Android - How can I call Java methods from C++?

August 1, 2012 - 12:11pm #1

The Vuforia Android samples are set up such that application lifecycle events are handled in Java, but tracking events and rendering are handled natively in C++. Additionally, users may want to leverage Android SDK functionality (such as touch handling or networking logic) while doing the low-level graphics work natively. All this requires that we have a means of communicating between Java and C++. This is provided by the JNI (Java Native Interface).

For a practical example of using the JNI to respond to native tracking events in Java see this article: AndroidUIChanges.xml

Calling Java methods from native

We have the following example in the ImageTargets.java class:

public int getTextureCount()

{

return mTextures.size();

}

Note this method doesn't include any special JNI syntax. We can call it natively by looking up the class that our ImageTargets Java object belongs to, and then by looking up the getTextureCount method in that class:

JNIEXPORT void JNICALL
Java_com_qualcomm_QCARSamples_ImageTargets_ImageTargets_initApplicationNative(
                            JNIEnv* env, jobject obj, jint width, jint height)
{
    ...

    jclass activityClass = env->GetObjectClass(obj);

    jmethodID getTextureCountMethodID = env->GetMethodID(activityClass,
                                                    "getTextureCount", "()I");
    if (getTextureCountMethodID == 0)
    {
        LOG("Function getTextureCount() not found.");
        return;
    }

    textureCount = env->CallIntMethod(obj, getTextureCountMethodID);

    ...
}

The last argument to the GetMethodID call - "()I" - needs some explanation. Inside the parentheses you place the argument types, in order, and after the parentheses you place the return type. So "(IF)Z" would represent a method that takes an int and a float and return a boolean. For more info and a table of the field descriptors see the jni documentation here: http://java.sun.com/docs/books/jni/html/types.html

Storing JNI references for later

Note that to call Java methods from C++ we need a handle on the JNIEnv object and the jobject (in this case the ImageTargets object). In the Vuforia samples, getTextureCount is called directly from a method that is called from Java (initApplicationNative). We automatically get the JNIEnv and jobject as the first two arguments to this function. If you want to call a Java method at a later time, however, you'll need to cache these values globally. It's somewhat dangerous to store the JNIEnv globally, mainly because it isn't thread safe. Here's a safe way to obtain the JNIEnv for the current thread at a later time:

JavaVM* javaVM = NULL;
jclass activityClass;
jobject activityObj;

JNIEXPORT void JNICALL
Java_com_qualcomm_QCARSamples_ImageTargets_ImageTargets_initApplicationNative(
                            JNIEnv* env, jobject obj, jint width, jint height)
{
    env->GetJavaVM(&javaVM);
    jclass cls = env->GetObjectClass(obj);
    activityClass = (jclass) env->NewGlobalRef(cls);
    activityObj = env->NewGlobalRef(obj);
}

void myNativeMethod()
{
    JNIEnv *env;
    javaVM->AttachCurrentThread(&env, NULL);
    jmethodID method = env->GetMethodID(activityClass, "myJavaMethod", "()V");
    env->CallVoidMethod(activityObj, method);
}

Also note the use of NewGlobalRef to make safe global references to the jclass and jobject objects.

 

 

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