How Can JHat and Other Tools Help Me Find and Fix Java Memory Leaks?

Finding a Memory Leak in Java with JHat

In Java, memory leaks can be challenging to locate. JHat, a tool included in the JDK, can assist in this task. The question poses several challenges in using JHat to identify the root cause of a memory leak.

Identifying Large Object Trees and Maps

JHat provides limited functionality for searching large object trees or finding general roots of large maps. However, it can identify objects that consume significant memory, including HashMap$Entry instances. To find objects with a large memory footprint:

1. Load the heap dump into JHat.
2. In the "Statistics" tab, navigate to "Classes By Bytes" or "Class Histogram by Size."
3. Sort the list by size to identify classes that occupy the most memory.

Using a Specialized Tool

While JHat can provide basic insight into memory leaks, it is often more effective to use a specialized tool such as JProfiler. These tools offer enhanced graphing capabilities and intuitive visualizations, making it easier to identify patterns and root causes.

Alternative Approach for Finding Leaks

If access to a commercial tool is not feasible, the following approach can be employed:

1. Capture a memory snapshot using a profiling tool or Java's HeapDumpOnOutOfMemoryError mechanism.
2. Perform the operation suspected of causing the leak.
3. Capture another memory snapshot.
4. Compare the two snapshots using a tool like Eclipse MAT, comparing objects by type and size.

This approach involves manually identifying objects that grow significantly between snapshots and analyzing their references to find the root cause of the memory leak.

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