My reference: Java Native Interface
This table was specifically handy:
| Java Type | JNI Type | machine dependent C/C++ typedef |
Signature | Call…Method Get…Field |
|---|---|---|---|---|
boolean |
jboolean |
unsigned char |
Z |
Boolean |
byte |
jbyte |
signed char |
B |
Byte |
char |
jchar |
unsigned short |
C |
Char |
short |
jshort |
short |
S |
Short |
int |
jint |
int |
I |
Int |
long |
jlong |
long |
J |
Long |
float |
jfloat |
float |
F |
Float |
double |
jdouble |
double |
D |
Double |
void |
void |
V |
Void |
|
| nonprimitive | jobject |
*… |
L...; |
Object |
Examples
| method definition | signature |
|---|---|
| int m1 () | ()I |
| double m2 (long l, char c) | (JC)D |
| void m3 (String s, int[] a) | (Ljava/lang/String;[I)V |
| String m4 (boolean b) | (Z)Ljava/lang/String; |
| Object m4 (BigDecimal b) | (Ljava/math/BigDecimal;)Ljava/lang/Object; |
My code testing implementing this field access:
HelloWorldNative.c (builds into .so linked from Java)
JNIEXPORT void JNICALL Java_helloworld_Main_nativePrintNumber
(JNIEnv *env, jobject obj, jint ji)
{
jfieldID fid;
jint version;
jclass c;
c = (*env)->GetObjectClass(env, obj);
fid = (*env)->GetFieldID(env, c, "version_number", "I");
version = (*env)->GetIntField(env, obj, fid);
(*env)->SetIntField(env, obj, fid, ji*ji);
printf("::: %d, %dn", fid, version);
hello_number(ji);
}
My Main.java, which the version_number integer field:
package helloworld;
/**
*
* @author thomas
*/
public class Main {
public int version_number;
private native void nativePrint();
private native void nativePrintNumber(int i);
/** Creates a new instance of Main */
public Main() {
version_number = 5;
}
static {
System.load("/home/thomas/testlib/HelloWorldNative/dist/HelloWorldNative.so");
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) {
Main me = new Main();
me.nativePrint();
me.nativePrintNumber(2);
me.nativePrintNumber(5);
me.nativePrintNumber(3);
// TODO code application logic here
}
}