In JNI, how do I cache the class, methodID, and fieldIDs per IBM's performance recommendations?

Question

I read on on IBM that

To access Java objects\' fields and invoke their methods, native code must make calls to FindClass(), GetFieldID(), GetMethod

Answer 1

You can have some utility structures like this:

typedef struct MYVARIANT_FID_CACHE {
    int cached;
    jclass clazz;
    jfieldID pAddress;
} MYVARIANT_FID_CACHE;

MYVARIANT_FID_CACHE VARIANTFc;

void cacheMYVARIANTFields(JNIEnv *env, jobject lpObject)
{
    if (VARIANTFc.cached) return;
    VARIANTFc.clazz = env->GetObjectClass(lpObject);
    VARIANTFc.pAddress = env->GetFieldID(VARIANTFc.clazz, "pAddress", "I");
    VARIANTFc.cached = 1;
}

VARIANT *getMYVARIANTFields(JNIEnv *env, jobject lpObject, VARIANT *lpStruct)
{
    if (!VARIANTFc.cached) cacheVARIANT2Fields(env, lpObject);

    lpStruct = (VARIANT*)(env->GetIntField(lpObject, VARIANTFc.pAddress));

    return lpStruct;
}

This is taken from my question: https://stackoverflow.com/questions/10617714/how-to-extend-swt-com-support

For some good examples look at the os_structs.c it is bundled with eclipse SWT implementation.

Note: The above code is just an example and could be adapted for different OS. Also it just shows "how to access java fields"; for methods you could follow the same approach.