How can I measure memory allocation of a Java Application?

Question

We built a web application that enables images to be loaded into a database through a servlet running on Tomcat. We have inconsistent behaivor when more than x number of people are loading images simultaneously. No doubt a memory allocation problem, but how can I measure it?
For example, I'd like to be able to create a graph showing the resources used against the number of people making uploads. I am using Linux and Tomcat/Apache.
I am hoping for something like Memory counters that are available for measurine Windows applications on MS Windows - comes with the Resource Kit. Does this type of tool exist for the Java/Linux market?

Answer 1

It is difficult to gauge memory usage from a Java application as the JVM handles garbage collection and even if objects are not referenced they might still be sitting in memory until the next garbage collect.
Anyhow, a way to query available memory is:
Runtime.getRuntime().freeMemory()
You can run this at the begining of a process and at the end and examine the difference. You should call System.gc() before each freeMemory() call in order to try to force garbage collection.
Another method would be to turn to the underlying OS. On linux you could use 'top' or 'ps'. With ps you could do something like:
ps -o rss,vsz,%mem <pid>
Where <pid> is the parent Tomcat process' id. The resulting three columns are:
- Amount of real memory used.
- Amount of virtual memory used.
- Percentage of real memory used.
'man ps' wil give you more options on how to use ps.
There are also fancy graphical stuff like 'gtop' as well.
Hope this helps.