了解JVM GC

Question

I am trying to understand how does my below written code works and outputs.

public static void getRunTimeMemoryConsumption(){

List<Integer> array = new ArrayList<Integer>();
for(int i = 0 ; i < 100000 ; i++){
    array.add(i);
}

List<Integer> array1 = new ArrayList<Integer>();
for(int i = 0 ; i < 100 ; i++){
    array1.add(i);
}

Runtime rt = Runtime.getRuntime();

//Run the GC
rt.gc();

long memoryFreed = rt.totalMemory() - rt.freeMemory();
System.out.println("Memory freed in Mbytes :: " + memoryFreed/(1024));

}

When I run this code, it always gives me 132 KB as output (9/10 runs). However, if I remove the rt.gc() call, I get more memory freed (>100 MB) written in my output statement. How can I understand this behavior of the Java Runtime?

Answer 1

I am not entirely sure what you are attempting.

You are not computing how much memory was freed, you are computing rt.totalMemory() - rt.freeMemory() , ie how much memory is used. With that in mind, the result you are getting makes a lot of sense, without garbage collection more memory is used then after gc. To find out how much memory was freed, you should compare rt.totalMemory() - rt.freeMemory() before and after the call to the garbage collector.

Also the integers you put in the lists in the for loops are not eligible for garbage collection, as they are references by live objects (ie the lists). Not sure what is garbage collected, as I don't see the rest of your program, but you should modify your test in a way that at the point of garbage collection you have objects which are not references by live objects.

Answer 2

You should just compare rt.freeMemory() before and after rt.gc() . Your expression prints the used memory. This is why the values are the opposite from what you expect.

Answer 3

Garbage collection here wont really remove anything because you dont have any objects that are really out of scope. The method scopes both Lists so they are both still accessible after the garbage collection call. Garbage collection is a scheduled task generally in java so it could run at any time theoretically. This means that the behaviour here is actually good because you wouldnt want garbage collection destroying objects that were still in scope (like you have here).

Answer 4

It seems like you got less memoryFreed because it was spent on garbage collector activities called by rt.gc(). Actually rt.gc() won't free memory allocated for your arrays. If you want to see how much memory was freed by GC, you should do the counting outside getRunTimeMemoryConsumption() method or assign arrays to null before the measurement.