JNI -- passing C struct from native method to java

see if it helps for You:-
 
 
You can do it as long as you have an API to the third party DLL. All you have to do is to
create your own DLL which contains native implementations of Java methods where C or C++
routines will have signatures generated by javah. All those routines would do is to invoke
 the appropriate functions in the target DLL with known API. I did it earlier for ODBC  
(where ODBC driver manager was the third party DLL) and we created our own DLL calling the
 ODBC methods.
 
 

Comment
 
 From: angelany
 Date: Thursday, September 17 1998 - 10:26AM PDT  
 
msmolyak,

Thanks for your answer. There are still a few things that I
don't quite understand. Would you please elaborate a little more?

Suppose I have these files:
test.dll -- the core dll file
TestMain.java -- the test application in Java, it calls class and methods in "Rate.java"
Rate.java -- the Java file that contains definition of native methods
Rate.h  -- generated by "javah -jni Rate.java"
Rate.c  -- generated by "javah -stubs Rate.java"

Now, should I write a code called something like "RateImp.c"
to implement all the signatures in "Rate.h"? How are the Java
specific types handled? How does the "Rate.c" come into play?

Thanks.

Angelany
 
 

Comment
 
 From: msmolyak
 Date: Thursday, September 17 1998 - 10:33AM PDT  
 
First of all you do need stubs if you are using JNI, stubs were needed for the old native
 interface. Thus you just take Rate.h and create a C file (e.g., RateImp.c) where you will
 implement the methods according to their signatures. You do not need your Rate.c
 
 

Comment
 
 From: angelany
 Date: Friday, September 18 1998 - 09:48AM PDT  
 
Hi msmolyan,

I still have trouble passing data types to and from Java.
Could you please give an example on passing an object and
a string? Thanks a lot.

Angelany
 
 

Comment
 
 From: msmolyak
 Date: Friday, September 18 1998 - 10:41AM PDT  
 
Here is an example. The first two parameters (JNIEnv and jobject) are present in all teh calls. The first is the environment, the second is pointer to the invoker object. In this example I pass file name as a parameter and convert it from Java string to C string. Then I invoke a C function using that string.

Go to the JNI Spec and tutorial on Javasoft's Web site, a lot of useful info.

/**
 * This function opens specified configuration file for reading or writing.
 */
JNIEXPORT void JNICALL Java_com_excalib_rw_CfgObjectAdapter_openConfigFiles
  (JNIEnv * env, jobject obj, jstring jFileName)
{
char *cfgFileName;

// Convert Java string to C string
cfgFileName = (*env)->GetStringUTFChars(env, jFileName, (jboolean)NULL);

// Call RetrievalWare API function openning configuration file.
cq_open_conquest_cfg(cfgFileName, FILE_OPEN_FOR_READ);

}
 

btw. take care of little and big endian

Comment accepted as answer

Exactly. Would you please share me the java class for manipulating the c-struct problem? Thanks!

I will send you the class tomorrow. I dont have it over here. Only at the PC at work.

Alexander.

What is your email address?

Alexander.