Waiting for input from PipedInputStream....thread yield?

[Grant Baxter]

Hello All,

I've written a test class to start playing around with PipedInputStreams and PipedOutputStreams. I need to work the kinks out of the functionality before I integrate it into my real app.

Essentially what I've done is created 2 threads -- one has a PipedOutputStream to which it writes messages, the other thread has a PipedInputStream from which it reads messages.

In my real application, I will need the information that comes in on the PipedInputStream as soon as it becomes available. However, I noticed that in my current setup, the tight "while(true)" loop of the thread that's reading the PipedInputStream is consuming huge amounts of processor time.

I thought that I understood that when I called the PipedInputStream.read() function, the thread would block and therefore yield until data became available on the PipedInputStream. Maybe it is blocking but the thread scheduler is passing control back to it so fast that it's still burning loads of cycles.

How is this general IO programming problem solved? You want your data in a timely fashion, but you don't want be behave badly and suck all of the CPU time away from other processes & threads! Should I cause my read thread to sleep for a specified time when it checks the input stream and finds no data? Any suggestions?

  
/*
 * Created on May 25, 2005
 *
 * TODO To change the template for this generated file go to
 * Window - Preferences - Java - Code Style - Code Templates
 */
 
import java.awt.Color;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.FocusEvent;
import java.awt.event.FocusListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.*;
 
import java.io.PipedInputStream;
import java.io.PipedOutputStream;
import java.io.*;
 
import javax.swing.*;
import javax.swing.text.BadLocationException;
import javax.swing.text.Document;
import javax.swing.text.SimpleAttributeSet;
import javax.swing.text.StyleConstants;
 
/**
 * @author Grant
 *
 * TODO To change the template for this generated type comment go to
 * Window - Preferences - Java - Code Style - Code Templates
 */
public class PlayingWithPipes {
 
private ThreadWithPipe twp1;
private ThreadWithPipe twp2;

public PlayingWithPipes(){

twp1 = new ThreadWithPipe();
twp2 = new ThreadWithPipe();

twp1.outPipe = new PipedOutputStream();
try {
twp2.inPipe = new PipedInputStream(twp1.outPipe);
} 
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
 

twp1.setSendMode();
twp2.setReceiveMode();

twp1.start();
twp2.start();

}

public static void main(String[] args){
PlayingWithPipes pwp = new PlayingWithPipes();
}


/*######### Begin ThreadWithPipe class ###############*/ 
class ThreadWithPipe extends Thread implements ActionListener{

JFrame frame;
PipedInputStream inPipe;
PipedOutputStream outPipe;
boolean sendMode = false;
boolean receiveMode = false;
 
int counter = 0;

JTextPane inOutScreen;
Document inOutDoc;
SimpleAttributeSet red;
SimpleAttributeSet blue;

String id;

JTextField cmdEntryField;
JButton sendButton;

public ThreadWithPipe(){

frame = new JFrame();
inOutScreen = new JTextPane();
inOutScreen.setEditable(false);
red = new SimpleAttributeSet();
blue = new SimpleAttributeSet();
StyleConstants.setForeground(red, Color.red);
StyleConstants.setForeground(blue, Color.blue);
inOutDoc = inOutScreen.getDocument();

Container c = frame.getContentPane();
c.add(inOutScreen);

frame.setSize(200,200);
frame.setVisible(true);

        frame.addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        });
}

/* (non-Javadoc)
 * @see java.lang.Runnable#run()
 */
public void run() {

while(true){
if(receiveMode){
receiveMessage();
}
}

}

public void setReceiveMode(){
id = new String("receive");
sendMode = false;
receiveMode = true;
frame.setTitle("Listen Mode");

try {
inOutDoc.insertString(inOutDoc.getLength(), "Receiving...\n", red);
} 
catch (BadLocationException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
}

public void setSendMode(){
id = new String("send");
receiveMode = false;
sendMode = true;
frame.setTitle("Send Mode");
 
try {
inOutDoc.insertString(inOutDoc.getLength(), "Sending...\n", blue);
} 
catch (BadLocationException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}

cmdEntryField = new JTextField("", 20);
sendButton = new JButton("Send");
JToolBar cmdBar = new JToolBar();

cmdBar.setFloatable(false);
cmdBar.add(cmdEntryField);
cmdBar.addSeparator();
cmdBar.add(sendButton);

Container c = frame.getContentPane();
c.removeAll();
c.setLayout(new BorderLayout());
c.add(cmdBar, BorderLayout.NORTH);
c.add(inOutScreen, BorderLayout.CENTER);

cmdEntryField.addKeyListener(new KeyAdapter() {
public void keyTyped(KeyEvent e){
char c = e.getKeyChar();
if(c == KeyEvent.VK_ENTER){
getCmdAndSend();
}
}
});
cmdEntryField.addFocusListener(new FocusListener(){
public void focusGained(FocusEvent e){
cmdEntryField.setText("");
}
public void focusLost(FocusEvent e){}
});

//frame.setVisible(false);
frame.setVisible(true);
//frame.repaint();
}

public void sendMessage(String msg){
counter++;
try {
inOutDoc.insertString(inOutDoc.getLength(), (counter + ": " + msg + "\n"), blue);
} 
catch (BadLocationException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}

msg = msg + "\n";
try {
outPipe.write(msg.getBytes());
outPipe.flush();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

}

public void receiveMessage(){
int readBufSize = 128;
byte[] readBuf = new byte[readBufSize];
int bytesRead = 0;
 
int availableBytes = 0;

try {
availableBytes = inPipe.available();
} 
catch (IOException e2) {
// TODO Auto-generated catch block
e2.printStackTrace();
}

if(availableBytes <= 0){
this.yield();
}

try {
bytesRead = inPipe.read(readBuf, 0, readBufSize);
} 
catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
try {
counter++;
inOutDoc.insertString(inOutDoc.getLength(), new String(readBuf, 0, bytesRead), red);
} 
catch (BadLocationException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
 

}
 
private void getCmdAndSend(){
String msg = cmdEntryField.getText();
cmdEntryField.setText("");
sendMessage(msg);
}

/* (non-Javadoc)
 * @see java.awt.event.ActionListener#actionPerformed(java.awt.event.ActionEvent)
 */
public void actionPerformed(ActionEvent arg0) {
if(arg0.getSource() == sendButton){
getCmdAndSend();
}
}
}
/*######### End ThreadWithPipe class ###############*/

}
 

[Henry Wong]

public void run() {
    while(true){
        if(receiveMode){
            receiveMessage();
        }
    }
}

What does the write (twp1) thread do?

Henry
[ May 26, 2005: Message edited by: Henry Wong ]

[Grant Baxter]

Hi Henry,

Sorry about the poor commenting of the code. The write thread is event driven -- when the object instance is configured to send mode (setSendMode()) it gets a JTextField where a user can enter text, and a JButton that the user clicks to cause the object to send the text out the PipedOutputStream.

There isn't anything that the write thread has to do during each pass through the run() while loop.

Grant

[Henry Wong]

There isn't anything that the write thread has to do during each pass through the run() while loop.

Your code is fine... sort of self documenting actually...

Please tell me exactly what the write thread does... (hint: I am trying to get you to find the problem yourself)

Henry

[Grant Baxter]

Hi Henry,

Found the bug w/ the help of some feedback on another forum -- I was two-timing by posting on the Java Forums as well ;-)

I was looking for the error in the wrong place. For some reason, I was thinking that once the thread finished executing its run() method, it would die. That's not the case. If I reverse the order of the while and if statements in the run() function, my problem is solved! ;-)

public void run() {

if(receiveMode){
while(true){
receiveMessage();
}
}
}

So the send thread executes run(), returns from the method and just sits in event driven mode. The receive thread has to continually poll the PipedInputStream by using PipedInputStream.read(). When it tries to read and no data is available it blocks and yields, just like I wanted it to.

Thanks