jStack
jstack One of the useful troubleshooting utilities in J2SETM 5.0 is jstack. It prints the stack traces of all java threads for a given process or core dump. The -m option will print a mixed-mode stack so that you can see native frames in addition to the java frames. jstack was originally created for troubleshooting problems involving hangs and crashes but developers have found it a useful utility to look at running applications too.
jstack isn't the only way to obtain a thread dump of a running application. Developers have long been accustomed to using Ctrl-\ (or Ctrl-Break if you are on Windows) to get a thread dump of an application that is running interactively. An alternative to the key sequence on Solaris (or Linux) is to send a QUIT signal to the target process. In that case the target processs prints a thread dump to its standard output (which can be pain if you don't know where the log file is).
If you are lucky to be on Solaris 10, then yet another way to obtain a stack trace is using the the pstack utility. This has been updated in Solaris 10 so that it prints java method names for interpreted, compiled and inlined java methods. pstack prints a mixed-mode stack and the output is similar to jstack -m.
Mustang (Java SE 6.0) brings more improvements. For starters the default mode for jstack is to work a bit like a remote Ctrl-\. This means that the output of the thread dump has changed a bit but there is more information on deadlocks and JNI Global References in additional to the thread stacks. If the VM is hung then a thread dump can still be forced using the -F option.
The second improvement is that jstack is included on Windows. It doesn't have all the features yet of the jstack utility on Solaris but it does solve a long standing requirement from developers to get a thread dump of an applications that run in the background as a Windows Service.
The third improvement is the "-l" option to print information about java.util.concurrent.locks. More specifically, it instructs the utility to look for ownable synchronizers in the heap. This should be useful for applications that make use of the concurrency APIs added in J2SE 5.0. Without the "-l" option the thread dump only includes information on monitors. One word of warning though, this option can be expensive if the heap is large