Java Tech - Chapter 23 : Outside the JVM

In the previous page we discussed classes CommPort and CommPortIdentifier, which provide information and access for communications ports. Here we look at how to use those classes.

The CommPortIdentifier class javax.comm.acts as both a source of information about ports on a system and also as a descriptor of an individual port. The static method getPortIdentifers() in the class provides a list of CommPortIdentifier objects for each port, serial and parallel, on the platform.

This enumeration lists the instances of CommPortIdentifier, one for each port. The methods of this class provide information about the particular port such as its name and type via getName() and getPortType(), respectively.

 
                  PortList
                

import javax.comm.*;
import java.util.*;

/** List all the ports available on the local machine. **/
public class PortList {
public static void main (String [] args) {

    Enumeration port_list = CommPortIdentifier.getPortIdentifiers ();

    while (port_list.hasMoreElements()) {
      CommPortIdentifier port_id =
        (CommPortIdentifier)portList.nextElement ();

      if (portId.getPortType () == CommPortIdentifier.PORT_SERIAL) {
          System.out.println ("Serial port: " + port_id.getName());
      }
      else if (port_id.getPortType () ==
CommPortIdentifier.PORT_PARALLEL) {
       System.out.println ("Parallel port: " + port_id.getName ());
      }
      else
        System.out.println ("Other port: " + port_id.getName ());
    }
} // main
} // class PortList

We used the constants PORT_SERIAL and PORT_PARALLEL from the CommPortIdentifier class to test for the port type. On a desktop machine with four serial ports and two parallel ports, the output would go as

Conversely, you can test for the presence of a port with a particular name as follows.

PortTest

 
                  import 
                    javax.comm.*;

                    import java.util.*;

                    /** Look for COM# ports on the local machine. **/ 

                    public class PortTest

                    {

                      public static void main (String 
                    [] args) {

                        String port_name;

                        int i = 0; 

                        while (true){

                          i++;

                          port_name 
                    = "COM" + i;

                          try {

                    CommPortIdentifier port_id = 

                    CommPortIdentifier.getPortIdentifier (portName);

                    System.out.println ("Port " + port_name + " exists");      
                      }

                            catch (NoSuchPortException 
                      e) {

                              System.out.println 
                      ("No port " + port_name);

                              break;

                            }

                          }

                        } // main

                      } // class PortTest

The following methods in CommPortIdentifier provide information about the status of a port:

Unfortunately, these methods only work properly if the port is owned by a Java application and not by a different type of program.

If a port is available you can take exclusive possession of it via one of the two overloaded open() methods and then use the port for reading and writing to the external device connected to the port.

The CommPortIdentifier method

CommPort open (String ownerName, int timeout)
throws PortInUseException

takes possession of the port and passes it the name of the owning application. The timeout parameter will determine how long in milliseconds the method will block (that is, not return from the method) while waiting for the port to become available.

For systems such as Unix where ports can be assigned a FileDescriptor, this overloaded open method is provided:

CommPort open (java.io.FileDescriptor fd)
throws UnsupportedCommOperationException

Below we show an example where ports are opened. If they are owned by some other application, a PortInUseException is caught. If the owner is a Java program, it can be identified by the name given in the open method, otherwise there is no name available.

PortListOpen

import javax.comm.*;
import java.util.*;

/** Check each port to see if it is open. **/
public class PortListOpen {

  public static void main (String [] args) {

    Enumeration port_list = CommPortIdentifier.getPortIdentifiers ();

    while (port_list.hasMoreElements ()) {
      // Get the list of ports
      CommPortIdentifier port_id =
        (CommPortIdentifier) port_list.nextElement ();

      // Find each ports type and name
      if (port_id.getPortType () == CommPortIdentifier.PORT_SERIAL) {
          System.out.println ("Serial port: " + port_id.getName ());

      } else if (port_id.getPortType () ==
                 CommPortIdentifier.PORT_PARALLEL) {
          System.out.println ("Parallel port: " + port_id.getName ());

      } else
          System.out.println ("Other port: " + port_id.getName ());

      // Attempt to open it
      try {
        CommPort port = port_id.open ("PortListOpen",20);
        System.out.println ("  Opened successfully");
        port.close ();

      }
      catch  (PortInUseException pe){
          System.out.println ("  Open failed");
          String owner_name = port_id.getCurrentOwner ();
          if (owner_name == null)
              System.out.println ("  Port Owned by unidentified app");
          else
              // The owner name not returned correctly unless it is
              // a Java program.
              System.out.println ("  " + owner_name);
      }
    }
  } //main
} // PortListOpen

Output from this application might go as

To monitor a port for changes in its ownership, you can implement the CommPortOwnershipListener interface. You must override the method ownerShipChange(int typeChange), which can detect three types of ownership (ownership attained, port now available, ownership requested) changes.

Port Communications Procedure

The steps to communicate over a port go as follows:

Invoke open() in the port's CommPortIdentifier instance to obtain a CommPort object for that port.
Invoke the getInputStream() and getOutputStream() in the CommPort object to obtain provide the streams for reading and writing data from and to the port.
Carry out the desired I/O with the streams.
Invoke close() with the CommPort object, which will release the port for other applications to use.

Other CommPort methods set parameters for the port communications such as the input/output buffer sizes and how long the read operation will wait for data before returning.