Measure memory usage of a certain datastructure

Question

I'm trying to measure the memory usage of my own datastructure in my Tomcat Java EE application at various levels of usage.

To measure the memory usage I have tried two strategies:

1. Runtime freeMemory and totalMemory:

   System.gc(); //about 20 times
   long start = Runtime.getRuntime().freeMemory();
   
   useMyDataStructure();
   
   long end = Runtime.getRuntime().freeMemory();
   
   System.out.println(start - end);
   

2. MemoryPoolMXBean.getPeakUsage():

   long before = Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory();
   List<MemoryPoolMXBean> memorymxbeans = ManagementFactory.getMemoryPoolMXBeans();
   for(MemoryPoolMXBean memorybean: memorymxbeans){
       memorybean.resetPeakUsage();
   }
   
   useMyDataStructure();
   
   for(MemoryPoolMXBean memorybean: memorymxbeans){
       MemoryUsage peak = memorybean.getPeakUsage();
       System.out.println(memorybean.getName() + ": " + (peak.getUsed() - before));
   }
   

Method 1 does not output reliable data at all. The data is useless.

Method 2 outputs negative values. Besides it's getName() tells me it's outputting Code Cache, PS Eden Space, PS Survivor Space and PS Old Gen seperately.

How can I acquire somewhat consistent memory usage numbers before and after my useMyDataStructure() call in Java? I do not wish to use VirtualVM, I prefer to catch the number in a long object and write it to file myself.

Thanks in advance.

---

edit 1:

useMyDatastructure in the above examples was an attempt to simplify the code. What's really there:

        int key = generateKey();
        MyOwnObject obj = makeAnObject();
        MyContainerClass.getSingleton().addToHashMap(key, obj);

So in essence I'm really trying to measure how much memory the HashMap<Integer, MyOwnObject> in MyContainerClass takes. I will use this memory measurement to perform an experiment where I fill up both the HashMap and MyOwnObject instances.

Answer 1

1st of all sizing objects in java is non-trivial (as explained very well here).

if you wish to know the size of a particular object, there are at least 2 open source libraries that will do the math for you - java.sizeof and javabi-sizeof

now, as for your specific test - System.gc() is mostly ignored by modern (hotspot) jvms, no matter how many times you call it. also, is it possible your useMyDataStructure() method does not retain a reference to the object(s) it creates? in that case measuring free memory after calling it is no good as any allocated Objects might have been cleared out.