How to find memory leaks - only using Java SE 6
I got an interesting comment as reaction for my last post. Vladimir (thank you) wrote how to monitor Java6 applications using only built-in tools. His howto is really great:
Here is a small HOWTO regarding the tools that help find memory leaks.
Note: Use the latest JDK 6, because it has the latest tools, with
lots of bugfixes and improvements. All the later examples assume that
JDK6's bin directory is in the PATH.
Step 1. Start the application.
Start the application as you usually do:
java -jar java_app.jar
Alternatively, you could start java with hprof agent. Java will run
slower, but the huge benefit of this approach is that the stack traces
for created objects will be available which improves memory leak
When the application is up, perform various actions that you think might lead
to memory leaks.
You might use jconsole from JDK 6 to see the memory consumption graph
to have a clue whether memory leak is present or not:
It will present a dialog with a list of java apps to connect to. Find
the one with java_app.jar and connect.
For example, if you open some documents in your app, the memory
graph could rapidly go up. If closing the docs and invocation of
full garbage collection did not bring the memory back to normal level,
there is probably a leak somewhere.
Jconsole allows to invoke full GC providing nice button just for that.
Step 2. Find the application pid.
Find out the application's process id via:
It will print something like:
In our case the pid is 15976.
Step 3. Dump the heap into file.
Dump heap into the file:
jmap -dump:format=b,file=/tmp/java_app-heap.bin 15976
We just told jmap to dump the heap into /tmp/java_app-heap.bin file,
in binary from (which is optimized to work with large heaps). The
third parameter is the pid we found in Step 2.
Alternatively, if you started java with hprof agent, you could just
use Ctrl-\ on Solaris/Linux or Ctrl-Break on Windows to dump heap into
the file, specified in hprof agent arguments.
Step 4. Visualize the heap.
Use jhat tool to visualize the heap:
jhat -J-Xmx326m /tmp/java_app-heap.bin
Jhat will parse the heap dump and start a web server at port 7000
Connect to Jhat server.
Point your browser to:
And start investigating. :)
Jhat allows you to see what obects are present in the heap, who has
references to those objects, etc.
Here are some tips:
* Investigate _instances_, not _classes_
* Use the following URL to see the instances:
* Use "Reference Chains from Rootset" (Exclude weak refs!!!) to see who's holding the instance
Vladimir -> thank you for the great comment