what are "live" objects in java heap? (heap dump with jmap)

Question

jmap help shows:

...

-dump:<dump-options> to dump java heap in hprof binary format
                    dump-options:
                      live         dump only live objects; if not specified,
                                   all objects in the heap are dumped.

...

Once I dump a Tomcat (with java param -Xmx384m) heap:

jmap -dump:file=dump.bin <pid>

I got a dump file of ~300M.

When I dump its heap with live objects only:

jmap -dump:live,file=live-dump.bin <pid>

I got a dump file of ~120M.

My guess of live objects may be:

1. Objects in young generation;

2. Objects that are used / referenced / reachable and will not be collected.

Which one is right?

UPDATE

My guess #2 seems correct, and thanks for Alexey Ragozin's explanation (live option will cause a full GC). I tested again according to his hint:

jmap -dump:file=dump.hprof <pid>
jmap -dump:live,file=live-dump.hprof <pid>
jmap -dump:file=after-live-dump.hprof <pid>

size of these 3 files are:

dump.hprof ~190MB
live-dump.hprof ~40MB
after-live-dump.hprof ~40MB

so after -dump:live, almost all objects in heap are live.

Ayrshire

@Holger sorry for my poor english. #2 I mean objects that are no longer used / referenced but not collected yet. – Ayrshire

sorry for my stupid mistake :-p – Ayrshire

Answer 1

jmap -dump:live,file=live-dump.bin <pid>

live option in jmap command below forces JVM to do a full GC before dumping content of heap into a file.

After full GC only objects transitively reachable from GC roots (definition of "live") are remaining in heap.

Zoospore

When you use live option, full GC is performed by the JVM which could literally hang the application until its complete, in some cases takes a long time and it may not even finish for application that use large heaps. In such cases, there is a good chance of getting the heap dump done if you drop the live option – Trampoline