[JNI] Local Reference cannot be stored in a static variable, Global Reference can be stored in a static variable

Look at the stringClass of the following code:

/* This code is illegal */
jstring MyNewString(JNIEnv *env, jchar *chars, jint len)
{
    static jclass stringClass = NULL;
    jmethodID cid;
    jcharArray elemArr;
    jstring result;
    if (stringClass == NULL) {
            stringClass = (*env)->FindClass(env,"java/lang/String");
            if (stringClass == NULL) {
                return NULL; 
            }
    }

    /* It is wrong to use the cached stringClass here, because it may be invalid. */
    cid = (*env)->GetMethodID(env, stringClass,"<init>", "([C)V");
    elemArr = (*env)->NewCharArray(env, len);
    result = (*env)->NewObject(env, stringClass, cid, elemArr);
    
    (*env)->DeleteLocalRef(env, elemArr);
    return result;
}

JNIEXPORT jstring JNICALL Java_C_f(JNIEnv *env, jobject this)
{
    char *c_str = ...;
    ...
    return MyNewString(c_str,len);
}

//Java层调用两次Java_C_f接口:
...
... = C.f();         // The first call is perhaps OK.
... = C.f();         // This would use an invalid local reference.
...

Explanation:

FindClass returns the localRef(stringClass) of java.lang.String. After the first Cf() method return, the virtual machine releases all localRefs created during Cf(), including stringClass.

But stringClass!=NULL, when Cf() is called for the second time, MyNewString() will no longer call FindClass and will not generate a new localRef. At this time, stringClass is an invalid localRef, which may cause system crash.

 

Global Reference can be stored in a static variable:

/* This code is OK */
jstring MyNewString(JNIEnv *env, jchar *chars, jint len)
{
    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);
        if (stringClass == NULL) {
               return NULL; /* out of memory exception thrown */
        }
        /* The local reference is no longer useful */
        (*env)->DeleteLocalRef(env, localRefCls);
    }
    ...
}

 

references:

[1] The Java™ Native Interface Programmer’s Guide and Specification.pdf