Writing a simple JNI program on OS/390

Table of Contents

• Step 1: Writing the class
• Step 2: Generating header and stub files
• Step 3: Creating the native method program
• Step 4: Compiling the native method program
• Step 5: Linking it into a dll
• Step 6: Running the class
• Trouble shooting
• Considerations for OS/390

Step 1: Writing the class

Write your java program that will call a native method. For example, HelloWorld.java:

    class HelloWorld {

        public native void displayHelloWorld();

        static {
            System.loadLibrary("HelloWorldNative");
            }

        public static void main(String[] args) {
            System.out.println("Class running");
            new HelloWorld().displayHelloWorld();
        }
    }

Compile the class:

    javac HelloWorld.java

Output:

    HelloWorld.class

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Step 2: Generating header and stub files

To generate the header file needed for the native method, run javah against the class file:

    javah -jni HelloWorld

Output:

    HelloWorld.h

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Step 3: Creating the native method program

#include "HelloWorld.h"

JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject 
 obj)
{
  printf("Hello world!\n");
  return;
}

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Step 4: Compiling the native method program

Compile the native method program HelloWorldNative.c as a dll, and specifying the directories to search for the java header files. For example:

    c89 -c -o HelloWorldNative.o -W c,expo,dll
-DNEEDSIEEE754 -DNEEDSLONGLONG
-I$JAVA_HOME/J1.1/include -I$JAVA_HOME/J1.1/include/mvs
HelloWorldNative.c

For an explanation of -DNEEDSIEEE754 and -DNEEDSLONGLONG see jni_oe.html shipped in $JAVA_HOME/J1.1/jni_example, or JNI Programming.

Output:

  HelloWorldNative.o

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Step 5: Linking it into a dll

Link the object files into a dll. For example:

c89 -W l,dll -o libHelloWorldNative.so ...
...HelloWorldNative.o

   libHelloWorldNative.so

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Step 6: Running the class

Set the LIBPATH environment variable to point to the directory containing your dll libHelloWorldNative.so.

Run the java program:

  java HelloWorld

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Trouble shooting

• Could not load dll. EDC5207S Load request for DLL load module unsuccessful.
  • Ensure the dll has the execute permission bit set.
  • Ensure the program has been compiled as a dll.
• java.lang.UnsatisfiedLinkError: no xyz in shared library path
  • Ensure the dll is named correctly:

    System.loadLibrary("HelloWorldNative");
    

    will load a dll called libHelloWorldNative .so
  • Ensure that the dll can be found, ie. that it is in a path pointed to by the LIBPATH environment variable.

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Considerations for OS/390

For an explanation of -DNEEDSIEEE754 and -DNEEDSLONGLONG see jni_oe.html shipped in $JAVA_HOME/J1.1/jni_example, or JNI Programming.

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