Returning strings from Java- C JNI calls

Question

<newbie to JNI>
I have an application written in Java that accesses a .DLL written in C. The .DLL does low-level communication to a hardware device. I've got it all working except for one little problem:
For all of the functions I need to return an integer return code and for some of them I also need to return a string, such as a serial number, version string, or whatever.
Try as I might, I can't find any information on how to return two values from a function call (pretty sure I can't in Java).
SO, I tried to find out how to stuff the version string into a string object variable in the class object the DLL API is defined in. I can't figure out how to do that either...
e.g. the following Java code:
class LLDev {
/* --- Load the .DLL -------------------- */
static { System.loadLibrary("LLDev"); }
/* --- Error Codes ---------------------- */
public static final int LLDOk = 0;
public static final int LLDInitErr     = -1;
/* --- Public Variables ----------------- */
public static String DLLVersion;
/* --- Public Methods ------------------- */
public native int InitLLDev();
When I invoke LLDev.InitLLDev() from Java I want the C .DLL function Java_LLDev_InitLLDev to put the DLL Version string in the LLDev object's DLLVersion field and return an error code as the function result.
Is this possible???? Is this the right way to do this?? I tried to define the API with methods that return strings instead of integer return codes but the customer using the class wants all of the methods to return a return code in case of some error (e.g. reading the serial number from the device could fail...)

Answer 1

1. In general, the java way to deal with errors is by throwing exceptions, not using return codes. But OK, you are stuck with the user requirement.
2. I suggest you have the native method take as an argument a java object which will accept a string as input. In other words, it has a string "setter".
3. So in your native method, if the function succeeds, it writes your string result to that object, and you can futrther process it when the native method returns a "success" code.
4. There are JNI methods for
o looking up the class of a java object.
o looking up the method of a java class.
o invoking the method.