static jclass stringClass = NULL;
...
if (stringClass == NULL) {
    jclass localRefCls =
        (*env)->FindClass(env, "java/lang/String");
    if (localRefCls == NULL) {
        return NULL; /* exception thrown */
    }
    /* Create a global reference */
    stringClass = (*env)->NewGlobalRef(env, localRefCls);
    /* The local reference is no longer useful */
    (*env)->DeleteLocalRef(env, localRefCls);
    /* Is the global reference created successfully? */
    if (stringClass == NULL) {
        return NULL; /* out of memory exception thrown */
    }
}