Newbie: Passing Structures between DLLs via Java

Question

A fairly high level question for you good people:
I have two DLLs. The first DLL is called by Java via JNI, and needs to return something akin to a C Structure (say an int, a double and a string for simplicity).
My Java code does not need to do anything with this data, except pass it to a second DLL for more processing (the second DLL for example needs to do something with just the double and the string).
My question is, is this sort of thing practical to do using JNI (I have the DLLs already but can easily add the JNIEXPORT etc functions) ?
(I have used JNI for a passing single values and arrays back and forth, but nothing with anything akin to structures).
Many thanks in advance.
Dave

Answer 1

My Java code does not need to do anything with this
data, except pass it to a second DLL for more
processing (the second DLL for example needs to do
something with just the double and the string).
My question is, is this sort of thing practical to do
using JNI (I have the DLLs already but can easily add
the JNIEXPORT etc functions) ?
Somewhere in memory a hunk is reserved for this. Your code does it or some external piece does.
You have a pointer to that.
It is very important that the memory does NOT go away until you tell it to.
You cast the pointer to a long and return that to java. Your java piece keeps track of that.
It passes the long off to the other dll. That dll casts it back to what is needed (a pointer) and uses it.
If the second dll disposes of the pointer then you are done. If not then you must dispose of the pointer.
If you want to keep it then you should add a 'destroy()' method in java that frees the pointer (native call) appropriately. This is also one of the few time where using finalize is probably appropriate as well.