How to get free memory info from Java

Question

I use the following lines to get the memory usage with Java :

Free_Memory=Run_Time.freeMemory()/1048576;           // 1024 x 1024 = 1K x 1K = 1Meg
Total_Memory=Run_Time.totalMemory()/1048576;         // 992 Total on a 4 GB PC

The Free_Memory I got was : 900, but it is way off, when Free_Memory goes down to around 600, my program ran out of memory and generated heap overflow message.

So I looked at the => Windows Task Manager : Performance : Physical Memory : Free, it's down to 1, 2 or 0, which is a more accurate reflection of my memory situation, and according to it, my Total Memory is : 4089, which is correct, while Java's Total_Memory=992 is incorrect.

So, my question now is : In my Java program how to get the memory usage numbers reflected in the Windows Task Manager : Performance : Physical Memory ? I need to depend on those numbers.

Answer 1

The JVM doesn't allow Java to consume all available system memory. Instead the JVM grabs a fixed chunk and allocates all of your objects within that chunk. If this area fills up, you're out of memory! There are commandline options to alternate the max/initial memory usage of the JVM.

The more important issue is that you should not be relying on tracking free/max memory. What are you doing that relies on tracking memory?

UPDATE:

Try 64bit if you need more memory than 1.5GB

If you're trying to track memory running out then consider figuring our WHY your program does this and if it can be prevented through different algorithms or better management of objects. When the memory reaches zero what do you expect to do? Popup a dialog and tell the user they're screwed and exit the program? I can understand a grateful shutdown but warning the user to run with a large -Xmx is not going to cut it.

Answer 2

If you want detailed Windows stats, you can use WMI and a .vbs script, executed via cscript.exe.

This link details a script that pulls more detailed memory stats than you could possibly want.

Execute this via the usual Process/Runtime combination, and simply read back what figures you require. These are system level stats, and not for the VM (although WMI can pull back per-process stats as well).