Why I should not reuse a jclass and/or jmethodID in JNI?

The rules here are clear. Method ID and field ID values are forever. You can hang onto them. The lookups take some time.

jclass, on the other hand, is generally a local reference. A local reference survives, at most, the duration of a single call to a JNI function.

If you need to optimize, you have to ask the JVM to make a global reference for you. It’s not uncommon to acquire and keep references to common classes like java.lang.String.

Holding such a reference to a class will prevent it (the class) from being garbage-collected, of course.

jclass local = env->FindClass(CLS_JAVA_LANG_STRING);
_CHECK_JAVA_EXCEPTION(env);
java_lang_string_class = (jclass)env->NewGlobalRef(local);
_CHECK_JAVA_EXCEPTION(env);
env->DeleteLocalRef(local);
_CHECK_JAVA_EXCEPTION(env);

The check macro calls:

static inline void
check_java_exception(JNIEnv *env, int line)
{
    UNUSED(line);
    if(env->ExceptionOccurred()) {
#ifdef DEBUG
        fprintf(stderr, "Java exception at rlpjni.cpp line %d\n", line);
        env->ExceptionDescribe();
    abort();
#endif
        throw bt_rlpjni_java_is_upset();
    }
}