You think that your shiny Java app has some memory issues but how do you find out if that is true and what is taking up all that memory? Knowing the potential problems is fine. Nevertheless you still have to find out your actual problems. There are several instruments available to help you analyse your Java application regarding its memory usage. I will tell you about increasing your maximum heap (most of you surely know about that), looking at the memory of a running app, making heap dumps (on demand or on OutOfMemoryException) and analyzing the dumps.
Increasing maximum heap
The Java VM has a setting that defines the maximum amount of heap memory available to your application. It defaults to 64MB which is enough for many programs. If you have a larger application you should try to start it with that value increased by passing the -Xmx<size>m parameter to the VM at startup. <size> is the value in MBytes so just fiddle around with that. If your app is leaking memory that won’t help you for long so you have to find out *if* it leaks.
Looking at memory usage of a running application
You can use jconsole for a quick look at your applications resource usage. jconsole is part of the Sun JDK since Java 6. You can connect the jconsole to any running java applications on your computer or even reachable over network and offering the Java Management Extensions (JMX) over TCP. Non-leaking programs should have a memory graph like this:
You can see, that the memory fluctuates over time because of the garbage collection cycles. But overall it does not grow. Next we will look at an application that leaks memory:
Above we see that the garbage collector (GC) tries its best but the used memory is growing over time. If we see such behaviour we probably need a heap dump to analyze the issue further.
Making a heapdump
Basically you have two nice ways to get a heap dump of your application which you can look into at a later time:
- Use jmap (which is also part of the Sun JDK 6) to dump the heap of a running application to a file using a command line like
jmap -dump:format=b,file=myheap.hprof <pid>
- Tell the VM to make a heap dump when an OutOfMemoryException occurs by adding
-XX:+HeapDumpOnOutOfMemoryErrorto the VM parameters at startup. With another switch you can specify the path for the dumps:
After you have obtained a dump of your application you certainly want to have a look at it and find the issues. You can start with Sun’s jhat which is also part of current JDKs. After supplying jhat the hprof-file you can point your browser to the integrated webserver of jhat and browse the heap looking for the objects that take up your memory.
We have seen many ways to perform memory diagnostics using only free tools which are part of the JDK from Sun. They are all nice but have their limitations. Especially jhat has problems with usability and performance when you examine larger heap dumps with it.
Next time I will show you how to use the Eclipse plugin MAT for analysis of heap dumps obtained in one of the above ways. So stay tuned!