How to diagnose memory leaks in java

Memory leaks doesn't have to be hard/scary/tedious problem to solve, if you follow below mentioned 3 simple steps:

Step 1: Capture baseline heap dump

You need to capture heap dump when it’s in healthy state. Start your application. Let it take real traffic for 10 minutes. At this point capture heap dump. Heap Dump is basically snapshot of your memory. It contains all objects that are residing in the memory, values stored in those objects, inbound & outbound references of those object. You can capture Heap dump can be captured using following command:

jmap -dump:format=b,file=<file-path> <pid>


pid: is the Java Process Id, whose heap dump should be captured

file-path: is the file path where heap dump will be written in to.

If you don't want to use jmap for capturing heap dumps, here are several other options to capture heap dumps

It’s always better to capture heap dump in the production environment (unless in the test environment you can mirror the exact production traffic pattern). Traffic type and its volume plays a primary role in the type and number of objects created in the memory.

Step 2: Capture troubled heap dump

After doing step #1, let the application run. Before application crashes take another heap dump once again. Often times it might be challenging to capture heap dumps before it crashes, because we don't know when application will crash. Is it after 30 minutes, 3 hours, 3 days? Thus, it's ideal to start your application with following JVM property:

-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=<file-path>

file-path: is the file path where heap dump will be written in to.

This property will trigger heap dump right when application experiences OutOfMemoryError.

Step 3: Compare heap dumps

Objects which are causing memory leaks grow over the period. If you can the identify objects whose size has grown between the heap dumps captured in step #1 and step #2, then those are the objects which is causing memory leak.