Monitoring and detecting memory leaks in java application

So your application is running out of memory, you’re spending days and nights analyzing your application hoping to catch the memory holes in your objects.

What is Memory Leak?
Memory leak is a bug that mainly occurs when a program does not release the memory it has obtained for temporary use.

Definition of Memory Leak: objects are no longer being used by the application, but Garbage Collector cannot remove them because they are being referenced.

How to determine if Memory Leak exists in a Java application?

If the application throws java.lang.OutOfMemoryError or if the program takes more time to execute than is required normally then there could be a memory leak in the application. There are various third party tools to detect and fix memory leaks but it is always better to prevent one from happening

The below steps will explain how to monitor and detect your memory leaks to make sure your app is on the safe side.

1. Memory leak suspicion

If you have a suspicion, there is a memory leak a convenient way to make sure it’s really there is using jconsole. You can locally or remotely connect jconsole to your app and let it monitor for a while (Hour, Half day, Overnight, Week.) After connecting jconsole to your app start analyzing the “Memory” tab. A memory leak suspicion would look like this:

2. How to find the leaking sources in your application

For this purpose, I recommend using jvisualVM. this tool is part of the JDK. Inside jvisualVM you can take Heap Dump (Inside the “Monitor” Tab). Please keep in mind that it’s not possible to create Heap-Dump remotely. You’ll need to either run jvisualvm on the same machine or execute jmap command to produce a Heap-Dump file and import it later into jvisualvm.

* Jmap is an oracle tool that prints all objects memory map tree for a given process.

So basically you run the jmap on your remote server (for instance your production environment) and then analyze that file locally. I recommend to do several Heap dumps. That will give you better picture whether you have memory leaks or not.

3. Analyzing the Heap dump file

I personally like to use MAT (Eclipse Memory Analyzer) (http://www.eclipse.org/mat/). MAT takes the heap dump file and helps you find memory leaks. MAT shows exactly which instances have memory growth suspects. You might notice Java libraries instances as a ‘Problem Suspect’ such as: “java.lang.Class” but this is normal.

Example for a leak detection

Here you can see the exact instance which is suspected as a leaking component.

4. Analyze suspected objects

Next step is to press on the details field of the suspected instance and investigate the objects inside:

In the above example we can see clearly that field of type TreeMap is growing.

5. Fix your leak and run the test again

Now what’s left is to understand and fix your leaking source – but of course this is individual for each object. These step-by step directions will help you detecting the leaking memory objects.