Thread: How to detect current memory free and used in java program ?

I would like to know how much of memory is free and used my java program.

I have used Runtime.totalMemory and freeMemory() functions. However, how come used memory is so different than java.exe process show in windows task manager ?

Thank !

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Windows is probably reporting the total amount of memory used by the JVM (as Java) rather than the memory requirements of your program. I haven't looked at the APIs for the memory methods you've mentioned, because I've never needed to and don't plan to. What are you trying to do, or why do you need to know these things?

Originally Posted by tanc08

I would like to know how much of memory is free and used my java program.

I have used Runtime.totalMemory and freeMemory() functions. However, how come used memory is so different than java.exe process show in windows task manager ?

Correct, because how the memory is used and reported by the VM is only partially related to how the underlying platform specific memory resource allocations work and are reported. Windows Task manager has it's own way of reporting on the total VM address space -- essentially the parent java or javaw process and all of its threads (if any).

The VM reports on various higher level and, maybe, implementation specific heap statistics it has gathered during the lifetime of the VM.

These two views will rarely match up except in the loosest sense.

This is an advanced topic that requires special tools and long experience with _specific_ VM internals and _special_ platform internals to fully grok. Useful, certainly, but unrelated to writing code in Java in the greatest majority of cases.

You should just make sure to practice good memory housekeeping. If you maintain a larger application that runs for a long time, or crunches bigger data sets, then you will have to learn the tools provided by the VM vendor to query the VM health to ensure you have a reasonably healthy heap churn over time.