Re: System.out and System.err
In article <6kj7spF86252U1@mid.individual.net>,
Jan Thom? <janthomae@janthomae.de> wrote:
On 2008-10-02 00:09:52 +0200, "Mike Schilling"
<mscottschilling@hotmail.com> said:
I think we all assume that access to System.out and System.err is
thread-safe; if we write to them from more than one threads, the
worst that will happen is output getting interleaved but not lost,
nor will exceptions be caused by unsynchronized access (e.g., we
don't expect internal bufferr-copying routines to get confused.)
Looking, though, I can't find this guarnteed anywhere. Can anyone
point to an offical statement about it?
Well the only "official" statement I have found looking at the
implementation of PrintStream which System.out and System.err are
internally. The write( .... ) methods are all internally
synchronized, so they will not produce weird results when multiple
threads access them:
[...]
Interesting. IIUC, not all implementations are so well behaved, though.
For some years, Michael B. Feldman has taught students of Ada and Java
about threads using code similar to that below, which he kindly shares
with us.
<code>
import java.util.Random;
/**---------------------------------------------------------------
* Demo of simple thread class
* Last Modified: March 2008
* @author Michael B. Feldman, mfeldman at gwu.edu
*--------------------------------------------------------------*/
public class ShowThreads {
public static void main(String args[]) {
SimpleThread ThreadA = new SimpleThread("A", 5, 1);
SimpleThread ThreadB = new SimpleThread("B", 7, 21);
SimpleThread ThreadC = new SimpleThread("C", 4, 41);
SafeScreen.clearScreen();
ThreadA.start();
ThreadB.start();
ThreadC.start();
}
} // end ShowThreads
/**---------------------------------------------------------------
* Simple Java thread example
* Last Modified: March 2008
* @author Michael B. Feldman, mfeldman at gwu.edu
*--------------------------------------------------------------*/
class SimpleThread extends Thread {
private static final Random random = new Random();
private String name = "Default";
private int count = 0;
private int column = 0;
// constructor
public SimpleThread(String name, int count, int column) {
this.name = name;
this.count = count;
this.column = column;
}
// run method invoked when 'start' called
public void run() {
for(int i = 1; i <= count; i++) {
int nap = random.nextInt(7) + 1;
SafeScreen.write(name + " naps " + nap + " secs", i, column);
try {
sleep(nap * 1000);
}
catch(InterruptedException e) {} // ignored
}
}
} // end class SimpleThread
/**---------------------------------------------------------------
* Thread-safe Mini-terminal controller for vt100 (ANSI) terminals
* Translated from C to Java February 2001
* @author Michael B. Feldman, mfeldman at gwu.edu
*--------------------------------------------------------------*/
class SafeScreen {
public static synchronized void clearScreen() {
System.out.print("\033" + "[2J");
}
public static synchronized void write(
String item, int row, int col) {
System.out.println("\033" + "[" + row + ";" + col + "f" + item);
}
} // End of class SafeScreen
</code>
[The code is Professor Feldman's; any errors are mine.]
--
John B. Matthews
trashgod at gmail dot com
home dot woh dot rr dot com slash jbmatthews