Recipe 12.7 Reading and Writing: Threads

Problem

After the connection is made, you don't know what order to read or write in.

Solution

Use a thread to handle each direction.

Discussion

When you have two things that must happen at the same time or unpredictably, the normal Java paradigm is to use a thread for each. We discuss threads in detail in Chapter 24, but for now, think of a thread as a small, semi-independent flow of control within a program, just as a program is a small, self-contained flow of control within an operating system. The Thread API requires you to construct a method whose signature is public void run( ) to do the body of work for the thread and call the start( ) method of the thread to "ignite" it and start it running independently. This example creates a Thread subclass called DataThread, which reads from one file and writes to another. DataThread works a byte at a time so that it works correctly with interactive prompts, which don't end at a line ending. My now-familiar converse( ) method creates two of these DataThreads, one to handle data "traffic" from the keyboard to the remote, and one to handle bytes arriving from the remote and copy them to the standard output. For each of these, the start( ) method is called. Example 12-9 shows the entire program.

Example 12-9. CommPortThreaded.java

import java.io.*; import javax.comm.*; import java.util.*; /**  * This program tries to do I/O in each direction using a separate Thread.   */ public class CommPortThreaded extends CommPortOpen {     public static void main(String[] ap)         throws IOException, NoSuchPortException,PortInUseException,             UnsupportedCommOperationException {         CommPortThreaded cp;         try {             cp = new CommPortThreaded( );             cp.converse( );         } catch(Exception e) {             System.err.println("You lose!");             System.err.println(e);         }     }     public CommPortThreaded( )         throws IOException, NoSuchPortException, PortInUseException,             UnsupportedCommOperationException {         super(null);     }     /** This version of converse( ) just starts a Thread in each direction.      */     protected void converse( ) throws IOException {         String resp;        // the modem response.         new DataThread(is, System.out).start( );         new DataThread(new DataInputStream(System.in), os).start( );     }     /** This inner class handles one side of a conversation. */     class DataThread extends Thread {         DataInputStream inStream;         PrintStream pStream;         /** Construct this object */         DataThread(DataInputStream is, PrintStream os) {             inStream = is;             pStream = os;         }         /** A Thread's run method does the work. */         public void run( ) {             byte ch = 0;             try {                 while ((ch = (byte)inStream.read( )) != -1)                     pStream.print((char)ch);             } catch (IOException e) {                 System.err.println("Input or output error: " + e);                 return;             }         }     } }