java JNI 函数是否有可能返回整数或布尔值？
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Is it possible that JNI function return integer or boolean?
提问by MOHAMED
JAVA Code
爪哇代码
boolean b = invokeNativeFunction();
int i = invokeNativeFunction2();
C code
C代码
jboolean Java_com_any_dom_Eservice_invokeNativeFunction(JNIEnv* env, jobject obj) {
bool bb = 0;
...
return // how can return 'bb' at the end of the function?
}
jint Java_com_any_dom_Eservice_invokeNativeFunction2(JNIEnv* env, jobject obj) {
int rr = 0;
...
return // how can return 'rr' at the end of the function?
}
Is it possible that JNI function return integer or boolean? If yes, How I can do that?
JNI 函数是否有可能返回整数或布尔值？如果是，我该怎么做？
回答by alijandro
Yes, just return the value directly.
是的，直接返回值即可。
JNIEXPORT jint JNICALL Java_com_example_demojni_Sample_intMethod(JNIEnv* env, jobject obj,
jint value) {
return value * value;
}
JNIEXPORT jboolean JNICALL Java_com_example_demojni_Sample_booleanMethod(JNIEnv* env,
jobject obj, jboolean unsignedChar) {
return !unsignedChar;
}
There is a map relation between Java primitive type and native type, reference here.
Java原始类型和原生类型之间存在映射关系，参考这里。
回答by Nathaniel D. Waggoner
I think your method signatures might be wrong...
我认为您的方法签名可能是错误的...
https://www3.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html
https://www3.ntu.edu.sg/home/ehchua/programming/java/JavaNativeInterface.html
If you'll notice a few things:
如果你会注意到一些事情：
1) the addition of JNIEXPORTand JNICALLaround methods..
2) the to j<object>type of arguments to be returned.
1) 添加JNIEXPORT和 JNICALL环绕方法.. 2)j<object>要返回的参数类型。
I think you need to mofidy your int example to:
我认为你需要修改你的 int 示例：
JNIEXPORT jint JNICALL Java_com_any_dom_Eservice_invokeNativeFunction2(JNIEnv* env, jobject obj) {
jint rr = 99;
...
return rr;
}
回答by Michael
Why not just do some static casts:
为什么不做一些静态转换：
return static_cast<jboolean>(bb);
return static_cast<jint>(rr);
In my copy of jni.hjintis defined as an int32_t, and jbooleanis defined as a uint8_t. The internal representations of trueand falseare the same in C++ and Java (at the VM level) AFAIK (i.e. 0==false, 1==true).
在我的副本中jni.hjint被定义为int32_t，并且jboolean被定义为uint8_t。的内部表示法true和false在C ++和Java相同的（在虚拟机级）AFAIK（即0 ==假，1 == TRUE）。
You can of course add some sanity checks if you want to, e.g.:
如果您愿意，您当然可以添加一些健全性检查，例如：
assert(numeric_limits<jint>::is_signed == numeric_limits<decltype(rr)>::is_signed &&
numeric_limits<jint>::min() <= numeric_limits<decltype(rr)>::min() &&
numeric_limits<jint>::max() >= numeric_limits<decltype(rr)>::max());
