Invoking Fortran functions from Java

// Assumption: I assume that you have already installed Java SDK, gfortran, g++ on your machine and you are using Linux.

1. Open a Terminal

2. Create a directory named src/com/sangeethlabs/jni/ to create the source files and bin to keep the compiled code and shared objects.

   mkdir -p src/com/sangeethlabs/jni
   mkdir bin
   

3. Open your favourite text editor, create a new file and write the following code

   package com.sangeethlabs.jni;
   
   public class Fortran {
   	static {
   		System.loadLibrary("FortranJNI");
   	}
   	
   	public synchronized native static int sum(int a, int b);
   	
   	public static void main(String[] args) {
   		int a = 10, b = 30;
   		int result = Fortran.sum(a, b);
   		System.out.printf("%s + %s = %s\n", a, b, result);
   	}	
   }
   

   Save the file as Fortran.java under src/com/sangeethlabs/jni/ directory.
   // NOTE: Ensure that the class name and the file name are same.

4. Create a new file and write the following Fortran code

         INTEGER FUNCTION FORTRANSUM(I,J)
         FORTRANSUM = I+J
         RETURN
         END
   

   Save the file as fortran.f under src/ directory.

5. Go back to your Terminal

6. Run the following command to compile the Java source code:

   javac -d bin src/com/sangeethlabs/jni/Fortran.java
   

   This generates a class file named Fortran.class under the directory bin/com/sangeethlabs/jni/.

7. Run the following command to compile the Fortran source code:

   gfortran -c -o "bin/fortran.o" src/fortran.f
   

   This generates the object file fortran.o under bin/ directory.

8. Now run the following command to generate header file for the Java Native Interface code.
   

   javah -d src -classpath bin com.sangeethlabs.jni.Fortran

   This results in a C/C++ header file named com_sangeethlabs_jni_Fortran.h under the directory src/ with the following contents

   /* DO NOT EDIT THIS FILE - it is machine generated */
   #include <jni.h>
   /* Header for class com_sangeethlabs_jni_Fortran */
   
   #ifndef _Included_com_sangeethlabs_jni_Fortran
   #define _Included_com_sangeethlabs_jni_Fortran
   #ifdef __cplusplus
   extern "C" {
   #endif
   /*
    * Class:     com_sangeethlabs_jni_Fortran
    * Method:    sum
    * Signature: (II)I
    */
   JNIEXPORT jint JNICALL Java_com_sangeethlabs_jni_Fortran_sum
     (JNIEnv *, jclass, jint, jint);
   
   #ifdef __cplusplus
   }
   #endif
   #endif
   

9. Open your favourite text editor, create a new file and write the following C++ code

   #include "com_sangeethlabs_jni_Fortran.h"
   
   #include <jni.h>
   
   extern"C" {
   	int fortransum_(int *, int *);
   }
   
   /*
    * Class:     com_sangeethlabs_jni_Fortran
    * Method:    sum
    * Signature: (II)I
    */
   JNIEXPORT jint JNICALL Java_com_sangeethlabs_jni_Fortran_sum(JNIEnv *env, 
                                                                jclass kclass, 
                                                                jint a, 
                                                                jint b)
   {
   	return fortransum_(&a,&b);
   }
   

   Save the file as com_sangeethlabs_jni_Fortran.cpp under src/ directory.

10. Run the following command to compile the C++ source code

    g++ -I$JAVA_HOME/include -I$JAVA_HOME/include/linux -fPIC -c -o "bin/com_sangeethlabs_jni_Fortran.o" src/com_sangeethlabs_jni_Fortran.cpp
    

    This generates an object file com_sangeethlabs_jni_Fortran.o under the directory bin/.

    The compiler option -fPIC is required to create a shared library with the object file com_sangeethlabs_jni_Fortran.o.

11. Now create a shared library named libFortranJNI.so with com_sangeethlabs_jni_Fortran.o and fortran.o under the directory bin/ using the following command

    g++ -shared -o "bin/libFortranJNI.so" bin/fortran.o bin/com_sangeethlabs_jni_Fortran.o
    

12. Now run the following command to execute the Java class.

    java -cp bin -Djava.library.path=bin com.sangeethlabs.jni.Fortran
    

    The output of the program will be as shown below,

    10 + 30 = 40
    

    // NOTE: “-Djava.library.path=.” option indicates to JVM that “libFortranJNI.so” can be located under the current directory. Another alternative is to set the environment variable LD_LIBRARY_PATH to include the path under which “libFortranJNI.so” can be located. By doing so, we can execute the Java code as shown below

    export LD_LIBRARY_PATH=bin:$LD_LIBRARY_PATH
    java -cp bin com.sangeethlabs.jni.Fortran
    

Now you have successfully invoked a Fortran function from a Java program using Java Native Interface !