奇怪:在JNI不能改变Integer的值 [英] Weird: can not change the value of Integer in jni
问题描述
我写了一个JNI函数来设置用C两个Java Integer对象的值,就像这样:
jni_do_something(JNIEnv的* ENV,jobject THIZ,jobject P1,P2 jobject){
JCLASS℃;
jfieldID ID;
C = env->的findClass(爪哇/郎/整型);
如果(C == NULL)
{
LOGD(findClass的失败);
返回-1;
} ID = env-> GetFieldID(三,价值,I);
如果(ID == NULL)
{
LOGD(GetFiledID失败);
返回-1;
} env-> SetIntField(P1,编号,5);
env-> SetIntField(P2,编号,10);
返回0;
}
在Java中,我调用此函数:
本地INT do_something(整数P1,P2的整数);整数P1 = 0,P2 = 0;
do_something(P1,P2);
Log.d(测试,P1+ P1);
Log.d(测试,P2+ P2);
输出均为10,为什么呢?
===============================================
我已经做了很多测试,并得到了以下几点。 (答案,评论,欢迎)
-
我不认为这JNI本机是无法改变不可变对象。毕竟,无论对象是从0变为10。
-
有一定的关系自动装箱(是吗?我不知道)。也许p1和p2是专门由JVM进行处理,制成指向单个对象如果与初始化
P1整数= 0,P2 = 0;
如果更改为:
整数P1 = 0,P2 = 1;
或
整数P1 =新的整数(0);
整数P2 =新的整数(0);
的结果是正确的(P1:5,P2:10)。任何人都可以澄清这一点?
也许我说的是不是正确的。我用Java编写以下code:
整数= 0;
整数B = 0;
B = 10;
Log.d(测试,A:+ A);
Log.d(测试,B:+ B);
据输出0,10所以,他们指出,没有一个单一的对象。我真的糊涂了。
整数I1 = 500,I2 = 10000;
testInts(I1,I2);
Log.d(日志,I1 =+ I1);
Log.d(日志,I2 =+ I2);
或
整数I1 =新的整数(0),I2 =新的整数(0);
testInts(I1,I2);
Log.d(日志,I1 =+ I1);
Log.d(日志,I2 =+ I2);
输出
I1 = 10
I2 = 10
I1 = 5
I2 = 10
I1 = 5
I2 = 10
工程(第一个结果使用您的例子)。我想起了什么,如果值是该字节(或?),那么范围Java使用一些怪异的优化。
编辑:看来它是自动装箱对于小数值的规则
C $ C $ç
JCLASS clazz所=(* ENV) - GT; GetObjectClass(ENV,I1);
jfieldID MI =(* ENV) - GT; GetFieldID(ENV,clazz中,价值,I); (* ENV) - GT; SetIntField(ENV,I1,MI,5);
(* ENV) - GT; SetIntField(ENV,I2,MI,10);
返回0;
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;
or
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.
Integer i1 = 500, i2 = 10000;
testInts(i1, i2);
Log.d("log", "I1 = " + i1);
Log.d("log", "I2 = " + i2);
or
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;
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