Java Thread won't pause on I/O operation

Question

I was under the impression that in Java, a Thread will pause and give other threads a chance to do some work during blocking I/O operations ( like Socket.read() or DataGramsocket.receive() ). For some reason in my multi threaded network application, a call to receive() is causing all my other threads to starve ( the thread that called receive() is becoming a big boss and never giving up control, thus blocking forever! )

Why would this happen? I used to have the same exact application, but instead of UDP it was TCP based. Socket.read() always paused the Thread and allowed others to work for a bit if it blocked for too long.

-- extra info -- The TCP version of my custom thread was this code: http://www.dwgold.com/code/proxy/proxyThread.java.txt

My new code ( UDP version ) is pretty much the same, but modified a bit to use UDP methods and style. I then created two of these threads and call start on both. The first thread always blocks and never lets the other do work in the UDP version.

Answer 1

It seems that you're using an InputStream, and according to the JDK docs for InputStream.read(), the read blocks exactly as you've described -- until the data is received, an end-of-file is reached, or an exception is thrown.

So for me, the question is: why does the TCP version of your code allow the block to get interrupted? That doesn't seem to make any sense. Perhaps your TCP code breaks the reads up into discrete, short-enough bursts that the thread has a chance to jump in between separate calls to read()?

Looking further, I see that the difference is that the TCP version of your code receives data via the InputStream that's provided by Socket, whereas the UDP version of your code receives data directly from the DatagramSocket's own receive() method. So I'd posit that this is just a fundamental difference between the functionality offered by the two (InputStream.read and DatagramSocket.receive). Have you thought about using DatagramPacket.setSoTimeout to set a timeout on the socket's receive block, and then catching the SocketTimeoutException when it's thrown by the call to receive() that times out? You should be able to implement this to achieve what you want.

UPDATE: looking further still, it appears that the DatagramSocket.receive method is synchronized. So the way to explain the behavior you're seeing is that you're starting two threads that are attempting to use the same DatagramSocket instance -- if two threads are attempting to execute receive() on the same DatagramSocket instance, one will block the instance until it's done, and the other will be blocked. (There's a post on the Sun forums that describes this, as well.) Might this be what's happening -- you're reusing the same DatagramSocket for both threads?

Answer 2

There's really no explanation for the behaviour you're describing, that a read on an InputStream from a Socket should cause "all other threads" to pause. Are you by any chance starting several threads, which all are reading from the same socket and what you actually mean is that all these threads are hanging? In that case, I would expect that the data read from the socket is divided arbitrarily among the threads.