HELP IN JNI (1.4) : how to get informations from a jclass pointer ?
Hi,
I am integrating a JVM inside an already built product in C and Objective C. I need to build hash tables of jclass pointers. The problem is :
On two different JNI interactions, when for instance you ask for the jclass pointer of "java/lang/String", you get different pointers. So I cannot built a hash key based on the jclass value. Only
(*env)->IsSameObject(env,jclass1,jclass2) can help to recognize that those pointers points to the same java class.
So it would be usefull for me to grab (efficiently) a stable information, such as the name of the class, to build a hash key from.
Anybody could tell me how can I do this ?
Thanks in advance,
Regards,
C.Dore
[731 byte] By [
cdore] at [2007-9-26 10:30:23]
env->CallStaticObjectMethod(env, cls, mID); whereenv is your JNIEnv*cls is your jclass argumentmID is the method ID env returned to you for getName, ()Ljava/lang/String; calling GetMethodID
This does not seem to work...
Here is what I tried :
jmethodID testMeth;
jclass testClass=(*env)->GetMethodID(env,(*env)->FindClass(env,"com/mycom/MyVcass"),
testMeth=(*env)->GetMethodID(env,(*env)->FindClass(env,"java/lang/Class"),"getName","()Ljava/lang/String;");
cname= (*env)->CallStaticObjectMethodA(env,testClass,testMeth,NULL);
the latest statement crashes....
Anybody can help ?
cdore at 2007-7-1 22:49:24 >
Yes, sorry, realized that myself and actually thought I would have posted the correct solution but I guess I forgot about it. Here it is:
[code]
import java.util.Hashtable;
public final class Test {
static { System.loadLibrary("test"); }
private static native Hashtable makeHash();
public static void main(String[] args) {
Hashtable h = makeHash();
h.put("ba", "ba");
System.out.println(h);
} // main(String[])
}
// NATIVE portion:
#include <jni.h>
/* Header for class Test */
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class:Test
* Method:makeHash
* Signature: ()Ljava/util/Hashtable;
*/
JNIEXPORT jobject JNICALL Java_Test_makeHash(JNIEnv *env, jclass klass) {
jclasshashClass = (*env)->FindClass(env, "java/util/Hashtable");
jmethodID hashConstr = (*env)->GetMethodID(env, hashClass, "<init>", "()V");
jobjecthash= (*env)->NewObject(env, hashClass, hashConstr);
return hash;
}
#ifdef __cplusplus
}
#endif
Well, now I know why I missed my post, I confused your thread again. But I also have a solution for you:
import java.util.Hashtable;
public final class Test {
static { System.loadLibrary("test"); }
private static native String getClassName(Object o);
public static void main(String[] args) {
System.out.println(getClassName(new String()));
} //* main(String[])
}
Native portion:
#include <jni.h>
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class:Test
* Method:makeHash
* Signature: ()Ljava/util/Hashtable;
*/
JNIEXPORT jobject JNICALL Java_Test_getClassName(JNIEnv *env, jclass klass, jobject o) {
jclassclassClass = (*env)->FindClass(env, "java/lang/Class");
jobjectoClass= (*env)->GetObjectClass(env, o);
jmethodID getName= (*env)->GetMethodID(env, classClass, "getName", "()Ljava/lang/String;");
return (*env)->CallObjectMethod(env, oClass, getName);
}
#ifdef __cplusplus
}
#endif
OK. It works fine.If I understand well, getObjectClass returns a jclass, which is in fact a reference ot the Class object we can grab through the Object::getClass() method.Thanks !C.Dore
cdore at 2007-7-1 22:49:24 >
Yes, you are correct. It actually returns a jclass, which is a subtype of jobject. This subtype relationship is modelled cleanly only if you use C++. In C it is simply realized by a typedef, which, in fact, makes both types identical. I chose to declare the variable only a jobject since I only access it as an instance of an object. If you need any functionality of the class itself, you are certainly free to declare it as jclass.
