in what way is java.net.Socket threadsafe?

Question

I have a Socket that I am both reading and writing to, via BufferedReaders and BufferedWriters. I'm not sure which operations are okay to do from separate threads. I would guess that writing to the socket from two different threads at the same time is a bad idea. Same with reading off the socket from two different threads at the same time. What about reading on one thread while writing on another?

I ask because I want to have one thread blocked for a long time on a read as it waits for more data, but during this wait I also have occasional data to send on the socket. I'm not clear if this is threadsafe, or if I should cancel the read before I write (which would be annoying).

Answer 1

Sockets are thread unsafe at the stream level. You have to provide synchronization. The only warranty is that you won't get copies of the exact same bytes in different read invocations no matter concurrency.

But at a Reader and, specially, Writer level, you might have some locking problems.

Anyway, you can handle read and write operations with the Socket's streams as if they were completely independent objects (they are, the only thing they share is their lifecyle).

Once you have provided correct synchronization among reader threads on one hand, and writer threads on the other hand, any number of readers and writers will be okay. This means that, yes, you can read on one thread and write on another (in fact that's very frequent), and you don't have to stop reading while writing.

One last advice: all of the operations involving threads have associated timeout, make sure that you handle the timeouts correctly.