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POJNI - GetObjectField returns NULL
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copied!<p>I'm currently working on <a href="http://mangler.org" rel="nofollow noreferrer">Mangler's</a> Android implementation.</p> <p>I have a java class that looks like so:</p> <pre><code>public class VentriloEventData { public short type; public class _pcm { public int length; public short send_type; public int rate; public byte channels; }; _pcm pcm; } </code></pre> <p>The signature for my pcm object:</p> <pre><code>$ javap -s -p VentriloEventData ... org.mangler.VentriloEventData$_pcm pcm; Signature: Lorg/mangler/VentriloEventData$_pcm; </code></pre> <p>I am implementing a native JNI function called getevent, which will write to the fields in an instance of the VentriloEventData class. For what it's worth, it's defined and called in Java like so:</p> <pre><code>public static native int getevent(VentriloEventData data); VentriloEventData data = new VentriloEventData(); getevent(data); </code></pre> <p>And my JNI implementation of getevent:</p> <pre><code>JNIEXPORT jint JNICALL Java_org_mangler_VentriloInterface_getevent(JNIEnv* env, jobject obj, jobject eventdata) { v3_event *ev = v3_get_event(V3_BLOCK); if(ev != NULL) { jclass event_class = (*env)->GetObjectClass(env, eventdata); // Event type. jfieldID type_field = (*env)->GetFieldID(env, event_class, "type", "S"); (*env)->SetShortField( env, eventdata, type_field, 1234 ); // Get PCM class. jfieldID pcm_field = (*env)->GetFieldID(env, event_class, "pcm", "Lorg/mangler/VentriloEventData$_pcm;"); jobject pcm = (*env)->GetObjectField( env, eventdata, pcm_field ); jclass pcm_class = (*env)->GetObjectClass(env, pcm); // Set PCM fields. jfieldID pcm_length_field = (*env)->GetFieldID(env, pcm_class, "length", "I"); (*env)->SetIntField( env, pcm, pcm_length_field, 1337 ); free(ev); } return 0; } </code></pre> <p>The code above works fine for writing into the type field (that is not wrapped by the _pcm class). Once getevent is called, data.type is verified to be 1234 at the java side :)</p> <p>My problem is that the assertion "pcm != NULL" will fail. Note that pcm_field != NULL, which probably indicates that the signature to that field is correct... so there must be something wrong with my call to GetObjectField. It looks fine though if I compare it to the <a href="http://java.sun.com/docs/books/jni/html/fldmeth.html" rel="nofollow noreferrer">official JNI docs</a>.</p> <p>Been bashing my head on this problem for the past 2 hours and I'm getting a little desperate.. hoping a different perspective will help me out on this one. </p> <p>edit1: This won't work with any nested object, even a simple 'Ljava/lang/String;' will fail.<br /> edit2: Both ExceptionOccurred and ExceptionCheck return JNI_FALSE.<br /> edit3: Won't work with static fields either.</p>

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