Using C++ Library in Java

Question

Hi,
How do I use a prewritten C++ API in Java? With C++ I have to use the include files that came with the DLL. I have no access to the code. I'd appreciate reasonably extensible code examples.
Thanks.

Answer 1

You can use JNI(Java Native Interface) to solve this problem.
I give an example below, you can do advanced functions in depth.
First, write a simle class named Test, it contains one method called getSquare which pass a int argument only. Of course It contains the program entry.
public class Test
public static void main(String args[])
//import dll and print returned result
try{
System.loadLibrary("Test.dll"); //we will name the compiled program as "Test.dll"
}catch(Exception e){}
System.out.println(Test.getSquare(5));
public static native int getSquare(int num);
As you can see, It is very easy. You need native method to access functions which wrote in C or C++. The native method like abstract do not have method body, it has signature only, we will write method body in other programming language such as C or C++.
Next step, likes we did normal, compile it(javac Test.java), then we will use JNI compiler tool which in Java bin directory compile the Test class(javah Test), note never specify the suffix ".class" or ".java". After the command executed, you will find Test.h in the same directory. That is, it is the header file of C or C++, it is the code skeleton, next step we will write function body.
You may complain the function's signature is too long to rember, don't worry, it is needless, just copy it to your C file or C++ file, then imply it. here the fountion returns the square of the argument.
JNICALL jint methodSignature(JNIEnv *env,jint arg)
return arg*arg;
Excuse me, I forget the functionsignature too, but there is a rule with it, so replace it in any signature(when you coding, never change the signature, it was created by compiler automatically).
At this step(imply the function), you can do as you like include invoke dlls, execute applications even thougn win32 founctions. Note, there are too many data types(jint, jshort,jstring), functions and structs and pointers defined in JNI environment, you must know the change C or C++ type to java types. If you know much about C or C++ and Java, you can read jni.h file which in Java include directory, it is important to read books or articles about JNI.
compile the code, name it with "Test.dll"
Execute your java class, what did you find?