Can't understand something of JNI

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cold80 - 23 Jul 2007 16:43 GMT

Today I've tried my first application with JNI. I made a simple Java
class with a native method, I generated the C stub with javah and
finally I wrote the implementation of the C function. Everythink works
just fine. In the implementation C file I had of course to include the
file jni.h. What I can't understand is where the JNI code is
written....I mean, where is the code that I can use to convert types
(like GetStringLength for example)? I could not understand very well
the content of the file jni.h but it doesn't seem to contain the real
code, just typedefs and structure definitions. I didn't link any
library into my VS2005 project, so I can't understand where the code
for the JNI functions is located.

Thank you in advance for your help

Cold

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paul.h.burns - 23 Jul 2007 17:47 GMT

I'm learning JNI myself, though using JNIWrapper from Teamdev. From my
nascent understanding of the situation, I believe what you're looking
for (functions such as GetStringLength) are JNI functions already
implemented in the JDK.

Hope that helps,
Paul

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Roedy Green - 24 Jul 2007 03:05 GMT

On Mon, 23 Jul 2007 09:47:39 -0700, "paul.h.burns"
<paul.h.burns@gmail.com> wrote, quoted or indirectly quoted someone
who said :

>I'm learning JNI myself, though using JNIWrapper from Teamdev. From my
>nascent understanding of the situation, I believe what you're looking
>for (functions such as GetStringLength) are JNI functions already
>implemented in the JDK.

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Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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Thomas Fritsch - 23 Jul 2007 17:54 GMT

> Today I've tried my first application with JNI. I made a simple Java
> class with a native method, I generated the C stub with javah and
[quoted text clipped - 5 lines]
> the content of the file jni.h but it doesn't seem to contain the real
> code, just typedefs and structure definitions.

Correct!
Note that the structure definitions contain mostly function pointers.

> I didn't link any library into my VS2005 project,

When Java calls your C function you get the "env" pointer.
From that "env" you get a pointer to the JNI function (pointing back
somewhere into Sun's C libraries):
(*env)->GetStringLength
and then you call the function lying at the pointed place:
(*env)->GetStringLength(...);
Note that "GetStringLength" is just a name inside a struct (handled by
the compiler), not an external function name (like "printf", handled by
the linker). Hence there is no need for any library to get your DLL linked.

> so I can't understand where the code
> for the JNI functions is located.

To satisfy your curiosity: The JNI functions are in one of the native
libraries coming with the JRE (on Windows: in "java.dll")

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Thomas

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Thomas Fritsch - 23 Jul 2007 23:08 GMT

[...]

> To satisfy your curiosity: The JNI functions are in one of the native
> libraries coming with the JRE (on Windows: in "java.dll")

... or in "jvm.dll"
(as the MSVC-debugger told me when stepping into the disassembled code)

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Thomas

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Roedy Green - 24 Jul 2007 02:58 GMT

>Today I've tried my first application with JNI. I made a simple Java
>class with a native method, I generated the C stub with javah and
>finally I wrote the implementation of the C function. Everythink works
>just fine. In the implementation C file I had of course to include the
>file jni.h. What I can't understand is where the JNI code is
>written

see http://mindprod.com/jgloss/jni.html

Also try downloading some of my code that uses JNI and see how it fits
together. e.g.

http://mindprod.com/products1.hml#FILETIMES

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Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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Can't understand something of JNI

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