What is the difference between Socket and ServerSocket?

Question

If Socket represents client side and ServerSocket represents server side, why Socket.read reads the data from server side? I'm really confused, Can you please clarify it to me?

Solution 1

(I post this answer because I always feel it's important to make the logic right.)

I suggest you take a look at the following sample.

http://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html

Admittedly, when carrying out TCP/IP communication, all the necessary information can be provided by the Socket class alone for the sole purpose of communication. No matter it is on server side or client side.

As you can see from the above link, server side use the following code to acquire its own Socket instance. That is, another socket is created on the same server local port and the client port pair.

Then, server use this Socket instance to talk to the client.

And to make the picture complete, below code snippet shows client's Socket instance.

So if Socket can do it all already, why do we still need the ServerSocket?

This is because of the working paradigm of communication over TCP/IP protocol.

When 2 programs talk over TCP/IP, usually one will passively listen/wait on a <IP:port> and the other one will actively connect to it.

So you can see, at this very starting phase of the communication, the 2 sides have very different behaviors. So 2 different classes are used to reflect this difference.

• Socket class encapsulates the behavior of the active side. (a.k.a. the client)
• ServerSocket class encapsulates the behavior of the passive side (a.k.a. the server)

Once the ServerSocket accomplished its listening task and detected an incoming connection, it will accept() it and create a new Socket instance to facilitate the communication.

Similarily, in java.nio package, you will find ServerSocketChannel and SocketChannel classes. And still, they behave like this:

ServerSocketChannel -------------> SocketChannel
                      accept()

So, to some extent, I agree with @JohnK as he pointed out in the comment, it's more or less just a 6-letter difference.

Solution 2

why socket.read reads the data from serverside

Because it is reading the data sent by the server through the network, it is not reading directly the server filesystem or resouces ( db , ram or anything like that ) it is reading the data that was already processed by the ServerSocket.

Think about the Socket as your web browser and the ServerSocket as the remote webserver.

When you request an image, page, etc, the webserver ( The ServerSocket ) writes the bytes to the client, in turn the client has to read them ( to know what the webserver sent right? ) and process them by displaying them to the final user.

The same happend with ServerSocket/Socket but at a lower level. The socket reads information from the ServerSocket.

Does it make sense?

Solution 3

First of all, let's clarify what IS Socket look like: in a common case, Socket is a concatenation of IP and port via :, for example: 127.0.0.1:8080.

So, you decided to make client-server application using Socket. There's nothing too much complicated. Here's short explanation about making connection between client and server:

1. First of all, let's clarify that fact, that our client have his own Socket and knows server IP address and port. For server there are provided only ServerSocket and port. In both cases port are the same number between 0 and 65535.

2. So, we decided to connect our client to our server:

   • client creates his Socket clientSocket object with known IP and port of our server.

   • server got incoming connection request with his ServerSocket.accept() method, which generates new Socket newClientSocket object (still on a server side (!) ).

   • Further data exchanging goes via clientSocket and newClientSocket objects (not between clientSocket and ServerSocket).

Here is almost perfect picture to understand the basic connection process (keep in mind, that Socket object on Client at this picture - same objects).

After you've made this simple structure, you need to open two streams on both Client.clientSocket and Server.newClientSocket sides for reading and writing information.

Solution 4

java.net.ServerSocket

This class implements server sockets. A server socket waits for requests to come in over the network. It performs some operation based on that request, and then possibly returns a result to the requester.

java.net.Socket

This class implements client sockets (also called just "sockets"). A socket is an endpoint for communication between two machines.

Solution 5

Take a look at http://java.sun.com/docs/books/tutorial/networking/sockets/

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Comments

[Kalle Richter]

Don't provide information as image which can be text.

[Kalle Richter]

Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference.

[smwikipedia]

@KarlRichter It's for better high-lighting.

[Sym-Sym]

Your Here link led me to the original post. codethat.wordpress.com/2010/01/14/… It is a wonderful socket/serversocket crash tutorial .... Thanks!

[Aniket Jadhav]

that image you attached cleared almost all my doubts. thank you so much.

[Matthias Braun]

"That is, another socket is created on another port." This might be misleading since the new socket uses the same port as the one the server is listening on. If your ServerSocket listens on port 443, the new Socket will also use port 443 and the client's ephemeral port.

[Matthias Braun]

Yes, I'm positive. You can convince yourself by cloning and starting this simple server which uses ServerSocket. Doing curl -k "https://localhost:8443/" will show that the socket (created from accept()) for sending TCP packets to the client and the serverSocket used for accepting new client connections both use 8443 as the local port. The code in question is here.

[smwikipedia]

@MatthiasBraun Thanks. You are correct. I refined the answer. It is the pair of ports defines the socket. The link in my answer also details that.

[wlnirvana]

Would it be better if the Socket class were named as Connection? It seems ServerSocket is really one socket (one IP + one port), whereas upon successfully creation, Socket is actually a TCP connection containing two sockets, i.e. both the client and the server endpoints.

[OAH]

@SuppieRK Assuming I have 100 Server.newClientSocket objects, all of themwill use the same server port for sending and receiving data to their corresponding client-sockets ?