Location of JavaGDBM.java is:
/opt/java/jdk1.5.0_01/jre/lib/ext/JavaGDBM.jar


The following is some sample JDBM code (not mine):

import java.io.PrintStream;

import java.util.Enumeration;

import au.com.pharos.test.Test;
import au.com.pharos.packing.Packing;
import au.com.pharos.packing.StringPacking;
import au.com.pharos.packing.RawPacking;

import au.com.pharos.gdbm.*;


public class GdbmFoo {

private GdbmFoo () {

       String a = System.getProperty ("java.library.path");
       System.out.println ("path = " + a );

       System.setProperty ("java.library.path", a + ":/opt/lib");

       a = System.getProperty ("java.library.path");
       System.out.println ("path = " + a );

       System.loadLibrary ("gdbmjava");


 }

public static void main (String [] argv) {


	try {
	    final String dbFile = "/tmp/java.gdbm";
	    String key, value;
	    boolean found;


	    // Test cases, inspired by db-hash.t in the Perl5.003_93
	    // distribution
	    System.out.print("GDBM Database tests:\n\t" +
			     GdbmFile.getLibraryVersion() + "\n\t" +
			     GdbmFile.getWrapperVersion() + "\n");


	    // Create a simple database
	    GdbmFile db = new GdbmFile(dbFile, GdbmFile.NEWDB);
	    Test.ok(1, db != null);

	    String s = System.getProperty ("au.com.pharos.gdbm.libraryFile");
	    System.out.println ("libfile = " + s + "\n");

	    s = System.getProperty ("java.library.path");
	    System.out.println ("libpath = " + s );

	} catch (Exception e) {
	    e.printStackTrace();
	}



}

}