Is PrintWriter threadsafe?

[Jeff Schwab]

Is it OK for multiple threads to use the same PrintWriter concurrently?

[Silvio Bierman]

"Jeff Schwab" <je******@comca st.net> wrote in message
news:rY******** ************@co mcast.com...

Is it OK for multiple threads to use the same PrintWriter concurrently?

What would you consider threadsafe behaviour in such a scenario?

Silvio Bierman

[Jeff Schwab]

Silvio Bierman wrote:

"Jeff Schwab" <je******@comca st.net> wrote in message
news:rY******** ************@co mcast.com...

Is it OK for multiple threads to use the same PrintWriter concurrently?

What would you consider threadsafe behaviour in such a scenario?

Good question. I just want to make sure the I/O system won't get
confused. It's OK if the output of the two threads is intertwined.

[Bob Sullivan]

Do you mind if characters within words or words are mixed?

Thread 1 trys to print "Thread 1 ABCDEF"
Thread 2 trys to print "Thread 2 GHIJKLM"

Is the following OK?

"TThhr 1 eadAe 2a GHd ABCIJKLMDEF"

I think you should synchronize the actual IO call. What we do is utilize a
message queue;
the different threads drop message objects in the queue while a low-priority
thread waits
on the queue, pops off a message, does the IO, then waits for the next
message. So only
one, low-priority, thread is actually doing the IO.
"Jeff Schwab" <je******@comca st.net> wrote in message
news:eu******** ************@co mcast.com...

Silvio Bierman wrote:

"Jeff Schwab" <je******@comca st.net> wrote in message
news:rY******** ************@co mcast.com...

Is it OK for multiple threads to use the same PrintWriter concurrently?

What would you consider threadsafe behaviour in such a scenario?

Good question. I just want to make sure the I/O system won't get
confused. It's OK if the output of the two threads is intertwined.

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[Jeff Schwab]

Bob Sullivan wrote:

Do you mind if characters within words or words are mixed?

Thread 1 trys to print "Thread 1 ABCDEF"
Thread 2 trys to print "Thread 2 GHIJKLM"

Is the following OK?

"TThhr 1 eadAe 2a GHd ABCIJKLMDEF"
Yes.
I think you should synchronize the actual IO call. What we do is utilize a
message queue;
the different threads drop message objects in the queue while a low-priority
thread waits
on the queue, pops off a message, does the IO, then waits for the next
message. So only
one, low-priority, thread is actually doing the IO.

That's a neat idea, and it does help, but I need to see some output
ASAP. It can't wait for a low-priority thread.

[Jeff Schwab]

Jeff Schwab wrote:

Bob Sullivan wrote:

Do you mind if characters within words or words are mixed?

Thread 1 trys to print "Thread 1 ABCDEF"
Thread 2 trys to print "Thread 2 GHIJKLM"

Is the following OK?

"TThhr 1 eadAe 2a GHd ABCIJKLMDEF"

Yes.

I think you should synchronize the actual IO call. What we do is utilize a message queue;the different threads drop message objects in the queue while a low-priority thread waits on the queue, pops off a message, does the IO, then waits for the next message. So only one, low-priority, thread is actually doing the IO.

That's a neat idea, and it does help, but I need to see some output ASAP. It can't wait for a low-priority thread.

To clarify: I don't need truly unbuffered output, but buffered only to
the level of the individual println. Since I don't println a character
at a time, is the above situation ("TThhr 1 eadAe 2a GHd ABCIJKLMDEF")
really possible?

[Silvio Bierman]

Hello Jeff,

You could do all println calls inside synchronized(so meSingleton) blocks.
This will result in almost-prompt printing and decent separation.

Silvio Bierman
"Jeff Schwab" <je******@comca st.net> wrote in message
news:3N******** ************@co mcast.com...

Jeff Schwab wrote:

Bob Sullivan wrote:

Do you mind if characters within words or words are mixed?

Thread 1 trys to print "Thread 1 ABCDEF"
Thread 2 trys to print "Thread 2 GHIJKLM"

Is the following OK?

"TThhr 1 eadAe 2a GHd ABCIJKLMDEF"

Yes.

I think you should synchronize the actual IO call. What we do is

utilize a message queue;the different threads drop message objects in the
queue while a low-priority thread waits on the queue, pops off a message,
does the IO, then waits for the next message. So only one, low-priority,
thread is actually doing the IO.
That's a neat idea, and it does help, but I need to see some output

ASAP. It can't wait for a low-priority thread.

To clarify: I don't need truly unbuffered output, but buffered only to
the level of the individual println. Since I don't println a character
at a time, is the above situation ("TThhr 1 eadAe 2a GHd ABCIJKLMDEF")
really possible?

[Jeff Schwab]

Silvio Bierman wrote:

Hello Jeff,

You could do all println calls inside synchronized(so meSingleton) blocks.
This will result in almost-prompt printing and decent separation.

Silvio Bierman
Yes, if necessary, I'll do that. I expect to have hundreds or thousands
of threads at a time vying for the same Writer, though, and I worry that
the performance detriment will be too great.

Is it safe to get the current time simultaneously in different threads,
without synchronization ? I only need to see error messages immediately
when the app ( a server ) is first starting. Once things get going,
it's just the order of the messages that needs to be preserved. Maybe I
could use the message-queue idea, and add time-stamps to the messages.
What do you think?


"Jeff Schwab" <je******@comca st.net> wrote in message
news:3N******** ************@co mcast.com...

Jeff Schwab wrote:

Bob Sullivan wrote:
Do you mind if characters within words or words are mixed?

Thread 1 trys to print "Thread 1 ABCDEF"
Thread 2 trys to print "Thread 2 GHIJKLM"

Is the following OK?

"TThhr 1 eadAe 2a GHd ABCIJKLMDEF"
Yes.
I think you should synchronize the actual IO call. What we do is
utilize a message queue;the different threads drop message objects in the
queue while a low-priority thread waits on the queue, pops off a message,
does the IO, then waits for the next message. So only one, low-priority,
thread is actually doing the IO.
That's a neat idea, and it does help, but I need to see some output

ASAP. It can't wait for a low-priority thread.

To clarify: I don't need truly unbuffered output, but buffered only to
the level of the individual println. Since I don't println a character
at a time, is the above situation ("TThhr 1 eadAe 2a GHd ABCIJKLMDEF")
really possible?

[Silvio Bierman]

"Jeff Schwab" <je******@comca st.net> wrote in message
news:Qa******** ************@co mcast.com...

Yes, if necessary, I'll do that. I expect to have hundreds or thousands
of threads at a time vying for the same Writer, though, and I worry that
the performance detriment will be too great.

Is it safe to get the current time simultaneously in different threads,
without synchronization ? I only need to see error messages immediately
when the app ( a server ) is first starting. Once things get going,
it's just the order of the messages that needs to be preserved. Maybe I
could use the message-queue idea, and add time-stamps to the messages.
What do you think?

You have that many threads shortly after starting your server? Impressive!

Using a synchronized queue would work fine and it would probably mean less
time lost due to synchronization when compared to synchronzied println's.
Your print-thread would be wait-ing on the queue and the error producers
would have to notify it to start the printing. The simpler alternative would
be using a timed wait but that would delay error logging and activate the
printing thread even when no messages are present.

You could probably wrap such a thing inside a SynchronizedQue uePrintStream
and use System#setErr to prevent code modifications.

Regards,

Silvio

[Jeff Schwab]

Silvio Bierman wrote:

"Jeff Schwab" <je******@comca st.net> wrote in message
news:Qa******** ************@co mcast.com...

Yes, if necessary, I'll do that. I expect to have hundreds or thousands
of threads at a time vying for the same Writer, though, and I worry that
the performance detriment will be too great.

Is it safe to get the current time simultaneously in different threads,
without synchronization ? I only need to see error messages immediately
when the app ( a server ) is first starting. Once things get going,
it's just the order of the messages that needs to be preserved. Maybe I
could use the message-queue idea, and add time-stamps to the messages.
What do you think?
You have that many threads shortly after starting your server? Impressive!

:) Not usually, but I have to be prepared for it.
Using a synchronized queue would work fine and it would probably mean less
time lost due to synchronization when compared to synchronzied println's.
Your print-thread would be wait-ing on the queue and the error producers
would have to notify it to start the printing. The simpler alternative would
be using a timed wait but that would delay error logging and activate the
printing thread even when no messages are present.
Thanks, Silvio, sounds good. Does this mean that PrintWriter isn't
synchronized on its own, or does this mean that we don't know, so I
should do possibly redundant synchronization just to make sure?
You could probably wrap such a thing inside a SynchronizedQue uePrintStream
and use System#setErr to prevent code modifications.

What is "System#setErr" ?