Printing Histogram of Java Object Heap

Java Tools Tutorials - Herong's Tutorial Examples

∟Printing Histogram of Java Object Heap

This section provides a tutorial example on how to print the histogram of Java object heap, a list of different types of objects and their counts and total sizes.

The first function of the "jmap" tool is to print histogram of object heap of a given JVM process. Now I am going to use a sample Java program, GarbageCollection.java, I wrote in another tutorial example in this book.

The first test is to print the heap histogram of a JVM process that runs GarbageCollection.java with the "jmap -histo pid" command:

herong> javac GarbageCollection.java

herong> java GarbageCollection
Free/total memory:
23725256   25034752
22710400   25034752
21618728   25034752
20523584   25034752
...

(Start another command window.)
herong> jps -l -m
2288 jdk.jcmd/sun.tools.jps.Jps -l -m
10012 GarbageCollection

herong> jmap -histo 10012

  num   #instances     #bytes  class name (module)
--------------------------------------------------
   1:         2928   27043688  [I (java.base@10.0.1)
   2:           80   19663104  [J (java.base@10.0.1)
   3:           75   19399856  [D (java.base@10.0.1)
   4:       302762    7266288  java.lang.String (java.base@10.0.1)
   5:         3319    4951872  [B (java.base@10.0.1)
   6:           74    4785312  [F (java.base@10.0.1)
   7:           86    4631088  [C (java.base@10.0.1)
   8:           88    1165048  [Ljava.lang.String; (java.base@10.0.1)
   9:         1478      97528  [Ljava.lang.Object; (java.base@10.0.1)
  10:          702      85984  java.lang.Class (java.base@10.0.1)
  11:         2398      76736  java.util.HashMap$Node (java.base@10.0.1)
  12:         1679      53728  java.util.concurrent.ConcurrentHashMap$N...
  13:          451      45376  [Ljava.util.HashMap$Node; (java.base@10....
  14:          477      22896  java.util.HashMap (java.base@10.0.1)
  15:           24      15616  [Ljava.util.concurrent.ConcurrentHashMap...
  16:          554      13296  java.lang.module.ModuleDescriptor$Export...
  17:          391      12512  java.lang.invoke.LambdaForm$Name (java.b...
  18:          227      10896  java.lang.invoke.MemberName (java.base@1...
  19:          146       9344  [Ljava.lang.ref.SoftReference; (java.bas...
  20:          242       7744  java.lang.module.ModuleDescriptor$Requir...
  21:          220       7040  java.lang.invoke.MethodType$ConcurrentWe...
  22:          173       6920  java.lang.invoke.MethodType (java.base@1...
  23:           80       6400  java.net.URI (java.base@10.0.1)
  24:          265       6360  java.util.ImmutableCollections$SetN (jav...
  25:          174       6144  [Ljava.lang.Class; (java.base@10.0.1)
  ...
Total       323237   89521384

The histogram gives a very good summary of heap objects used in my GarbageCollection.java program:

Most of the objects were String objects. There were 302762 of them.
"[Ljava.lang.String;" is a special class name representing a java.lang.String[] array.
"[I" is a special class name representing a int[] array.