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Q: Calling Java classes from C programs ( Answered 4 out of 5 stars,   0 Comments )
Subject: Calling Java classes from C programs
Category: Computers > Programming
Asked by: lk_sf-ga
List Price: $10.00
Posted: 19 May 2006 13:58 PDT
Expires: 18 Jun 2006 13:58 PDT
Question ID: 730502
If I have a C program that needs to call a method in a java class (and
receive back the results), how do I do it? I need some sample code.
Subject: Re: Calling Java classes from C programs
Answered By: maniac-ga on 19 May 2006 15:52 PDT
Rated:4 out of 5 stars
Hello Lk_sf,

There is a pretty comprehensive article describing this method and
source code available at
or equivalently at
which is an article titled "Calling Java from C" [published in two
magazines?]. The direct link to the source code (a zip file), batch
files to build / run the applications under Windows 2000, and sample
files to run is:
Note that the command files are for Windows, the article indicates
that the code should be platform independent (if that is important to

In the middle of the article is a good introduction into the issues
related to getting data back to your C program. This example uses a
string, but other methods (such as using JNI to call back the C code)
could be used as well.

There are several other good references - try searching with phrases such as
  calling "java from c"
  calling java from c
The first is more specific, the second will also bring up references
of Java calling C. In addition, a phrase like
  java jni
  calling jni
brings up general information related to the Java Native Interface.

Please make a clarification request if the answer is incomplete or unclear.

Good luck with your work.


Request for Answer Clarification by lk_sf-ga on 19 May 2006 19:27 PDT
Thank you,
So is it always calling the 'public static void main(String args[])' of the class?
What if my class is a library?
I was looking for a way to somehow instanciate an object and call a
public method, not through 'void main'.
Am I missing anything?

Clarification of Answer by maniac-ga on 20 May 2006 12:06 PDT
Hello Lk_sf,

The method described in the article which implements this by calling a
java "main program" is the simplest to implement. The steps that get
done "behind the scenes" include:
 - starting / connecting to the Java virtual machine
 - running the code within that environment
 - destroying all the objects and other required clean up steps

What you have asked for (creating a Java object and calling a public
method) is much more complicated and involves several pages of code.
Let me provide some references on that approach:

[1] Starting the Java VM is described at
as JNI_CreateJavaVM. This was done inside the code referenced in the
original answer but a simple version is also provided at

[2] Creating the java object (I assume you want constructors to run...)
is done using a call to NewObject (or one of the variants). See
for one of the simplest examples I could find for this (and the
related code). There is a more complicated example (in four parts)
starting at
Its this interface (and the next one) that has pretty meager references.

[3] Calling a method for that object is described at
and in the previous two code references. See also
which indicates that four JNI calls are required to make a method
call. There is some additional source code at
(doing matrix multiplication) that may be helpful. Try searching with phrases like
  callobjectmethod jni
  callobjectmethod jni source code
for more examples.

[4] At this point, you should clean up the Java environment. This is
also described at
as DestroyJavaVM. This is a pretty straight forward call - several of
the code files already referenced will call this.

Let me know if you need additional references to this (more complicated) method.

lk_sf-ga rated this answer:4 out of 5 stars

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