1. Introduction

Memory leaks in Java applications can cause severe performance degradation, frequent OutOfMemoryErrors, and even complete system crashes. While Java has garbage collection, leaks can still occur if references to unused objects are unintentionally held.

2. What Is a Memory Leak in Java?

In Java, a memory leak occurs when an object is no longer needed but is not garbage collected because it is still referenced (directly or indirectly) from reachable code.

Example

List<String> cache = new ArrayList<>();

public void addToCache(String item) {
    cache.add(item);  // Keeps growing indefinitely if not managed
}

If cache keeps growing, this can cause a heap memory leak.

3. Symptoms of a Memory Leak in Production

You may be facing a memory leak if you observe:

• Increasing heap usage over time (monotonic growth)
• Frequent Full GCs with little memory reclaimed
• OutOfMemoryError: Java heap space
• Application becomes slow or unresponsive
• Thread dumps show GC taking too long

4. Step-by-Step: How to Debug a Memory Leak in Production JVM

Step 1: Monitor the Application

Plot heap memory over time. If it’s continuously rising and never going down even after GC, this indicates a leak.

Step 2: Enable Heap Dumps on OutOfMemoryError

In production, always configure JVM to dump heap when it crashes:

-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=/var/log/myapp/heapdump.hprof

The heap dump will be a .hprof file which contains the entire memory snapshot of the application at crash time.

Step 3: Analyze Heap Dump

Use a heap analysis tool to inspect the .hprof file.

Focus on:

• Large Map or List objects that grow unchecked
• Custom caches without eviction
• Static fields holding large object graphs
• Listeners or observers that aren’t deregistered
• ThreadLocal leaks
• Inner classes referencing outer class unintentionally

Step 4: Take Live Heap Snapshots (Optional)

In some cases, you can connect a profiler to the live production server:

• Use JMX + VisualVM
• Or JFR (Java Flight Recorder) in Java 11+

With jcmd you can trigger dumps live:

jcmd <pid> GC.heap_dump /path/to/heap.hprof

Make sure to avoid excessive overhead on a live system.

5. Common Causes of Memory Leaks

Source	Description
Static Collections	Holding references to objects forever
Unbounded Caches	No eviction policy leads to ever-growing memory
Listeners not removed	Event listeners (e.g., Swing, UI, observer patterns)
ThreadLocal	Threads live long and hold large objects
ClassLoader Leaks	Common in web applications that reload classes
Improper use of Weak/Soft References	Not understanding their semantics
Infinite data structures	E.g., growing HashMap, ArrayList, queues