Heap dumps - looking at all objects and fields in a live system, without a debugger

Do you have a java server - maybe in production - and you want to look at the value of some field ? You can't attach a debugger, this would require starting the server with various flags - but you can get a heap dump, which includes all objects, including the value of their fields. 

The other use is to generate a heap dump before and after running a load test - and look at garbage to evaluate how much work you put on the garbage collector and find leaks. There are profilers that can get better results - but this is pretty fast and free way to get the same result, and you can use it against running systems. 

Getting heap dumps:

 jmap -dump:format=b,file=heap3.bin PID_OF_JAVA_PROCESS

Using code ( in a servlet, etc ):

server = ManagementFactory.getPlatformMBeanServer();
server.invoke(new ObjectName("com.sun.management:type=HotSpotDiagnostic"),
              "dumpHeap",
              new Object[] {fileName, Boolean.TRUE}, 
              new String[] {String.class.getName(), "boolean"});

Looking at the data:

  Eclipse MemoryAnalyzer 

or 

  jhat heap3.bin 

  open http://localhost:7000