Why does my JNI code not successfully find a jthrowable's getMessage method?
exception, java, java-native-interface
Solution
Okay, so it looks like my problem was that `GetObjectClass` doesn't act the way you'd expect it to on a jthrowable, or at least the results of it are not useful for the purposes of getting methods. Replacing that portion of the code with this works:
java_class = (*jEnv)->FindClass (jEnv, "java/lang/Throwable");
method = (*jEnv)->GetMethodID (jEnv, java_class, "getMessage", "()Ljava/lang/String;");
Damnably odd, that. I hope this helps someone else in the future, though.
Problem
I'm trying to access the message in a jthrowable while handing an exception generated when I fail to find a class. However, I am unable to access the message ID of getMessage() on the jthrowable object, and I don't know why. I've tried changing the signature of getMessage to "()Ljava/lang/String" (without the semicolon at the end, but that's necessary, right?) with no joy. I'm confused as hell about this. I even tried replacing getMessage with toString, and that didn't work. Obviously I'm doing something trivially wrong here. Here's the code I'm using: ``` jthrowable java_exception; jclass java_class; jmethodID method; java_exception = (*jEnv)->ExceptionOccurred(jEnv); assert (java_exception != NULL); java_class = (*jEnv)->GetObjectClass (jEnv, java_exception); assert (java_class != NULL); method = (*jEnv)->GetMethodID (jEnv, java_class, "getMessage", "()Ljava/lang/String;"); if (method == NULL) { printf ("Seriously, how do I get here?!\n"); (*jEnv)->ExceptionDescribe (jEnv); return; } ``` The output of this code (amongst other things) looks like this: Seriously, how do I get here?! Exception in thread "main" java.lang.NoClassDefFoundError: com/planet/core360/docgen/Processor `javap -p -s java.lang.Throwable` gives me this: Compiled from "Throwable.java" public class java.lang.Throwable extends java.lang.Object implements java.io.Serializable{ ... public java.lang.String getMessage(); Signature: ()Ljava/lang/String; ...