Heap Histogram
The easiest way to analyse which objects consume the most of your memory is via heap histograms.
Heap histogram can be obtained by running jcmd (here with process 875)
>jcmd 875 GC.class_histogram
num #instances #bytes class name ---------------------------------------------
1: 789087 31563480 java.math.BigDecimal
2: 237997 22617192 [C
3: 137371 20696640 <constMethodKlass>
4: 137371 18695208 <methodKlass>
5: 13456 15654944 <constantPoolKlass>
6: 13456 10331560 <instanceKlassKlass>
7: 37059 9238848 [B
Klass related data are often at the top of the list, and so that strings or character arrays
Similar output can be obtained by running jmap command
>jmap -histo:live 875
where :histo suffix is used to force GC so that dead objects could be removed from histogram
Heap histogram are quite small so don't be shy introducing them into your integration tests
Heap Dumps
For a deeper analysis of live objects heap dumps should be obtained. To generate heap dump from a command line, run following command:
> jcmd PID GC.heap_dump /path/to/dump/heap_dump.hprof
or
>jmap -dump:live, file=/path/to/dump/heap_dump.hprof PID
Such obtained heap dump can be analysed using tools like:
jhat - analysis dump and runs a small http server to let you see results
mat - more sophisticated heap analysis tool with better queries support and reporting
These programs will give you information about objects: Retained Size, Shallow Size, Deep Size
Retained Size - this is the object size + referenced object sizes included (but excluding shared objects i.e. if the referenced objects is referenced from another object, its size won't be included here)
Shallow Size - this object size + reference pointer sizes i.e. 4 or 8 bytes)
Deep Size - this is the object size + referenced objects sizes included + shared object sizes included
