Debugging the Java Heap (jmap and other tools)

How to debug the Java heap?

There are many tools, one is jmap (shows you the heap, gc algorithm etc).

jmap -histo pid

shows the number of objects of each class in a given java process.

Debug Java Threads

jstack -l -m

-m means mixed mode (Java and native C/C++ stack frames)

Jargon

In output of jmap you'll see some jargon. For example:

CMS - Concurrent Mark Sweep (aka low latency collector). computes transitive closure of live references for garbage collection.
gc "ergonomics" - automatically selecting a gc algo based on conditions at runtime

Other Java Tools

jinfo - tells you the VM info you are running with
jps - ps for java processes, jps -l gives you the fully qualified classname
jstat - performance statistics for JVM