Better alternative for PipedReader/PipedWriter?

Question

I need to have a buffered char stream, into which I write in one thread and from which I read in another thread. Right now I\'m using PipedReader and PipedWriter fo

Answer 1

I implemented something a little similar, and asked a question whether anyone else had any better thought out and tested code.

Answer 2

It should be fairly easy to wrap a char stream API around BlockingQueue.

I must say, however, it seems quite perverse that PipedReader would use polling to wait for data. Is this documented somewhere, or did you discover it for yourself somehow?

Answer 3

@Esko Luontola, I've been reading through your code in the sbt package to try to understand what you are doing. It seems like you want to start up a Process and pass input to it, and have the result of the action be teed to different places. Is this at all correct?

I would try modifying the main loop in ReaderToWriterCopier so that instead of doing a read() - a blocking operation that apparently when a PipedReader is involved causes polling - you explicitly wait for the Writer to flush. The documentation is clear that flush causes any Readers to be notified.

I'm not sure how to run your code so I can't get deeper into it. Hope this helps.

Answer 4

The problem was that when something is written to the PipedWriter, it does not automatically notify the PipedReader that there is some data to read. When one tries to read PipedReader and the buffer is empty, the PipedReader will loop and wait using a wait(1000) call until the buffer has some data.

The solution is to call PipedWriter.flush() always after writing something to the pipe. All that the flush does is call notifyAll() on the reader. The fix to the code in question looks like this.

(To me the PipedReader/PipedWriter implementation looks very much like a case of premature optimization - why not to notifyAll on every write? Also the readers wait in an active loop, waking up every second, instead of waking only when there is something to read. The code also contains some todo comments, that the reader/writer thread detection which it does is not sophisticated enough.)

This same problem appears to be also in PipedOutputStream. In my current project calling flush() manually is not possible (can't modify Commons IO's IOUtils.copy()), so I fixed it by creating low-latency wrappers for the pipe classes. They work much better than the original classes. :-)