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javaout-of-memoryheap-dump

What is the breakdown of the statistics in a Java heap dump

Given this heap dump

size         no. of obj class
515313696       2380602 char[]
75476832        614571  * ConstMethodKlass
57412368        2392182 java.lang.String
44255544        614571  * MethodKlass
33836872        70371   * ConstantPoolKlass
28034704        70371   * InstanceKlassKlass
26834392        349363  java.lang.Object[]
25853848        256925  java.util.HashMap$Entry[]
24224240        496587  * SymbolKlass
19627024        117963  byte[]
18963232        61583   * ConstantPoolCacheKlass
18373920        120113  int[]
15239352        634973  java.util.HashMap$Entry
11789056        92102   ph.com.my.class.Person

And only 1 class is from my app, ph.com.my.class.Person. The class definition is follows:

public class Person {
 private String f_name;
 private String l_name;
}

In the heap dump, does the Person size (11789056) include the memory that the 2 string variables occupying? Or will the f_name and l_name be counted in the String class instead, in this case size 57412368?

UPDATED - added followup question:

So let's say each instance of:

  1. f_name size is 30
  2. l_name size is 20
  3. Person size is 75

If there where 10 instances of Person, there will be

  1. 10 * (30+20) = 500
  2. 10 * 75 = 750

Will the 500 be counted in String or char[]? And subsequently, will 750 be counted in Person?

Solution

  • The size of an object in the heap dump is the number of bytes allocated as a block on the heap to hold that instance. It never includes the bytes of the whole graph reachable through the object. In general that could easily mean that the size of the object is the entire heap. So in your case it takes into account the two references, but not the String instances themselves. Note also that even the String size doesn't reflect the size of the represented string -- that's stored in a char[]. The char[] instances are shared between strings so the story isn't that simple.