Matty wrote:
> Howdy,
>
> I am trying to use the hotspot provider's object-alloc probe to
> display the number of objects allocated, and the number of bytes
> associated with these objects. When I run the attached [1] DTrace
> script to capture this information, I periodically see classes named
> "[" followed by a character:
>
> $ dtrace objects.d
> [Ljava/lang/Object; 204216
> java/util/HashMap$EntryIterator 262656
> [Ljava/lang/reflect/Field; 300000
> java/lang/String 480864
> java/lang/reflect/Field 1620000
> [I 1636752
> [C 11512240
> [B 15426184
>
> Does anyone happen to know why some classes show up this way?
>
> Thanks for any insight,
> - Ryan
A possibly more convenient way to find out how many objects
of a given type you have is to use the "jmap -histo" command.
That will tell you how many instances of each type you have
in the heap, and how much space those instances are taking
up. You give it an argument which is is the pid of the java
process you want to examine. E.g.,
$ jmap -histo 698
num #instances #bytes class name
--------------------------------------
1: 478 85224 [C
2: 12 73296 [I
3: 12 34256 [B
4: 324 13848 [Ljava.lang.Object;
5: 32 10240 <objArrayKlassKlass>
6: 352 8448 java.lang.String
7: 43 4128 java.lang.Class
8: 37 2664 java.lang.reflect.Field
9: 56 1880 [Ljava.lang.String;
10: 58 1392 java.util.Hashtable$Entry
11: 16 1280 [Ljava.util.HashMap$Entry;
12: 9 1216 <constMethodKlass>
13: 10 1040 [Ljava.util.Hashtable$Entry;
14: 24 912 <symbolKlass>
15: 56 896 java.lang.StringBuilder
....
Total 2111 259760
"jmap -histo" shows you what's in the heap right now. If you
are only interested in the live objects, use "jmap -histo:live",
which does a full collection, then reports the statistics. E.g.,
for the same application, at the same point shows
$ jmap -histo:live 698
num #instances #bytes class name
--------------------------------------
1: 190 44672 [C
2: 9 25168 [B
3: 313 13048 [Ljava.lang.Object;
4: 32 10240 <objArrayKlassKlass>
5: 186 4464 java.lang.String
6: 43 4128 java.lang.Class
7: 31 2232 java.lang.reflect.Field
8: 48 1640 [Ljava.lang.String;
9: 6 1376 [I
10: 57 1368 java.util.Hashtable$Entry
11: 16 1280 [Ljava.util.HashMap$Entry;
12: 9 1216 <constMethodKlass>
13: 24 912 <symbolKlass>
14: 2 816 <constantPoolKlass>
15: 9 720 <methodKlass>
....
Total 1428 128096
If you want both sets of numbers, get the total occupancy first,
then just the live objects, since getting just the live objects
forces a full collection.
jmap has the advantage over dtrace that it has no overhead until
you want to iterate the heap. A disadvantage is that it only
shows the current occupancy of the heap, rather than the total
number of instances allocated over time. If you make your heap
large enough, those numbers are similar. :-)
jmap produces type names the way Keith described them per the spec.
In addition it shows instances of classes that the VM itself uses,
in brokets, e.g., "<symbolKlass>". Those objects are in the
"permanent generation", rather than in the Java object heap.
... peter
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