Calling a native library from Java

Question

Hello,
I'm trying to call a native method (windows dll) from a web service implemented in Java. I've confirmed that the class I've created to call the native method works when used outside of my web service (ie. in a standard Java application). However, when I try to use the class in the web service, an exception is thrown when
System.loadLibrary("MyLibrary");
is executed. The exception thrown is:
InvocationTargetException
JAXRPCSERVLET28: Missing port information
Does any one have any suggestions as to what might be causing this error?
Thanks

Answer 1

There are basically 3 steps to calling a native method from your Java code.
1. Create a C/C++ stub function that will translate between your Java call and the native C method.
2. Create the dll that exports the stub function.
3. Invoke the System.loadLibrary("myDllName") method.
Here's what I did to learn how to use the JNI.
I first created the class that would be calling the dll:
public class CallDll
/** Creates a new instance of CallDll */
public CallDll()
static
//LVtoJava is my dll name.
System.loadLibrary("LVtoJava");
//AddDll is the name of the function I'm exporting from my Dll
//It does not have to be static
public native static double AddDll(int func, double x, double y);
I then used the javah utility in the jdk/bin directory to create the C stub header file. Once you have the generated stub header file, you can create an implementation file, and compile it into a dll.
//My C++ stub, generated by javah utility
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class LabViewDll_CallDll */
#ifndef IncludedLabViewDll_CallDll
#define IncludedLabViewDll_CallDll
#ifdef __cplusplus
extern "C" {
#endif
* Class: LabViewDll_CallDll
* Method: AddDll
* Signature: (IDD)D
JNIEXPORT jdouble JNICALL Java_LabViewDll_CallDll_AddDll
(JNIEnv *, jclass, jint, jdouble, jdouble);
#ifdef __cplusplus
#endif
#endif
You may want to note the stub function's signature and how it decorates the function name that you created from your Java class. Do not modify it, as the format is required by the JNI. The javah utility appends the fully qualified package name and the word Java, separated by underscores, to your original function name. You do not need to change the name in your Java class.
All the other special key words in the function's signature are defined in the jni.h or the jni_md.h (if your using windows).
You may want to refer to the JNI documention on Sun's website. The book I learned out of is the Core Java Volume 2, published by Sun Microsystems Press. It goes through the details of invoking your first native function and I've found it to be a good reference.
Hope this helps,
PS. I seemed to have found the issue with calling my dll from a web service. My dll is actually calling another dll and that seems to be the source of my problems. When I removed the call to the 2nd dll, everything worked fine. So now I need to figure out why the 2nd dll call is an issue.
Any suggestions?