Need help on calling java from C using JNI

[Maki]

Could you help me on this...
Why is the following two calls returning "NULL" when called from JNI.I
tried lot of ways but failed to reslove the issue.
Thread.currentThread().getContextClassLoader(),
getClass().getClassLoader().
/>./a.out
Main-Class:class Test
Thread:Thread[main,5,main]
Thread-current-Thread:null
Main-ClassLoader:null

But,when I call Test.class file from java("java Test") its working
fine...!!!!
Main-Class:class Test
Thread:Thread[main,5,main]
Thread-current-Thread:sun.misc.Launcher$AppClassLoader@1d10ed
Main-ClassLoader:sun.misc.Launcher$AppClassLoader@1d10ed



Please find here the programs
*****************How to compile******************
#javac Test.java
#/usr/local/bin/gcc -I /usr/java1.2/include -I
/usr/java1.2/include/solaris -L/usr/java1.2/jre/lib/sparc -ljava
-ljvm invoke.c
#export LD_LIBRARY_PATH=/usr/java1.2/jre/lib/sparc
************************End of Compile******************

My java Programe is
***************************Test.java***************************

import java.io.InputStream;
import java.net.URL;

public class Test{

public static void main(String[ ] args){
try{

Test c = new Test();

System.out.println("Main-Class:"+c.getClass());

System.out.println("Thread:"+Thread.currentThread());

System.out.println("Thread-current-Thread:"+Thread.currentThread().getContextClassLoader());

System.out.println("Main-ClassLoader:"+c.getClass().getClassLoader());

}
catch(Exception e)
{
System.out.println("Error\n"+e);
e.printStackTrace();
}
}
}
******************************************End of
Test.java*********************

******************************************invoke.c****************************
#include <jni.h>

#ifdef _WIN32
#define PATH_SEPARATOR ';'
#else /* UNIX */
#define PATH_SEPARATOR ':'
#endif

#define USER_CLASSPATH "." /* where Prog.class is */

main() {
JNIEnv *env;
JavaVM *jvm;
JDK1_1InitArgs vm_args;
jint res;
jclass cls;
jmethodID mid;
jstring jstr;
jobjectArray args;
char classpath[1024];
jthrowable exc;


/* IMPORTANT: specify vm_args version # if you use JDK1.1.2 and
beyond */
vm_args.version = 0x00010001;

JNI_GetDefaultJavaVMInitArgs(&vm_args);

/* Append USER_CLASSPATH to the end of default system class path
*/
sprintf(classpath, "%s%c%s",
vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH);
vm_args.classpath = classpath;

//JNI_CreateJavaVM(JavaVM **pvm, void **penv, void *args);
/* Create the Java VM */
res = JNI_CreateJavaVM(&jvm,&env,&vm_args);
if (res < 0) {
fprintf(stderr, "Can't create Java VM\n");
exit(1);
}

cls = (*env)->FindClass(env, "Test");
if (cls == 0) {
fprintf(stderr, "Can't find Prog class\n");
exit(1);
}

mid = (*env)->GetStaticMethodID(env, cls, "main",
"([Ljava/lang/StringV");
if (mid == 0) {
fprintf(stderr, "Can't find Prog.main\n");
exit(1);
}

jstr = (*env)->NewStringUTF(env, " from C!");
if (jstr == 0) {
fprintf(stderr, "Out of memory\n");
exit(1);
}

args = (*env)->NewObjectArray(env, 1,
(*env)->FindClass(env,
"java/lang/String"), jstr);
if (args == 0) {
fprintf(stderr, "Out of memory\n");
exit(1);
}

(*env)->CallStaticVoidMethod(env, cls, mid, args);
exc = (*env)->ExceptionOccurred(env);
if(exc)
{
(*env)->ExceptionDescribe(env);
(*env)->ExceptionClear(env);
printf("Sorry Failed in adding user\n");
}
(*jvm)->DestroyJavaVM(jvm);
}
***************************************End of
invoke.c************************************

Thanks,
Ranga

 

[Gordon Beaton]

[ comp.lang.java.developer does not exist ]

Why is the following two calls returning "NULL" when called from
JNI.I tried lot of ways but failed to reslove the issue.

You might check that vm_args.classpath contains something valid before
you attempt to use the value, here:

sprintf(classpath, "%s%c%s",
vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH);

When I tried this using your code, vm_args.classpath contained
garbage.

Using the Blackdown 1.4.1 JVM on UltraSparc Linux, I get identical
results (no NULL) with your code and the regular java launcher.

I made the following changes however:

- define _REENTRANT when compiling the launcher
- change to JNI_VERSION_1_2
- use JavaVMInitArgs instead of JDK1_1InitArgs

You should condider using (*env)->ExceptionDescribe() when your JNI
calls fail, instead of just reporting "Out of memory" or "Can't find
prog".

I'd also suggest you turn on compiler warnings (-Wall), since there
are several. For example main() must return an int, and you should
include <stdio.h> as well as <stdlib.h>.

/gordon