | 500 MB


4. Analyze Class Loaders

Histogram Filter: Filter for java.lang.ClassLoader in the Histogram view to see class loader instances and their retained memory.


Regex: .*ClassLoader

We see multiple instances of our custom class loader:


Class Name | Instances | Shallow Heap | Retained Heap -----------------------------|-----------|--------------|--------------- com.example.PluginClassLoader| 1,500 | 120,000 B | 500 MB

Diagram Tip: Create a pie chart of class loader instances and their retained heap sizes.


5. Examine Dominator Tree

In the dominator tree, we find:


Class Name | Retained Heap -----------------------------|--------------- com.example.PluginClassLoader| 500 MB |- com.example.PluginA | 100 MB |- com.example.PluginB | 150 MB |- ...


Diagram Tip: Create a tree diagram showing the dominator hierarchy.


6. Inspect Class Loader Retention


Analyzing the path to GC roots for a PluginClassLoader instance:

PluginClassLoader@0x12345678

<- static field PluginRegistry.loadedPlugins

<- PluginRegistry class

Diagram Tip: Create a chain diagram showing the reference path from GC root to the class loader.

7. Identify Unnecessary References

We discover thatPluginRegistryholds static references to all loaded plugins:

public class PluginRegistry {

private static final Map<String, Plugin> loadedPlugins = new HashMap<>();

public static void registerPlugin(String name, Plugin plugin)

{ loadedPlugins.put(name, plugin); }

// Missing: Method to unregister plugins }


8. Analyze and Fix the Leak

The issue: Plugins (and their class loaders) are never unregistered, causing a memory leak.

public static void unregisterPlugin(String name) {

loadedPlugins.remove(name);

}


Verification

After implementing the fix:

  1. Redeploy the application.
  2. Monitor memory usage (it should stabilize).
  3. Generate a new heap dump and analyze:

Class Name | Instances | Shallow Heap | Retained Heap -----------------------------|-----------|--------------|--------------- com.example.PluginClassLoader| 10 | 800 B | 5 MB


Diagram Tip: Create a before/after bar chart comparing class loader instances and retained heap.


References

https://stackoverflow.com/questions/40079616/what-is-classloader-leak

https://www.dynatrace.com/resources/ebooks/javabook/class-loader-issues/

https://docs.oracle.com/en/java/javase/17/troubleshoot/troubleshooting-memory-leaks.html

https://www.eclipsecon.org/2020/sessions/hunting-down-osgi-classloader-leaks





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