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Message:

Blocking? readLine method in BufferedReader class

Posted by Pedro Borges on October 04, 2000 at 1:05 PM

Hello there, I am posting this unusual behavior I got from combining BufferedReader.ready() and BufferedReader.readLine() when trying to patch an example from SUN's JAVA tutorial.
The example in question is "Reading from and Writing to a Socket"

-------------------- ORIGINAL EXAMPLE CODE

import java.io.*;
import java.net.*;

public class EchoClient {
public static void main(String[] args) throws IOException {
Socket echoSocket = null;
PrintWriter out = null;
BufferedReader in = null;
try {
echoSocket = new Socket("your.mail.server", 25);
out = new PrintWriter(echoSocket.getOutputStream(), true);
in = new BufferedReader(new InputStreamReader(
echoSocket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Don't know about host: taranis.");
System.exit(1);
} catch (IOException e) {
System.err.println("Couldn't get I/O for "
+ "the connection to: taranis.");
System.exit(1);
}
BufferedReader stdIn = new BufferedReader(
new InputStreamReader(System.in));
String userInput;
while ((userInput = stdIn.readLine()) != null) {
out.println(userInput);
System.out.println("echo: " + in.readLine());
}
out.close();
in.close();
stdIn.close();
echoSocket.close();
}
}

-------------------

This example works fine except for the fact it expects exactly one line of text as the reply to any single line of command. So I felt compelled to patch the code to alow multiple response lines to a single command, thus having:

1
2 while ((userInput = stdIn.readLine()) != null) {
3 out.println(userInput);
4 while(in.ready()){
5 System.out.println("echo: " + in.readLine());
6 }
7 }

Now, for my surprise I get in.readLine() to block immediately, I think, well I think, there could be the case when I have data ready for reading, but not \n \r or \r\n terminated (wich is not the case), so I pached on (I'll spare you from the code) and found that if I have in BufferedReader a "foo\n" string waiting, I have .ready() == true; Ok, if I .read() 4 times I get f o o \n respectively and .ready() == false; Ok, but if I .readLine() it, I get "foo\n" but .ready() STILL ON !!!

Also weird is the fact of in.ready() == false in line 1, and true in line 3 !
On my tests I connected to a local sendmail port, thus having an instant response line from the server - in.ready() on line 1 SHOULD be true (!)

Any info on this is highly appreciated,
Thank you in advance for your time,

Pedro Borges @ Portugal