Use JNI to Create, Populate and Return a Java Class Instance

Question

I\'m trying to use a JNI function to create a Java class and set some properties of that class using the DeviceId.java constructor method. I\'m able to get the constructor

Answer 1

When you called GetMethodID, you provided the signature for the two-arg constructor. Thus, you just need to pass your jstring and a jbytearray when you call NewObject - for example:

return (*env)->NewObject(env, c, cnstrctr, id, cache);

You don't need to call the setId and setCache methods unless you decide to call the 0-arg constructor - and that just complicates your code since you'll have to call GetMethodID for those and call them. Simpler to continue down the route you're on.

Answer 2

I wanted to return a custom Java object from JNI's cpp code back to Java. The solution is to return a jobject from cpp function and use our custom Java object in native method declaration.

public class PyError {

    public String message;

    public boolean occurred;

    public PyError(boolean occurred, String message){
        this.message = message;
        this.occurred = occurred;
    }
} 

and method declaration in Java:

native PyError nativePythonErrorOccurred();

on the cpp side:

extern "C" JNIEXPORT jobject JNICALL
Java_com_your_package_nativePythonErrorOccurred(JNIEnv *env, jobject obj) {

    jclass javaLocalClass = env->FindClass("com/your/package/PyError");

    if (javaLocalClass == NULL) {
        LOGP("Find Class Failed.\n");
    } else {
        LOGP("Found class.\n");
    }

    jclass javaGlobalClass = reinterpret_cast<jclass>(env->NewGlobalRef(javaLocalClass));

    // info: last argument is Java method signature
    jmethodID javaConstructor = env->GetMethodID(javaGlobalClass, "<init>", "(ZLjava/lang/String;)V");

    if (javaConstructor == NULL) {
        LOGP("Find method Failed.\n");
    } else {
        LOGP("Found method.\n");
    }

    jobject pyErrorObject = env->NewObject(javaGlobalClass, javaConstructor, true, env->NewStringUTF("Sample error body"));
    return pyErrorObject;
}

Determine the signature of the method using javap -s java.your.package.YourClass. Also, have a look here.

If you encounter error similar to: JNI ERROR (app bug): attempt to use stale Global 0xf2ac01ba your method signature is wrong, you're passing wrong arguments to env->NewObject() or you're not using global state of jni objects - more here.