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In our first example, the one we called "simple TCP program",

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we could only run one client at a time.

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In this example, our goal is going to be to enable several clients to talk to the server,

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In parallel. We'll achieve that by using threads, and I'll try to explain the concept of multi threading

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real quick right now. A thread in our example is just going to be an instance of the same application.

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It's the same code, same operations, same methods - we just run them separately for each new client. On the

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server side,

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we're going to have one main class, where we're going to have our main method.

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The first thing we're going to do in this class is - we're going to open the server socket.

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Then we're going to run an infinite loop, where for each incoming connection, we're going to open a new

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thread.

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Now I repeat - this is a very shortened version.

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We're going to go through the entire code later. For now,

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this is what threads look like for every new client that connects to the server -

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we run a new thread,

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this calls a run method in a server thread that we create for every new connection.

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So to sum up, whenever a new connection comes to the server, we create a regular socket,

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by accepting that connection - that's coming through the server socket, and then we create a thread for

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that particular connection.

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After that, we just go back to the loop and wait for the new connection to show up.

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All right.

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Hopefully this will clear up as we go through the code.

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I'm going to open Eclipse.

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I'm going to create a new package and I'm going to name it simple_tcp_multithread. This time we'll need

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two server classes. One is ServerMain,

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and the other one is ServerThread.

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We'll copy the Client class from our previous example, because the code in the client is not really going

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to change.

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We're only changing the server.

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We're only enabling the server to accept several connections at a time.

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So I'm just going to copy the client into our new package.

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All right, let's get going.

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I'm going to start from the ServerMain class.

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The first thing we're going to need is going to be the main method.

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And then inside our main method, we're going to have the try-catch block

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again - we're familiar with that.

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We're going to do the printStackTrace() in the catch block, and in the try block we're going to call

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new ServerMain(). Then, we're going to create this ServerMain method.

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It's going to be public,

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and the first thing we're going to need here is the server socket.

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We'll use 2020 for our port number again.

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And I'm going to press

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Ctrl + Shift +O to import all and necessary packets.

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Let's print out a message just so we know where we are when the program runs.

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We're gonna say "Port 2020 is now open."

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Now we need that infinite while loop,

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I'm going to add a comment here -

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you create an infinite loop by just typing "true" in the condition.

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So whatever happens inside the loop, we're always going to come back and do another run through the loop.

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So the point of this loop is to create a new thread for every new client that shows up. The first step

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would be to create a socket for every new client.

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So we're going to create the socket object, and we get the socket object when we accept a connection

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coming into the server socket. Again, I'm going to press Ctrl + Shift + O to import the socket package,

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and now I'm going to work on the thread.

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So now you're going to have to bear with me, I'm going to explain the code as I go, and some of it will

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only become clear later.

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So let's just type it in and see what happens.

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First I'm going to create a ServerThread object, and I'm going to have to use a constructor from the

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ServerThread class.

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Now since we don't have anything in that class yet, we're going to have this warning throughout typing

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this code, but don't worry about that - we're gonna resolve that later.

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So here's our object. We're going to say new ServerThread and we're going to forward the socket as the

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argument.

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Here's that error that I was talking about.

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We're just gonna ignore that for now and it's actually going to resolve itself.

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Now, we're going to need a Thread - Java already has a Thread class, since threads are a pretty common conception.

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We're going to forward our server_thread object as an argument, and then we're going to say thread.start().

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Now what this means is - whenever we run into this line, we're going to call a run() method in the ServerThread

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ServerThread class - you'll realize later on what that means since right now we haven't built our run() method

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yet.

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So let's go to our ServerThread class, and get into it right now. Our ServerThread class is going

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to have to "implement Runnable".

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By the way,

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this could be done in a different manner, but we're going to use Runnable. What Runnable is - it's pretty

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much an interface and is like a template for an object that is intended to be executed by a thread.

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It defines a single method which is >>run()<< - we haven't defined it yet, but we are going to - and that run() method

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is meant to contain the code that is executed by the thread.

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This is going to make much more sense when we run the code so let's get into it and hopefully it will

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clear up. Now, before we add that run() method,

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we need to create the constructor that I mentioned in our previous class.

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If you're not familiar with constructors - a constructor is kind of a special method, that is called when

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an object is initiated.

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So a class constructor initializes instances - or objects - of that class.

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In our case, we need to initialize the object that we forwarded from our ServerMain class, and that

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would be our socket object.

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We're going to define that object as private, because we don't want other threads to be able - other threads

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or classes - to be able to access a socket in a particular thread.

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So it's going to be private Socket socket, and then we're going to say public server_thread.

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Now this method has to be public because we need to access it - or we need to call it from another class,

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and that class is the ServerMain class.

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And here we are defining the object that is being forwarded, and then we're going to say this.socket

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this.socket = socket.

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We're using the >>this<< keyword, because our objects are named the same.

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But if you're wondering which socket is which, you just need to click - or at least, if you're using Eclipse -

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you just need to click on one of the objects and it's going to tell you which one of them is being referred

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here.

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So this is the socket we're talking about.

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This socket - this object - is going to become this newly defined object in our constructor.

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I'm going to press Ctrl + Shift + O to import the Socket package, and then we can move on to our run()

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method.

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I'm going to have to use @Override here because Java's Thread class already has its own run() method.

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So in order for us to be able to use our own run() method ,we're just going to use this @Override thing.

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run() is going to be - public void run() - and then we're going to have the try-catch block inside of it.

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Our catch block is going to have the regular printStackTrace(), and our try block is actually going to

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have all of the code that defines the protocol for the communication between the server and our client.

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I'm actually going to be a bit lazy and copy that from our previous example.

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I'm going to copy.

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No I'm actually going to go to the first one and I'm going to copy all of this.

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I'm going to edit this a bit.

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I'm going to add a printout here, that a new client has connected.

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And then the server is going to say to the client:

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"Welcome! What's your name?"

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"Welcome! What's your name?"

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I'm going to save that, and I'm going to go back to ServerMain to check things here.

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Everything seems OK.

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I'm going to save that too and then I'm going to open the client.

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The client is not going to be guessing any numbers this time.

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So we're going to delete all of this instead.

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We're going to just accept the message from the server,

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print it out in our console.

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We're going to say -

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and then we're going to store whatever the user types in into the same string.

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And then we're going to send that out to the server.

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That's it.

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That should work.

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I'm going to save this and then run it. We're going to run ServerMain. So we're again running only the

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classes that have the main method.

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And then let's run our first client.

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I'm going to switch to the client's console.

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It's asking me what's my name.

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I'm going to say "Luke" for example, and then I'm going to run another client.

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This time, I'm not going to answer to server's question, and I'm going to run another client.

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So at this point we have two clients running.

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Let's check that out in the console.

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So we have client three here OK.

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So this is the second client and this is the third client.

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I'm going to type another name into the second client and now my second client has disconnected.

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And then I'm going to type in something else into the third client.

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Let's say "Laura" for example.

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All right.

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Now all of my three clients have disconnected but the server is actually still running.

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If we go to "Clients" - it says "terminated" here.

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So I'm going to close this console, and then I'm going to close the third client's console, and I'm only

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left with my server's console. And I can see here that it's still running, because I have this "terminate"

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button still available.

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I'm going to click here and now we're done.

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Now I can close the console for the server too.