To get a heap dump of an existing application you need to know the PID of your java process.
To do this, you can use the jps command
jps -l
So for above, I can see the PID for the process is 5776
Once you know your PID you can use the jcmd to generate the heap dump
jcmd <pid> GC.heap_dump <file-path>
For above PID I can do
jcmd 5776 GC.heap_dump c:/temp/dump.hprof
To open the the heap dump, you can use YourKit or Open source tools such as Eclipse MAT tool
Using Eclipse MAT Tool we can open the Heap dump
Using the Eclipse MAT tool you can see the drill in the pie chat to list Objects and to see the incoming references, see the retain size of objects, see the paths to GC Root. MAT tool also has leak hunter tool to try to find problem areas.