Identifying Memory Leaks in Java with JHat

Finding memory leaks in Java can be challenging, but JHat, a tool included in the JDK, provides valuable insights into heap usage. While JHat provides a basic view of heap allocation, pinpointing the root cause of a memory leak can be arduous. This article offers a systematic approach to identify large object trees and locate potential references leading to memory leaks.

Steps to Identify Leaks:

1. Create a Memory Snapshot:

   • Use JHat to create a heap dump of the running application in a stable state.

2. Simulate Memory Leak:

   • Trigger the suspected memory leak-causing operation multiple times, allowing for caching and initialization.

3. Generate Another Snapshot:

   • Create a second heap dump after the leak-inducing operation has been performed.

4. Diff the Snapshots:

   • Compare the two heap dumps using JHat's diff feature. Focus on the objects that have experienced the largest positive difference in allocation.

5. Trace Root References:

   • Investigate the objects with significant memory changes. Look for objects with multiple references or complex relationships that may be holding onto memory unnecessarily.

6. Analyze Object Trees:

   • Inspect large object trees to identify any potential circular references or unreachable objects that could lead to a memory leak.

Tips:

• If using large maps, search for specific types of maps, e.g., java.util.HashMap.
• Consider using JProfiler for more advanced features and specialized profiling capabilities.
• If unable to modify code for logging, consider using other techniques such as heap dumping and analysis at different stages of the application's execution.