I'm looking for JNI real world examples

[Frank Hale]

I've been looking around the net for quite a while on real world
JNI examples. I am really getting tired of all the Hello,World examples that I keep finding. What I am looking for is how to get at C structures from Java code. I would like to build a JNI wrapper around an existing C Library but I cannot find enough good information beyond Hello,World to get me started. Does anyone have any good links to some tutorials which go beyond Hello,World using Java's Native Interface?
I am using JDK 1.3.
Thanks,
Frank

[Harish Kashyap]

There is a very good example of jni that we use in jdbc applications. that is the use of jdbc-odbc bridge.
the jdbc accesses the odbc methods or the resources through the jni(native methods).
If u could explore these native methods u'll explore the real application use of jni.
the jdbc drivers and classes are available with jdk. use reflection to explore it.

[Frank Hale]

Do you have the C code for the JDBC-ODBC bridge? The part which implements the native calls?
I wanna know how I map C structures to Java objects.
Frank

[Amit Zzz Kulkarni]

Frank,
What exactly do you mean by this ? I assume that you want to pass the structures to Java using JNI ,if so store your C structs in some memory allocated with malloc() or if using C++ an object allocated by new. Then cast the pointer to this (C or C++) object to a jint and return the jint to your java code to be used as a native structure handle.

Then when ever the java code calls back to your C or C++ code pass the jint handle to the C code and cast it back to the C object and use it.
If you are running on a system with 64 bit pointers (not likely) then use a jlong instead of a jint. Or just use a jlong to start with so that you can upgrade easily to 64 bits in the future.

Originally posted by Frank Hale:
What I am looking for is how to get at C structures from Java code. I would like to build a JNI wrapper around an existing C Library

[Amit Zzz Kulkarni]

Or you can convert ALL your structs to Java classes and then map them,member by member.Please ask a specific question in future so somebody may help you out faster.

[Frank Hale]

Next time I will be more clear.
Okay what I am doing is trying to create a binding to a C lib (Xlib - X Windows System). This is a huge library and very complex. I've figured out how to cast some pointers to jint and pass them back to java as a handle. I've created a few native wrapper functions around Xlib functions which then cast the jint back to the original type, struct, or whatever. I am having a problem with XEvent as its a union and when I try to cast it, it will not fit inside a jint or a jlong. If I try to pass a reference and then cast that back I crash the VM everytime.
I've probably went into to much detail. Can anyone help? I can post code if you like.
Another question if I was to create java objects for C structs how would I go about passing the data to the java object?
Frank

[Carl Trusiak]

Originally posted by Frank Hale:
Next time I will be more clear.
Okay what I am doing is trying to create a binding to a C lib (Xlib - X Windows System). This is a huge library and very complex. I've figured out how to cast some pointers to jint and pass them back to java as a handle. I've created a few native wrapper functions around Xlib functions which then cast the jint back to the original type, struct, or whatever. I am having a problem with XEvent as its a union and when I try to cast it, it will not fit inside a jint or a jlong. If I try to pass a reference and then cast that back I crash the VM everytime.
I've probably went into to much detail. Can anyone help? I can post code if you like.
Another question if I was to create java objects for C structs how would I go about passing the data to the java object?
Frank

I haven't tried it but, one solution I heard of is to create a new C DLL that converts to types that do map to java so esentually your java program calls a DLL which calls the DLL that you need. In the case of a Union, send it back as seperate elements. (Which is basically what a union is anyway, seperate types pointing to the same location in memory)

[This message has been edited by Carl Trusiak (edited July 12, 2000).]

[Frank Hale]

I haven't tried it but, one solution I heard of is to create a new C DLL that converts to types that do map to java so esentually your java program calls a DLL which calls the DLL that you need. In the case of a Union, send it back as seperate elements. (Which is basically what a union is anyway, seperate types pointing to the same location in memory)

small problem with this Union

/*
 * this union is defined so Xlib can always use the same sized
 * event structure internally, to avoid memory fragmentation.
 */
typedef union _XEvent {
        int type;/* must not be changed; first element */
XAnyEvent xany;
XKeyEvent xkey;
XButtonEvent xbutton;
XMotionEvent xmotion;
XCrossingEvent xcrossing;
XFocusChangeEvent xfocus;
XExposeEvent xexpose;
XGraphicsExposeEvent xgraphicsexpose;
XNoExposeEvent xnoexpose;
XVisibilityEvent xvisibility;
XCreateWindowEvent xcreatewindow;
XDestroyWindowEvent xdestroywindow;
XUnmapEvent xunmap;
XMapEvent xmap;
XMapRequestEvent xmaprequest;
XReparentEvent xreparent;
XConfigureEvent xconfigure;
XGravityEvent xgravity;
XResizeRequestEvent xresizerequest;
XConfigureRequestEvent xconfigurerequest;
XCirculateEvent xcirculate;
XCirculateRequestEvent xcirculaterequest;
XPropertyEvent xproperty;
XSelectionClearEvent xselectionclear;
XSelectionRequestEvent xselectionrequest;
XSelectionEvent xselection;
XColormapEvent xcolormap;
XClientMessageEvent xclient;
XMappingEvent xmapping;
XErrorEvent xerror;
XKeymapEvent xkeymap;
long pad[24];
} XEvent;

That is the way its defined in Xlib.h and now you typically do this:
XEvent ev;
And then pass ev to functions like so:
XNextEvent(display, window, &ev);
The problem is how do I know what value to pass back to java and when?
This is becoming a very difficult issue because nowhere have I found information on JNI interfacing C code with Java which talks about what to do in these situations.
Frank

[Frank Hale]

Okay I figured out how to pass the XEvent pointer over to java as a handle and now I can perform event loops in Java instead of in pure C. If anyone wants to see the magic I performed let me know.
Thanks,
Frank

[Amit Zzz Kulkarni]

Frank,
This site is no good in sending notifications that this question was updated. I am glad that you solved it and am interested in how you got it ?
Regards
Amit

[Frank Hale]

Well I allocated memory for the C struct using calloc then passed the value of the pointer back to Java and stuff it in an int. Then when I need to make other native calls from java I just pass back this value (which I call a handle) and then cast it to its type in the C native method. I use the finalize() method in java to free the allocated memory.
Frank

[chickmath vidyanand]

Could you please send me the whole stuffs like C source code and Java code.
Actually I am also in need of the same requirements like passing structures (data) from C++ to JAVA.
Please send me some simple samples so that I can understand easily.
It should have C++ program having structure defined(should contain data). Then this data should be passed to JAVA and printed in JAVA program.
Can anyone knows how to do this, please help me....
Urgent requirement...
rgds,
Vidyanand

[Jesper de Jong]

cagri sener,
Your post was moved to a new topic.

[Jesus Angeles]

Amit Zzz Kulkarni wrote:Frank,
This site is no good in sending notifications that this question was updated.

You can set things in your Profile, or click ~'Watch Topic' at the bottom.

[Campbell Ritchie]

Jesus Angeles wrote:

Amit Zzz Kulkarni wrote:Frank,
This site is no good in sending notifications that this question was updated.

You can set things in your Profile, or click ~'Watch Topic' at the bottom.

If he has waited 11 years for a notification of a reply, he is a very patient person

[Ulf Dittmer]

You can set things in your Profile, or click ~'Watch Topic' at the bottom.

That works now, but neither option existed when Amit was around.

[Jesus Angeles]

My bad. I didnt notice that it was posted in 2000.

[Campbell Ritchie]

You can tell from his middle name Zzz which proves he has slept nearly as long as Rip van Winkle.