从C ++ JNI获取Java类中的实例变量

Question

Good day, Sorry am just a noob in JNI so bear with me if it is a rather silly question :D now For calling a method in a java class from C++ using JNI, you can do this:

Java class:

public int getCount() {     
    return mCount; 
}

JNI:

JNIEXPORT void JNICALL
Java_com_example_init(JNIEnv* env, jobject obj, jint number)
{
    ...
    jclass Class = env->GetObjectClass(obj);
    jmethodID getCountMethodID = env->GetMethodID(Class,
                                                    "getCount", "()I");
    if (getCountMethodID == 0)
    {
        LOG("Function getCount() not found.");
        return;
    }
   Count = env->CallIntMethod(obj, getCountMethodID);
    ...
}

but how do you just get the instance variable from the java class directly? cannot seem to find an example for this. or is it very straightforward?

Answer 1

You can refer the JNI Docs for more details http://docs.oracle.com/javase/6/docs/technotes/guides/jni/spec/functions.html#wp16536 .

Get the jfieldID of the desired instance variable from jclass using following method

jfieldID GetFieldID(JNIEnv *env, jclass clazz, const char *name, const char *sig);

Once you have the jfieldID you can access object instance variable using following method. But you need to know upfront the type of field that you are going to access.

NativeType Get<type>Field(JNIEnv *env, jobject obj, jfieldID fieldID);

Answer 2

You use GetFieldID() to get the field ID; if you're going to be doing this multiple times or on multiple objects, be sure to cache that field ID instead of looking it up every time. Then, use the Get<type>Field() functions to get the field's value:

jfieldID field = env->GetFieldID(Class, "myCount", "I");  // "I" = int field
if (field == NULL)
    /* Handle error */;

jint myCount = env->GetIntField(obj, field);
// Use field value...