how to fork JVM? [duplicate]

Question

Possible Duplicate:
Java - C-Like Fork?

I wanted to know how is it possible to fork a child JVM from a JDK or even is it possible to do so?

Some frameworks like hadoop fork a child JVM for specific tasks thus Please throw some light on the subject.

Thanks!

Answer 1

Fork is commonly confused with spawn. Spawn is fork + exec (which means start a process which replaces the current one and inherits some of its external resources, such as open sockets).

Spawn is available in Java (via System.exec and the like).

Fork is not in Java, because it would be extremely problematic.

Fork requires cloning of the address space. The kernel support to do this efficiently is not available in most non-Unix OSes; e.g. U/win and Cygwin have struggled to emulate fork under Win32. In a language such as Java, the garbage collector & JIT compilers would tend to touch Vmem pages such that the space would not long remain shared after fork.

Cloning has many side effects. E.g., input or outputs buffers will be processed in all of the forked children. SysV shared memory will be detached.

Most languages and many libraries simply refuse to support forking. This includes the (POSIX) threading API ; The child is not allowed to use threads until it execs (i.e., becomes a spawn) !

Perl uses fork because its internals are extremely close to C/Unix, and its multithreading is abysmal.

Unix shell uses fork by design, which is how it snapshots all local variables and state. That's also why there is no decent Unix shell for a non-Unix environment.

Answer 2

In general, I don't believe this is possible in the sense you mean. You can System.exec() to spin off a new process, and can call a new JVM that way. Note that you can certainly call fork() directly from native code, but in the words of a poster here, "Don't. Just don't."