Re: Non-blocking and semi-blocking Sockets class.

Knute Johnson <>
Wed, 17 Jan 2007 21:15:14 -0800
nukleus wrote:

I am working on a network related application.
There is an issues with sockets blocking and how to handle it.
There are several approaches:

1. Use blocking sockets and set timeout.
    In this case, if connection is lost or socket object is not created
    yet as we are in the middle of connect operation, the process
    effectively hangs and user has to wait for timeout expiration.
    Not a very desirable approach.

2. Use byte reads and buffered input stream and check
    if data is available. In case of text, if it is, then read it
    until you get a line separator character, and then return
    a text line to a caller of getLine().

    If eof condition happens, returned by -1 on a read,
    that could mean the server closed a connection for whatever reason.
    In this case, simply return. Non blocking operation.

    If there is nothing in the buffer, but you are waiting for a server
    standard response or data, you loop untill such data arrives.
    This is blocking operation and has all sorts of nasty side effects.
    The loop uses a separate timer from the socket timeout timer.
    When that timer expires, the getLine() returns with error or
    throws a TimeOut exception.

    But it could be resolved usint a separate thread for a blocking socket.

    One of the alternatives here is to use a byte read loop with that
    could be interrupted, say, if user realizes there is error, and hits
    Cancel button. In this case, the event handler sets the event,
    that is tested in a read loop, and if the flag is set, the loop
    This is interruptible blocking operation, which should effectively
    behave like a non-blocking operation for practical purposes.

3. Blocking socket running as a separate thread.
    This seems to be the best overall approach, as we can allow
    the socket thread to block, but we are not block on the main
    thread level and all our gui stuff operations as advertised.

    But there are issues with this. Since you are running a separate
    thread, how do you tell it what command we issue to the server
    and how does the main thread knows when operation is completed
    successfuly. Anotherwords, how do you exchange the information
    and various error conditions between the main object and the socket?

    From windows C++ standpoint, you can go asynchronous using
    overlapped structures for one thing.

    Question: what is the best approach with java and what are the
    exact steps to design the separate socket thread?
    Do we use Events?
    What happens to Exceptions running in a separate thread that
    need to be communicated to a different thread of different frame object?

    Since we create a separate thread for a socket that has its own
    run() loop and we could be performing either send or receive
    network operation, how do we exchange data between socket
    thread and a parent class?

4. Fully asynchronous design.
    This design, by definition, is non-blocking.
    The overall architecture may be implemented with a state machine
    approach. In this case, there is never any blocking on a socket
    object. But how do you do it in Java?
    It seems events are the way to go, but how do you set up your
    architecture to do it?

    Related question:
        Anyone can suggest a good state machine design in Java?
        I used my own design in C++ that had a vector of state class
        objects and a state machine iterating code. Each state object
        had a paramter telling it which is the next state to proceed to
        in case of successful result, which state to proceed to in case
        of error result, and which alternative states to proceed to in
        case of minor error conditions, such as incorrect user command
        issued to the server in our situation, or incorrect value of data
        passed to server.

        This design is flexible in that the state machine could even be
        substituted for another state machine, need be. For example,
        if you are trying to send email, and you made a mistake in your
        configuration that tells the server that you are connecting to
        NNTP server, than the server log-in response code would be
        different. In this case, you could switch the state machine on
        the fly and perform the NNTP related operation. This is just
        and abstract example. You wouln't probably want to do that
        in real life. :--}

Does anyone have any feedback on this all?

Thanks in advance.

Use 3. I assume your data is some kind of message? Just have a read
thread that takes in the data and calls methods appropriate to the
message. The output can be handled from the GUI. The simpler you make
it the more reliable and maintainable it will be.

Just my 2 cents.


Knute Johnson
email s/nospam/knute/

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