PipeReader and PipeWriter

[memati bas]

Hello,
Although I read lots of tutprials about PipeReader and PipeWriter, I could not exactly understand its purpose and especially usage.
Would you explain it and send sample explanatory code snippet for this ??

Thanks

[Joe Ess]

Pipes are used to channel the output from one thread into the input of another

The Java Tutorial: IO: How to Use Pipe Streams
There's an example on that page as well. Does that all make sense?

[Edwin Dalorzo]

What happens when you want to read in a InputStream (or Reader) every byte (or character) that is being written to an OutputStream (or Writer)?

That's what pipes are good for. You have one thread procesing information, and writing that information into a PipeOutputStream (PipedWriter). At the other side of the pipe there is another thread, reading the information through a PipedInputStream (PipedReader).

For instance, you have this thread, that simply receives a list of
Strings, reverse them and then prints them on the screen.

public static class Reverser extends Thread{
private List<String> words;

public Reverser(List<String> words){
this.words = words;
}

public void run(){
for(String word: words){
System.out.println(reverse(word));
}
}

public String reverse(String word){
return new StringBuilder(word).reverse().toString();
}
}

Now, instead of printing them in the screen, we are going to send them on pipe
to other thread

You do that like this:

public static class Reverser extends Thread{
private List<String> words;
private PipedWriter writer;

public Reverser(List<String> words, PipedWriter writer){
this.words = words;
this.writer = writer;
}

public void run(){
try{
for(String word: words){
writer.write(reverse(word));
writer.flush();
}
writer.close();
}catch(IOException e){
e.printStackTrace();
}
}

public String reverse(String word){
return new StringBuilder(word).reverse().toString();
}
}

But, how do we connect the pipes?. That's to say, how do we make another thread to
receive the reversed strings. Well, take a look at this example. In this case, the
thread that connects to the other end of the pipe is the main thread.

public static void main(String[] args) {
List<String> jedis = new ArrayList<String>();

//The list to be reversed
jedis.add("Obi-wan");
jedis.add("Anakin");
jedis.add("Qui-gon");
jedis.add("Mace-Windu");


PipedWriter pw = null;
PipedReader pr = null;

try{
//Create and connect the pipes
pw = new PipedWriter();
pr = new PipedReader(pw);

//pass one pipe to one thread
new Reverser(jedis,pw).start();

//the main thread keeps the other side of the pipe
BufferedReader reader = new BufferedReader(pr);

//print the reversed items sent through the pipe
String line = null;
while( (line = reader.readLine())!=null){
System.out.println(line);
}
reader.close();

}catch(IOException e){
e.printStackTrace();
}
}

I hope this helps!
[ April 27, 2006: Message edited by: Edwin Dalorzo ]