Java - 关闭没有异常的ServerSocket？

Question

For years I've been wondering what the correct way to close a listening ServerSocket in Java is. My implementations always work like this, they:

1. Create a new ServerSocket(int).
2. Start a thread that calls its accept() method in a while (true) loop.
3. Start another thread when accept() returns (client connects) that reads from the client until it disconnects. The accept thread then continues with another accept() call.

But when I want to close the ServerSocket because my application is exiting, I've never found another way of doing so other than calling it's close() method (after I've closed all client Sockets), which causes accept() to throw a SocketException, I catch that and break from the while (true) loop in the accept thread, causing all my threads to exit.

I think this is ugly, strictly speaking there is no exception occurring, closing my ServerSocket is part of my programs normal operation.

Is there really no other way of doing this without causing an Exception to be thrown?

Thanks.

Answer 1

I've never found another way of doing so other than calling it's close() method (after I've closed all client Sockets),

You can set a flag closed = true; and open a dummy connection to wake up the accept()ing thread which checks the flag before continuing.

which causes accept() to throw a SocketException,

A SocketClosedException which is expected behaviour here.

I catch that and break from the while (true) loop in the accept thread,

If you catch it outside the loop, you don't need to also break out of the loop. If the exception is thrown and you have set closed = true you can discard the exception.

BTW I would do

while(!serverSocket.isClosed()) {

causing all my threads to exit.

There is no particular reason you need to have this, but you can choose to do this if you want.

I think this is ugly, strictly speaking there is no exception occurring,

An exceptional condition is happening. What is not happening is an Error.

closing my ServerSocket is part of my programs normal operation.

Or you could say it is operating normally when it is running and not shutting down.

Answer 2

Thanks for your comment.

You can set a flag closed = true; and open a dummy connection to wake up the accept()ing thread which checks the flag before continuing.

Opening a new Socket to close an existing one? IMO that is one of the worst sins ever.

A SocketClosedException which is expected behaviour here.

Actually it doesn't throw a SocketClosedException but a SocketException, the following code:

try {
    final ServerSocket ss = new ServerSocket(1234);
    new Thread(new Runnable() {

        @Override
        public void run() {
            try {
                ss.accept();
            } catch (Exception ex) {
                ex.printStackTrace();
            }
        }

    }).start();
    ss.close();
} catch (Exception ex) {
    ex.printStackTrace();
}

Produced the following output:

java.net.SocketException: Socket is closed
at java.net.ServerSocket.accept(ServerSocket.java:494)
at Main$1.run(Main.java:12)
at java.lang.Thread.run(Thread.java:744)

If you catch it outside the loop, you don't need to also break out of the loop. If the exception is thrown and you have set closed = true you can discard the exception.

BTW I would do

while(!serverSocket.isClosed()) {

That would work, but I think this should work without setting flags and catching Exceptions.

An exceptional condition is happening. What is not happening is an Error.

Or you could say it is operating normally when it is running and not shutting down.

I don't agree. It is normal behaviour for applications to be exiting. They may always do so.

Why doesn't the accept() method just returns null when close() is called? That would be so much easier.