jni——如何转换有符号与无符号数

java数据结构默认均为有符号数，而通过jni转换到c/c++层，却不一定是有符号数。

如若在java中存储的即为无符号数，则在jni中可将jbyte直接进行类型转换。

若进行操作，则可在计算时，先将byte&0xff，这样即可转换为32位数据，而后再进行计算。

转换方式如下：

1、jbyteArray转换为unsigned char*

Java

1 public class example {
2 public final static native void set_Foo_array(long jarg0, short[] jarg1);
3 public final static native short[] get_Foo_array(long jarg0);
4 }

c++ code:

class Foo {
public:
unsigned char array[10];
};

extern "C"{
JNIEXPORT void JNICALL Java_example_set_1Foo_1array(JNIEnv *jenv, jclass jcls, jlong jarg0, jshortArray jarg1) {
Foo *arg0 ;
unsigned char *arg1 ;
int i ;
jshort* jarg1_carray ;
jsize jarg1_len = jenv->GetArrayLength(jarg1) ;

arg0 = *(Foo **)&jarg0;
jarg1_carray = jenv->GetShortArrayElements(jarg1, 0);
arg1 = (unsigned char *) malloc(jarg1_len * sizeof(unsigned char ));
for(i=0; i<jarg1_len; i++)
arg1[i] = (unsigned char )jarg1_carray;
{
int i;
for (i=0; i<10; i++)
arg0->array[i] = arg1[i];
}
jenv->ReleaseShortArrayElements(jarg1, jarg1_carray, 0);
free(arg1);
}}


extern "C"{
JNIEXPORT jshortArray JNICALL Java_example_get_1Foo_1array(JNIEnv *jenv, jclass jcls, jlong jarg0) {
jshortArray jresult = 0 ;
Foo *arg0 ;
unsigned char *result ;
jshort* jnitype_ptr = 0 ;
int k ;

arg0 = *(Foo **)&jarg0;
result = (unsigned char *)(unsigned char *) (arg0->array);
jresult = jenv->NewShortArray(10);
jnitype_ptr = jenv->GetShortArrayElements(jresult, 0);
for (k=0; k<10; k++)
jnitype_ptr[k] = (jshort)result[k];
jenv->ReleaseShortArrayElements(jresult, jnitype_ptr, 0);
return jresult;
}}

The code comes from the output of swig (www.swig.org) - a tool which generated the JNI and Java code given the C++ class definition above.

2、char*转jbyte

 1 static .. jvm; // ref to jvm
 2 static jobject printAPI; // the static class ref
 3 static jmethodID loadBuffer; // the static method ref
 4 
 5 void loadImage(int x, int y, int width, int height, char* pixels, int maxWidth, int maxHeight){
 6      JNIEnv* env;
 7      jint result = (*jvm)->GetEnv(jvm, &env, JNI_VERSION_1_2);
 8      if(result < 0 || env == NULL){
 9        fprintf(stderr, "Error finding JNI environment\n");
10      }else{
11           int i, size = width*height*3;
12           jbyteArray rgb;  // the byte array
13           jbyte* v; // array of jbytes for transfer operation
14 
15           /* Transfer char* to jbyteArray */
16        rgb = (*env)->NewByteArray(env, width * height *3);
17        v = malloc( (width * height *3) * sizeof(jbyte));
18           if( !rgb || !v ){
19             fprintf(stderr, "Error allocating memory in loadImage(..)");
20             return;
21           }
22        for(i=0;i<size; ++i) v[i] = (jbyte) pixels;
23 
24      (*env)->SetByteArrayRegion(env, rgb, 0, size, v);
25 
26 
27 
28 /* send pixel dump to gui */
29 
30 (*env)->CallStaticIntMethod(env, printAPI, loadBuffer, x, y, width, height, rgb);
31 
32      }
33 
34 }
35 
36 
37 
38 the method on the java side is
39     static void loadByteArray( int x, int y, int w, int h, byte[] rgb ){..}
40 Hope it helps.