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Accessing a Java object in a Java object in C using JNI

I am relatively new to JNI and started to master the basics of mess with integers and arrays in Java objects using JNI. Now I am trying to modify / access a Java object in a Java object.

I searched on the Internet and in Qaru and have not yet figured out how to do this.

Here is an example.

In Java:

public class ObjectOne
{
    private byte[] buff;
    ...
    ...
}

public class ObjectTwo
{
    private ObjectOne obj;
    ...
    ...
}

In JNI, how do I access the "buff" from ObjectOne through ObjectTwo? I tried something like this ...

JNIEXPORT void JNICALL Java_accessBuffThroughObjectTwo(JNIEnv *env, jobject obj, jobject objectTwo)
{
    jclass clazz;
    jclass bufferClazz;
    jobject bufferJObject;

    clazz = (*env)->GetObjectClass(env, objectTwo);
    fid = (*env)->GetFieldID(env, clazz, "obj", "Ljava/lang/Object;");
    bufferJObject = (*env)->GetObjectField(env, javascsicommand, fid);
    bufferClazz = (*env)->GetObjectClass(env, bufferJObject);  <-- Fails here for Access Violation
    fid = (*env)->GetFieldID(env, bufferClazz, "buff", "[B");
}

Any help on what I'm doing wrong?

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1 answer

When trying your code, you can easily add some statements like this:

JNIEXPORT void JNICALL Java_accessBuffThroughObjectTwo(JNIEnv *env, jobject obj,  jobject objectTwo) {
    jclass clazz;
    jclass bufferClazz;
    jobject bufferJObject;
    jfieldID fid;

    clazz = (*env)->GetObjectClass(env, objectTwo);
    assert(clazz != NULL);
    fid = (*env)->GetFieldID(env, clazz, "obj", "Ljava/lang/Object;");
    assert(fid != NULL);
    bufferJObject = (*env)->GetObjectField(env, javascsicommand, fid);
    assert(bufferJObject != NULL);
    bufferClazz = (*env)->GetObjectClass(env, bufferJObject);
    assert(bufferClazz != NULL);
    fid = (*env)->GetFieldID(env, bufferClazz, "buff", "[B");
    assert(fid != NULL);
}

, , fid NULL. , ObjectTwo java.lang.Object. , ( com/package):

fid = (*env)->GetFieldID(env, clazz, "obj", "Lcom/package/ObjectOne;");

, , fid null, .

, , javascsicommand ObjectTwo.

, , bufferJObject. , , NULL, Java-, , obj null.

Java - :

public class ObjectTwo
{
    private ObjectOne obj = new ObjectOne();
    ...
    ...
}

.

, null :

bufferClazz = (*env)->GetObjectClass(env, bufferJObject); <-- The bufferJObject was NULL
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