Weird: can not change the value of Integer in jni

问题

I write a jni function to set the value of two java Integer object in C, just like this:

jni_do_something(JNIEnv *env, jobject thiz, jobject p1, jobject p2) {
    jclass c;
    jfieldID id;
    c = env->FindClass("java/lang/Integer");
    if (c==NULL)
    {
        LOGD("FindClass failed");
        return -1;
    }

    id = env->GetFieldID(c, "value", "I");
    if (id==NULL)
    {
        LOGD("GetFiledID failed");
        return -1;
    }

    env->SetIntField(p1, id, 5);
    env->SetIntField(p2, id, 10);
    return 0;
}

In java, I called this function:

native int do_something(Integer p1, Integer p2);

Integer p1=0, p2=0;
do_something(p1, p2);
Log.d("test", "p1: "+p1);
Log.d("test", "p2: "+p2);

The output are both '10', why?

===============================================

I have done many tests, and got the following points. (Answer, comments are welcome)

I don't think this jni native is unable to alter immutable object. After all, the both objects are changed to 10 from 0.
There is some relation to auto-boxing (yes? I'm not sure). Maybe p1 and p2 are specially processed by jvm, made pointed to a single object if initialized with:

Integer p1=0, p2=0;

If change to:

Integer p1=0, p2=1;

Integer p1 = new Integer(0);
Integer p2 = new Integer(0);

The result is right (p1:5, p2:10). Can anyone clarify this?

Maybe what I said is not correct. I have write the following code in java:

Integer a = 0;
Integer b = 0;
b = 10;
Log.d("test", "a: "+a);
Log.d("test", "b: "+b);

It output 0, 10. So, they point to not a single object. I'm really confused.

回答1:

Integer i1 = 500, i2 = 10000;
testInts(i1, i2);
Log.d("log", "I1 = " + i1); 
Log.d("log", "I2 = " + i2);

Integer i1 = new Integer(0), i2 = new Integer(0);
testInts(i1, i2);
Log.d("log", "I1 = " + i1); 
Log.d("log", "I2 = " + i2);

output

I1 = 10
I2 = 10
I1 = 5
I2 = 10
I1 = 5
I2 = 10

Works (first result using your example). I have remembered something that if value is in byte (or?) range then java uses some weird optimization. EDIT: appears it is a rule for autoboxing for small values.

c code

    jclass clazz = (*env)->GetObjectClass(env, i1);
    jfieldID mi = (*env)->GetFieldID(env, clazz, "value", "I");

    (*env)->SetIntField(env, i1, mi, 5);
    (*env)->SetIntField(env, i2, mi, 10);
    return 0;

回答2:

I don’t think Integer is mutable. You’ll see it has no set method.

来源：https://stackoverflow.com/questions/10650230/weird-can-not-change-the-value-of-integer-in-jni

标签

android

java-native-interface