43.3.2 Multi Threaded Example

Following is a Java version of the train example.

This is a multi threaded version of the train example. In this mode several threads can access the SICStus runtime via a Prolog interface. The static method Jasper.newProlog() returns an object that implements a Prolog interface. A thread can make queries by calling the query-methods of the Prolog object. The calls will be sent to a separate server thread that does the actual call to SICStus runtime.

                        
// MultiSimple.java
     
     import se.sics.jasper.Jasper;
     import se.sics.jasper.Query;
     import se.sics.jasper.Prolog;
     import java.util.HashMap;
     
     public class MultiSimple
     {
         class Client extends Thread
         {
             Prolog jp;
             String qs;
     
             Client(Prolog p,String queryString)
             {
                 jp = p;
                 qs = queryString;
             }
             public void run()
             {
                 HashMap WayMap = new HashMap();
                 try {
                     synchronized(jp) {
                         Query query = jp.openPrologQuery(qs, WayMap);
                         try {
                             while (query.nextSolution()) {
                                 System.out.println(WayMap);
                             }
                         } finally {
                             query.close();
                         }
                     }
                 } catch ( Exception e ) {
                     e.printStackTrace();
                 }
             }
         }
         {
             try {
                 Prolog jp = Jasper.newProlog(argv,null,"train.sav");
     
                 Client c1 =
                     new Client(jp,"connected('Örebro', 'Hallsberg',
                                Way1, Way1).");
                 c1.start();
                 // The prolog variable names are different from above
                 // so we can tell which query gives what solution.
                 Client c2 =
                     new Client(jp,"connected('Stockholm', 'Hallsberg',
                                Way2, Way2).");
                 c2.start();
             }
             catch ( Exception e ) {
                 e.printStackTrace();
             }
         }
     
         public static void main(String argv[])
         {
             new MultiSimple(argv);
         }
     }

1. The Prolog object jp is the interface to SICStus. It implements the methods of interface Prolog, making it possible to write quite similar code for single threaded and multi threaded usage of Jasper. The static method Jasper.newProlog() returns such an object.
2. In this example, the Prolog code is loaded by the server thread just after creating the SICStus object (which is invisible to the user). The third argument to the method Jasper.newProlog is the .sav file to restore. Two threads are then started, which will make different queries with the connected predicate.
3. openPrologQuery is not recommended in multi threaded mode, but if you must use it from more than one Java thread, you should enclose the call to openPrologQuery and the closing of the query in a single synchronized block, synchronizing on the Prolog object. See SPTerm and Memory, for details on one of the reasons why this is necessary.