Example: NativeHello.java

The NativeHello.java source code shows the Java code that is used to demonstrate ILE native methods for Java. The native method setTheString() is called by the Java code from within the main() method. The result of the call is that the function Java_NativeHello_setTheString(), defined in the C implementation code, gets control and uses Java Native Interface (JNI) to call back into the Java code to set the value of the Java string variable theString. Control is then returned to the Java code, and the string variable is written to stdout.

public class NativeHello {
 
  //  Declare a field of type 'String' in the NativeHello object.
  //  This is an 'instance' field, so every NativeHello object 
  //  contains one.
  public String theString;           // instance variable
 
  //  Declare the native method itself.  This native method 
  //  creates a new string object, and places a reference to it 
  //  into 'theString'
  public native void setTheString(); // native method to set string
 
  //  This 'static initializer' code is called before the class is
  //  first used.
  static {
     //  Attempt to load the native method library.  If you do not
     //  find it, write a message to 'out', and try a hardcoded path.
     //  If that fails, then exit.
     try {
        //  System.loadLibrary uses the java.library.path property or 
        //  the LIBPATH environment variable.
        System.loadLibrary("NATHELLO");
     }
     catch (UnsatisfiedLinkError e1) {
        //  Did not find the service program.
        System.out.println("I did not find NATHELLO *SRVPGM.");
        System.out.println("(I will try a hardcoded path)");
 
        try {
           //  System.load takes the full integrated file system form path.
           System.load ("/qsys.lib/ileexample.lib/nathello.srvpgm");
        }
        catch (UnsatisfiedLinkError e2) {
           //  If you get to this point, then you are done!  Write the message
           //  and exit.
           System.out.println
              ("<sigh> I did not find NATHELLO *SRVPGM anywhere. Goodbye");
           System.exit(1);
        }
     }
  }
 
  //  Here is the 'main' code of this class. This is what runs when you
  //  enter 'java NativeHello' on the command line.
  public static void main(String argv[]){
    
     //  Allocate a new NativeHello object now.
     NativeHello nh = new NativeHello();
 
     //  Echo location.
     System.out.println("(Java) Instantiated NativeHello object");
     System.out.println("(Java) string field is '" + nh.theString + "'");
     System.out.println("(Java) Calling native method to set the string");
 
     //  Here is the call to the native method.
     nh.setTheString();
 
     //  Now, print the value after the call to double check.
     System.out.println("(Java) Returned from the native method");
     System.out.println("(Java) string field is '" + nh.theString + "'");
     System.out.println("(Java) All done...");   
  }
}