Getting the text of a java exception into C++ via JNI - how ?

Question

I have a C++ application that needs to create and call Java objects. If there is an exception in a Java method I want to be able to get the text of the exception's message for reporting an error to the C++ application user. The problem is, I can't get the methodID of the 'getMessage' method in the Throwable class.
This is how I am tryng to get it:
// Relevant exception-handling variables:
jthrowable exceptionHandle;
jclass jThrowableClass;
jmethodID methodID;
//Invoke the Java object
env->CallVoidMethod( obj, methodID, args );
//Check for an exception
if ( (exceptionHandle = env->ExceptionOccurred()) != NULL )
jThrowableClass = env->GetObjectClass( exceptionHandle );
methodID = env->GetMethodID( jThrowableClass, "getMessage", "()Ljava/lang/String;" );
// methodID is always NULL -- why??At this point, it can't go any further because the methodID is always NULL.
I've tried several things, such as explicitly including rt.jar in the classpath when initializing the VM (I'm using JDK 1.3.1 and JNI 1.2) as well as copying rt.jar to the directory where the application's java VM code is at, but no luck there. I've also tried FindClass( "java/lang/Throwable") but that returned NULL so doesn't get as far as the code example here.
How do you get the methodID of a method in a java language class like java.lang.Throwable ? Is there some initialization you need to do with the classpath ?
Many thanks,
Colm.

Answer 1

When you program in Java you can use java.lang.reflect.
For example, having a method named "met1" with 2 parameters, you can assign an object Method in this way:
import java.lang.reflect.Method;
Method comp = o1.getClass.getMethod("met1", param1.getClass(), param2.getClass());So with Throwable you could try something like: Throwable.class.getMethod("getMessage")Sorry but I am not so strong in C++ and I cannot give you help about it.
The Java API documentation for the reflect package is at: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/reflect/package-frame.html
Hoping to be helpful
giangio