Java Sockets - Need help understanding them better

Question

Okay, so I've read around on the Oracal site and some questions on this site. I'm still having kind of a hard time understanding a few things about sockets so I'll see if anyone here could spend the time to explain it to my slow brain. What I'm doing is setting up a chat client and chat server (To learn swing and sockets in one swoop). Despite all the examples I've seen, I still don't quiet grasp how they work. I know how 1 socket with an input stream and 1 socket with an output stream work, but beyond that I'm having trouble understanding because that is as far as most the resources I find explain. Here is my volley of questions regarding this.

1. If I want to be able to handle input and output to a client at the same time what would I do? Wait for out, then if there is a change in the server switch to input stream and get the changes, then switch back to output stream? Or can I run both an input and output stream at once?
2. Lets say the server has to handle several clients at once. I'll have to make a socket for each client right? What would you suggest is a good way handle this?
3. Lets say the client wants to change the IP address or port of their current socket and connect to a different server. Would I just create a new socket, or is there some way to change the current one?

That's the main questions I have. If I can get that much understood I'm pretty sure I could figure out the rest I need on my own.

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Answer 1

1.- If I want to be able to handle input and output to a client at the same time what would I do? Wait for out, then if there is a change in the server switch to input stream and get the changes, then switch back to output stream? Or can I run both an input and output stream at once?

It depends on your protocol, if your client start the connection, then your server waits for an input before going to the output stream and sends something. Every connection, being a tcp connection or even working with files have an input stream and an output stream.

2.- Lets say the server has to handle several clients at once. I'll have to make a socket for each client right? What would you suggest is a good way handle this?

There are different strategies for this that include multithreading so for now focus on streams.Or keep it with one server one client.

3.- Lets say the client wants to change the IP address or port of their current socket and connect to a different server. Would I just create a new socket, or is there some way to change the current one?

Yes, the definition of a socket is a connection made by an IP address through a specific port if any of those change you need a new socket.

Answer 2

I'll try to do my best here, but I really don't think this is the place for that kind of questions:

First of all, you need to understand that sockets are an abstraction of the underlying operating system sockets (unix socket, win socks, etc).

These kinds of sockets are to model connection-oriented services of the transport layer (look at the OSI model). So this means that sockets offer you a stream of bytes from the client and a stream of bytes to the client, so to answer your first question, these streams are independent. Of course it is your responsibility for the design of the protocol you speak over these streams.

To answer your second question you need to know how TCP connections work, basically your server is listening over one or more network interfaces in one port (ports are the TCP addressing mechanism) and can handle a configurable backlog of incoming simultaneous connections. So the answer is, it is common that for any incoming connection a new Thread on the server gets created or obtained from a Thread pool.

To answer your third question, connections are made between hosts, so if you need to change any of them, there will be the need of creating a new connection.

Hope this helps.

Cheers

Answer 3

Here's an excellent guide to sockets. It's not "Java sockets" per se, but I think you'll find it very useful:

Beej's Guide to Network Programming

To answer your questions:

Q: If I want to be able to handle input and output to a client at the same time what would I do?

A: You don't have to do anything special. Sockets are automatically "bi-modal": you can read (if there's any data) or write at any time.

Q: Lets say the server has to handle several clients at once. I'll have to make a socket for each client right?

A: Actually, the system gives you the socket for each new client connection. You don't "create" one - it's given to you.

Each new connection is a new socket.

Often, your server will spawn a new thread to service each new client connection.

Q: Lets say the client wants to change the IP address or port of their current socket and connect to a different server. Would I just create a new socket, or is there some way to change the current one?

A: The client would terminate the existing connection and open a new connection.