Suppose you need to send a command to a device and get a response
back, and then send another, and get another. This has been called a
“lock-step” protocol, since both ends of the
communication are locked into step with one another, like soldiers on
parade. There is no requirement that both ends be able to write at
the same time (see Recipes 10.7 and 10.8 for this), since you know
what the response to your command should be and don’t proceed
until you have received that response. A well-known example is using
a standard Hayes-command-set
modem to just dial a phone number.
In its simplest form, you send the command string ATZ and expect the
response OK, then send ATD with the number, and expect CONNECT. To
implement this, we first subclass from
CommPortOpen to add two functions,
send and expect, which perform
reasonably obvious functions for dealing with
such devices. See Example 11-5.
Example 11-5. CommPortModem.java
import java.awt.*;
import java.io.*;
import javax.comm.*;
import java.util.*;
/**
* Subclasses CommPortOpen and adds send/expect handling for dealing
* with Hayes-type modems.
*
*/
public class CommPortModem extends CommPortOpen {
/** The last line read from the serial port. */
protected String response;
/** A flag to control debugging output. */
protected boolean debug = true;
public CommPortModem(Frame f)
throws IOException, NoSuchPortException,PortInUseException,
UnsupportedCommOperationException {
super(f);
}
/** Send a line to a PC-style modem. Send
, regardless of
* what platform we're on, instead of using println( ).
*/
protected void send(String s) throws IOException {
if (debug) {
System.out.print(">>> ");
System.out.print(s);
System.out.println( );
}
os.print(s);
os.print("
");
// Expect the modem to echo the command.
if (!expect(s)) {
System.err.println("WARNING: Modem did not echo command.");
}
// The modem sends an extra blank line by way of a prompt.
// Here we read and discard it.
String junk = is.readLine( );
if (junk.length( ) != 0) {
System.err.print("Warning unexpected response: ");
System.err.println(junk);
}
}
/** Read a line, saving it in "response".
* @return true if the expected String is contained in the response, false if not.
*/
protected boolean expect(String exp) throws IOException {
response = is.readLine( );
if (debug) {
System.out.print("<<< ");
System.out.print(response);
System.out.println( );
}
return response.indexOf(exp) >= 0;
}
}Finally, Example 11-6 extends our
CommPortModem program to
initialize the
modem and dial a
telephone number.
Example 11-6. CommPortDial.java
import java.io.*;
import javax.comm.*;
import java.util.*;
/**
* Dial a phone using the Java Communications Package.
*
*/
public class CommPortDial extends CommPortModem {
protected static String number = "000-0000";
public static void main(String[] ap)
throws IOException, NoSuchPortException,PortInUseException,
UnsupportedCommOperationException {
if (ap.length == 1)
number = ap[0];
new CommPortDial().converse( );
System.exit(0);
}
public CommPortDial( )
throws IOException, NoSuchPortException, PortInUseException,
UnsupportedCommOperationException {
super(null);
}
protected void converse( ) throws IOException {
String resp; // the modem response.
// Send the reset command
send("ATZ");
expect("OK");
send("ATDT" + number);
expect("OK");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
// nothing to do
}
is.close( );
os.close( );
}
}