Mapping struct dataype in C to Java JNI

Question

Do anyone have idea how to map struct datatype in C to Java JNI?
I've got idea and examples of mapping primitive datatype and array in C to Java JNI but none for struct datatype in C?
Hope I can got an example of it! Thanks!!!

Answer 1

The first way - by the book
Read Sheng Liang's book (downloadable from ftp://ftp.javasoft.com/docs/specs/jni.pdf )
In brief - and explaining only the main steps - no detailed instructions - please read the book
You want to return an array of the following objects (they have only primitive data, for easing the explanation)
//-- C++
struct MyData {
    int firstField;
    char *secondField;
//-- Java
public class MyData {
    public int firstField;
    public String secondField;
}in the native Java method "doSomething" of the class MyProgram
public class MyProgram {
    static {
        System.loadLibrary ("myjni");
    public static native MyData[] doSomething() ;
}You will have to call JNI functions for creating the MyData[] array, for creating individual MyData objects, and for setting the individual fields of each MyData object. Very boring indeed.
The second way - the hard one
Instead of returning a Java array of Java objects, study the serialization format of Java, and return a Java array of bytes. Then use ByteArrayInputStream and ObjectInputStream, or DataInputStream, for deserializing the objects you do need to get in your program. It will be slower than creating directly the Java objects in your C++ program, but you can save a few lines of code in your C++ program. (You will maintain the program yourself, don't you?)
The third way - the lazy one
Instead of returning a Java array of Java objects, return a Java String (it could be a XML-formatted string, or a string using delimiters - what you think that is easier to encode in C++ and decode in Java.)
The third way - the better way
Use the SWIG framework ( http://www.swig.org ). Your code will be very maintenable, you will not have to deal with JNI issues (there are lots and lots of JNI details you will have to bother with, solve all that problems with SWIG). Take care with spelling - SWIG is not SWING.