Tutorial :Programmatically calculate memory occupied by a Java Object including objects it references [duplicate]


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I need to programmatically find out exactly how much memory a given Java Object is occupying including the memory occupied by the objects that it is referencing.

I can generate a memory heap dump and analyse the results using a tool. However it takes a considerable amount of time to generate a heap dump and for such a tool to read the dump to generate reports. Given the fact that I will probably need to do this several times, my work would be a lot more agile if I could throw in some code in my project that would give me this value "runtime".

How could I best achieve this?

ps: Specifically I have a collection of objects of type javax.xml.transform.Templates


You will need to use reflection for that. The resulting piece of code is too complicated for me to post here (although it will soon be available as part of a GPL toolkit I am building), but the main idea is:

  • An object header uses 8 bytes (for class pointer and reference count)
  • Each primitive field uses 1, 2, 4 or 8 bytes depending on the actual type
  • Each object reference (i.e. non-primitive) field uses 4 bytes (the reference, plus whatever the referenced object uses)

You need to treat arrays separately (8 bytes of header, 4 bytes of length field, 4*length bytes of table, plus whatever the objects inside are using). You need to process other types of objects iterating through the fields (and it's parents fields) using reflection.

You also need to keep a set of "seen" objects during the recursion, so as not to count multiple times objects referenced in several places.


Looks like there is already a utility for doing this called Classmexer.

I haven't tried it myself, but I would go that route before I rolled my own.


A good generic solution is to use heap size delta. This involves minimal effort and is re-usable between any type of object / object graph. By instantiating and destroying your objects many times and garbage collecting in between, and then taking the average, you avoid compiler and JVM optimizations that alter results and get a fairly accurate result. If you need an EXACT answer down to the byte then this may not be the solution for you, but for all practical applications that I know of (profiling, memory requirement calcualtions) it works extremely well. The code below will do just that.

    public class Sizeof {        public static void main(String[] args)            throws Exception {          // "warm up" all classes/methods that we are going to use:          runGC();          usedMemory();            // array to keep strong references to allocated objects:          final int count = 10000; // 10000 or so is enough for small ojects          Object[] objects = new Object[count];            long heap1 = 0;            // allocate count+1 objects, discard the first one:          for (int i = -1; i < count; ++i) {            Object object;        //// INSTANTIATE YOUR DATA HERE AND ASSIGN IT TO 'object':                object=YOUR OBJECT;      ////end your code here            if (i >= 0) {              objects[i] = object;            }            else {              object = null; // discard the "warmup" object              runGC();              heap1 = usedMemory(); // take a "before" heap snapshot            }          }            runGC();          long heap2 = usedMemory(); // take an "after" heap snapshot:            final int size = Math.round(((float)(heap2 - heap1)) / count);          System.out.println("'before' heap: " + heap1 +                             ", 'after' heap: " + heap2);          System.out.println("heap delta: " + (heap2 - heap1) +                             ", {" + objects[0].getClass() + "} size = " + size + " bytes");        }          // a helper method for creating Strings of desired length        // and avoiding getting tricked by String interning:        public static String createString(final int length) {          final char[] result = new char[length];          for (int i = 0; i < length; ++i) {            result[i] = (char)i;          }            return new String(result);        }          // this is our way of requesting garbage collection to be run:        // [how aggressive it is depends on the JVM to a large degree, but        // it is almost always better than a single Runtime.gc() call]        private static void runGC()            throws Exception {          // for whatever reason it helps to call Runtime.gc()          // using several method calls:          for (int r = 0; r < 4; ++r) {            _runGC();          }        }          private static void _runGC()            throws Exception {          long usedMem1 = usedMemory(), usedMem2 = Long.MAX_VALUE;            for (int i = 0; (usedMem1 < usedMem2) && (i < 1000); ++i) {            s_runtime.runFinalization();            s_runtime.gc();            Thread.currentThread().yield();              usedMem2 = usedMem1;            usedMem1 = usedMemory();          }        }          private static long usedMemory() {          return s_runtime.totalMemory() - s_runtime.freeMemory();        }          private static final Runtime s_runtime = Runtime.getRuntime();        } // end of class  

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