Working With Messages

Many different types of fields can be appended to a message’s data. These field types include three sizes of integers, two sizes of real numbers, character strings, and arrays of the scalar field types (such as an array of four-byte integers). These field types are listed in the online documentation for the TipcMsg class and can be recognized by their FT_ prefix. Messages themselves can even be used as fields within other container messages; this allows operations such as large transactions to be represented with a single message. Once a message is constructed, it can be published to other RTclients using the TipcSrv.send method.

To get a better feel for working with the SmartSockets Java API for building and sending messages, you will enhance your sending program from the previous lesson to send a NUMERIC_DATA message. The data part of a NUMERIC_DATA message consists of one or more variable-value pairs, where a variable is a text string and a value is a floating point number.

%RTHOME%\java\tutorial\lesson3

$RTHOME/java/tutorial/lesson3

Step 1

//---------------------------------------------------------- 
// send2.java -- write a NUMERIC_DATA message 
 
1 import java.io.*; 
2 import com.smartsockets.*; 
 
3 public class send2 { 
 
4   public static void main(String[] argv) { 
      try { 
5       Tut.setOption("ss.project", "smartsockets"); 
 
6       TipcSrv srv=TipcSvc.getSrv(); 
7       if (!srv.create()) { 
8         Tut.exitFailure("Couldn't connect to RTserver!"); 
        } 
        // create a message of type NUMERIC_DATA 
9       TipcMsg msg = TipcSvc.createMsg(TipcMt.NUMERIC_DATA); 
 
        // set the destination subject of the message 
10      msg.setDest("/ss/tutorial/lesson3"); 
 
        // build a NUMERIC_DATA msg with 3 variable-value pairs, 
        // set as follows (X, 10.0), (Y, 20.0) and (Z, 30.0)  
11      msg.appendStr("X"); 
12      msg.appendReal8(10.0); 
13      msg.appendStr("Y"); 
14      msg.appendReal8(20.0); 
15      msg.appendStr("Z"); 
16      msg.appendReal8(30.0); 
 
        // send the message 
17      srv.send(msg); 
18      srv.flush(); 
 
   // disconnect from RTserver 
19      srv.destroy(); 
      } catch (TipcException e) { 
20      Tut.warning(e); 
      } // catch 
    } // main 
  } // send2

Step 2

You now need to modify your receiving program so that it can read and print the contents of the NUMERIC_DATA message you are sending.

Step 3

Line 9	Now creates a message of type NUMERIC_DATA instead of type INFO.
Lines 11-16	The call to TipcMsg.appendStr is replaced with multiple calls to TipcMsg.appendStr and TipcMsg.appendReal8 to build the data part of the message.

Copy the file receive2.java into your working directory. This is an example of the receive2.java program:

//---------------------------------------------------------------- 
// receive2.java -- receive a NUMERIC_DATA message 
 
1 import java.io.*; 
2 import com.smartsockets.*; 
 
3 public class receive2 { 
 
4   public static void main(String[] argv) { 
5     TipcMsg msg = null; 
6     TipcSrv srv = null; 
 
      // set the ss.project 
      try { 
7       Tut.setOption("ss.project", "smartsockets"); 
8       srv=TipcSvc.getSrv(); 
 
        // connect to RTserver 
9       if (!srv.create()) { 
10        Tut.exitFailure("Couldn't connect to RTserver!"); 
        } // if 
        // subscribe to the appropriate subject 
11      srv.setSubjectSubscribe("/ss/tutorial/lesson3", true); 
12      msg = srv.next(TipcDefs.TIMEOUT_FOREVER); 
13    }  
        catch (TipcException e) { 
14      Tut.fatal(e); 
      } // catch 
 
      // position the field ptr to the beginning of the message 
      try { 
15      msg.setCurrent(0); 
16   } catch (TipcException e) { 
17     Tut.fatal(e); 
     } // catch 
 
18   System.out.println("Contents of NUMERIC_DATA message:"); 
 
     // read the data part of the message 
     try { 
19     String var_name; 
20     while (true) { 
21       var_name = msg.nextStr(); 
22       double var_value; 
23       var_value = msg.nextReal8(); 
24       System.out.println("Var name = " + var_name +  
                 ", value = " + var_value); 
        } // while 
      } 
        // catch end-of-message-data exception, do nothing. 
25      catch (TipcException e) {  
          // we expect this exception from the while loop 
        } // catch 
 
        // drop our connection to RTserver 
        try { 
26        srv.destroy(); 
27      } catch (TipcException e) { 
          // not concerned with problems dropping connection 
        } // catch 
      } // main 
 
    } // receive2

Let’s look at how this program extracts information from the data part of the message:


Lines 7-14	Set the project to "`smartsockets`" and connect to RTserver.
Line 20	Beginning of loop over fields in the data part of the message.
Lines 21-23	The TipcMsg.nextStr method retrieves the variable name and the TipcMsg.nextReal8 method retrieves the variable value. When TipcMsg.nextStr throws a NoSuchFieldException, there are no more fields to access in the message, and the enclosing while loop is exited. This is an expensive way to end the loop (throwing exceptions takes a lot of time), and you can also use the TipcMsg.getNumFields method to retrieve the field count and loop with a counter instead. For more details, see the descriptions of the TipcMsg.next* methods in the online Java documentation.

Step 4

$ javac receive2.java

Step 5

To demonstrate that everything is still working, start the receiving and sending programs in separate windows as you did earlier. First start the receive2 program using the command: