Runtime methods

[adithya narayan]

Hi,

Runtime class provides api methods like
1)maxMemory()
2)totalMemory() and
3)freeMemory()

I was under the impression that :
(1)maxMemory() is the max memory which JVM can work upon (say 128 MB)
(2)totalMemory() is the available memory with JVM (say 100MB)
(3)freeMemory() is the memory which JVM can work upon if it needs to allocate memory to objects,etc. (say 28MB).

I was guessing that
maxMemory = totalMemory + freeMemory.

but it isn't so !!

1)Can anyone explain why the above equation isn't true ?How do we figure out the relation between these (we need to have some calculation in place so that we know how much memory we are using or how much memory we will require more) ?

2)How do we decide the max memory we can allocate to the JVM ?


Following is the code i wrote to check that.

public class GCTest 
{
//	GCTest gcTest;
	
	public static void main(String[] args) 
	{
		Runtime runtime = Runtime.getRuntime();
		
		System.out.println("Max mem JVM can use = "+runtime.maxMemory());
		System.out.println("Total available mem with JVM = "+runtime.totalMemory());
		System.out.println("Free mem for future use by JVM = "+runtime.freeMemory());
		
		//allocate memory to date objects
		Date date = null;
		for(int i=0;i<10000;i++)
		{
			date = new Date();
			date = null;
		}
		System.out.println("Free mem for future use by JVM 'after memory allocation' = "+runtime.freeMemory());
		runtime.gc();
		System.out.println("Free mem for future use by JVM 'after calling GC' = "+runtime.freeMemory());
		
		/*GCTest gcTest1 = new GCTest();
		GCTest gcTest2 = new GCTest();
		GCTest gcTest3 = new GCTest();
		
		gcTest1.gcTest = gcTest2;
		gcTest2.gcTest = gcTest2;
		gcTest3.gcTest = gcTest1;*/
	}
}

Cheers,
Adithya.

[Stephan van Hulst]

I'm not exactly sure, but my guess is that totalMemory() is the amount of memory that is available to the JVM right now and maxMemory is the maximum amount of memory that the JVM will request from the OS.

The JVM doesn't immediately request all the memory it can get. It will request some, use that memory (which affects freeMemory()) and when it needs more free memory, but there aren't any objects to be garbage collected, it will request more memory from the OS, adding this to totalMemory(). It will keep doing this until it hits maxMemory(), at this point it will throw an OutOfMemoryError.

Again, this is only my best guess.

[Ernest Friedman-Hill]

Stephan is correct. freeMemory() is the amount that could be allocated right now without running the garbage collector.