Calling C from Java - passing an int

Suppose we want to implement the method int toDouble(int) in C, instead of Java. The steps required are:

Declare a native Java method with the desired signature (line 5)
Add a static initialization block to the Java class (lines 6-8)
Use javac to produce a .class file
Use javah to produce a C header file (lines 16-34)
Copy the C function signature from the C header file into a C source file (line 39)
#include the generated C header file in the C source file (line 37)
Implement the body of the C function (lines 40-41)
Compile the C source file into a .dll
Use java to run the .class file (line 44)

Step 2 above is required because Java expects a dynamic link library to be available when the native method is invoked. This is set-up by calling System.loadLibrary() and passing it the name of a .dll file (line 7). We can ensure that this happens prior to the first use of the Java class that contains the native method by placing the call in a static initialization block.

Rather than generate the cryptic C function name and signature by hand, it is a lot easier to have Java do it. The command for step 4 is

   javah JNIdemos

If javah has a problem, make sure the CLASSPATH environment variable includes the currect directory. Notice that:

the name of the header file is the argument specified in the System.loadLibrary() call,
the name of the C function is mangled from the name of the Java native method, and
the argument list of the C function has had two extra parameters prepended

The command for step 8 is

   cl -LD JNIdemos.cpp -FeJNIdemos.dll /ML

Notice how the single int parameter to the native method (line 5) has been mapped to the third parameter of the C function (line 39). Its type is jint, which is defined in jni.h (included at line 18 via line 37). The return value is also of type jint.

The first two parameters defined for the C function are not required in this example.

[source: Horstmann98, pp581-588]