Java Support for developing TCP Network Based Programs

Transcription

1 Java Support for developing TCP Network Based Programs 1

2 How to Write a Network Based Program (In Java) As mentioned, we will use the TCP Transport Protocol. To communicate over TCP, a client program and a server program establish a software connection to one another. Each program binds a software socket to its end of the connection. To communicate, the client and the server each reads from and writes to the socket bound to the connection. 2

3 General Method: Used by Java For the server program, initially, a software structure, called a Server Socket must be created. Once a server socket has been created and started, then the server software invokes the listen method ( accept method ) of the server socket to listens for and accept client connections. Once a client has connected, the server software starts another socket, called a Client Socket, through which the server exchanges data with the connected client. Meanwhile, the server socket continues to monitor for new connection requests. The type of socket used must be TCP socket, because, as discussed previously, we are using TCP. Note: there are UDP sockets, which we will not be using. 3

4 General Method: Used by Java For the client program, initially, a software structure, called a Client Socket must be created and connected to a server. Once a client socket has connected to a server, then the client may exchange data with the server through the client socket. Each program is said to bind a software socket to its end of the connection. To communicate, the client and the server each reads from and writes to the socket bound to the connection. 4

5 General Method: Used by Java Server Side Client Side ServerSocket listen() Socket (For Client 1)... Socket (For Client N) <network> <connection request> <network> <exchange data> <network> <exchange data> Client1 (Host 1) Socket... ClientN (Host N) Socket Each program is said to bind a software socket to its end of the connection. To communicate, the client and the server each reads from and writes to the socket bound to the connection. 5

6 Server Software (Uses Both Socket Types: ServerSocket and Socket) The Server software creates two types of sockets 1. First, the server software creates a ServerSocket to listen for and accept connection requests from remote clients. The class ServerSocket implements one part of the server side of the communications, i.e., to listen for and accept connection requests from clients. 2. After the ServerSocket accepts a connection from a remote client, the server software then creates a Socket to exchange data with the connected client. Input and output communications streams are created under the Socket that is now connected to the client. Client Software (Uses One Socket Type: Socket) Client software creates a Socket to make a connection request with a server. If and when a connection request is accepted, the Socket is used to exchange data with a server. Input and output communications streams are created under the Socket that is now connected to the server. In other words, a Socket provides an interface through which a client program can connect to and exchange data with a server program on the network. Client and Server programs each use their created Socket to exchange data. 6

7 Establishing a Connection in Java The java.net package must be imported. SERVER SOFTWARE Before a connection can be established, the server program must create a ServerSocket and bind it to a port. ServerSocket serversocket = new ServerSocket(port); Once a server socket is created, the protocol stack will listen for client-connection requests. The server software must invoke the accept method, which will wait for the protocol stack to indicate a clientconnection request has been received. Socket asockettoconnecttoaclient = serversocket.accept(); Upon receiving a connection request, the accept() method accepts the connection and returns a Socket, which can then be used to create I/O streams to communicate data with the connected client. CLIENT SOFTWARE Client software creates a Socket to connect to a server: Socket asockettoconnecttoaserver = new Socket(host, port); The arguments, host, port, are the host id and port number of the server, respectively. 7

8 UML Sequence Diagram: Scenario: Two Client-server Connections Server Side Client Side (Client A) Client Side (Client N) Application ServerSocket ClientA:Socket ClientN:Socket Application Socket Application Socket create create listen connect create create connection request connection established connection request create connect message ClientA message ClientN connection established message senddata message senddata 8

9 Exchanging Information In Java (Java.Io) Each program (i.e., both server and client) uses an instance of InputStream to receive messages from the other program. OutputStream to send messages to the other program. output = clientsocket.getoutputstream(); input = clientsocket.getinputstream(); getoutputstream and getinputstream create an instance of InputStream and OutputStream, respectively, and they connect the instance to the previously created socket. Exchanging raw bytes is the lowest level of communication. Java allows higher levels of communication using (1) Primitives and (2) Objects. 9

10 OutputStream output = clientsocket.getoutputstream(); output.write(msg); Passes the msg (Byte) to the OuputStream object, which transmits the byte through the Socket. Previously created Socket. Returns an OuputStream object, which is linked to the Socket. 10

11 InputStream input = clientsocket.getinputstream(); msg = input.read(); Previously created Socket. Returns an InputStream object, which is linked to the Socket. InputStream captures bytes, which have been received through the clientsocket. The read() method returns a byte from the InputStream. read() blocks until a byte is available. If end-ofstream is detected, -1 is returned. 11

12 Exchanging Primitive Variables (such as int, float, and double) Creates a DataOutputStream object (in this case, named output). This object acts as a converter: it accepts a primitive type, converts the primitive to a sequence of raw bytes (a process called Serialization), and then passes the sequence to the OutputStream (which is provided in the constructor s argument). output = new DataOutputStream ( clientsocket.getoutputstream() ); This method creates an OutputStream object, and returns a reference to it, which is linked with the previously created clientsocket. This reference is passed to the DataOutputStream constructor. 12

13 Creates an ObjectOutputStream object. This object acts as a converter: it accepts any Java object that supports serialization, converts the object to a sequence of raw bytes (called Serialization), and then passes the sequence to the OutputStream (which is provided in the constructor s argument). output = new ObjectOutputStream ( clientsocket.getoutputstream() ); This method creates an OutputStream object, and returns a reference to it, which is linked with the clientsocket. This reference is passed to the ObjectOutputStream constructor. 13

14 Sending and Receiving Messages Without any filters (raw bytes) output.write(msg); msg = input.read(); Using the Primitive Filter with Serialization (e.g.:) output.writedouble(msg); msg = input.readdouble(); Using the Object Filter with Serialization output.writeobject(msg); msg = input.readobject(); 14

15 Concurrency Requirements of the Server (Thread 1: Server-User Interaction) One thread is required to interact with the server user. Many (but not all) server programs allow a user to interact with the server software. The user may issue commands to the server software, such as Start/Stop listen, restart, shutdown, display list of connected clients, kill a client. Without this thread the server program may not be able to interact satisfactorily with the user if the program spends most of its time listening for client connections or exchanging data with a connected client. Note that this thread s processing of any user commands should not take long; else the thread would not be able to interact with the user satisfactorily. Some Server programs don t have a requirement to have a server user. These servers, therefore, don t require a separate thread to interact with a user. 15

16 Thread 1: Server-User Interaction Core Code public void run() { //To be run as a concurrent thread. String userinput = "no input"; while (true) { try { userinput = console.readline(); //Waits for user input } catch (IOException e) { System.err.println( Network Error, exiting program."); System.exit(1); } myusercommandhandler.execute(userinput); //Even though this is a concurrent thread, execute method //should not take too long, else user-interface will be //non-responsive. } } 16

17 Concurrency Requirements of the Server (Thread 2: Listening for Connection Requests) One thread is required to listen for and accept client connections. Without this thread the server program may not be able to listen for and accept client connections. For example, if the server spends all of its time interacting with the server user, it may not have time to respond to a connection request, requested by a client. Once a connection has been established, this thread passes the connection information to another thread to handle communications with the connected client, and then it continues listening for new connections. 17

18 Thread 2: Listening for Connection Requests public void run() { while (true) { try { Core Code clientsocket = serversocket.accept(); //Next: send clientsocket to a thread that will //handle communications with the connected client. } } } catch (IOException e) { //Respond accordingly 18

19 Concurrency Requirements of the Server (Thread n: Connection To Clients) When a client connection is accepted, a connection-to-client thread is required to interact exclusively with that client. If there are n clients connected, there will be n connection-to-client threads, one thread for each connected client. A connection-to-client thread essentially monitors incoming events at the socket: looking for client messages or responding to network disconnections. Whenever a connected client sends a message to the server, the thread captures the message and hands the message off to another server program object, accordingly. Note that this server object is part of the thread, and so it should not take long to process the message, else subsequent messages from the client may be missed or their processing delayed. When an unexpected network disconnection occurs, the stack will detect it and notify the thread, which will handle it, appropriately. 19

20 Thread n: Connection to Clients Core Code public void run() { byte msg; String thestring; while (stopthisthread == false) { try { msg = (byte) input.read(); thestring = bytetostring(msg); myclientcommandhandler.execute(this,thestring); } catch (IOException e) { //handle exception appropriately; } } } 20

21 Summary: Server Program Threads In general, a server program needs threads: One thread to interact with a server-user; One thread to listen for and accept client connection requests; n connection-to-client threads, one thread to interact with each connected client. 21

22 Activities of a Server: Represented by an UML State Diagram States of the Server as a Whole: Application starts here Initializing main() thread starts here Open Socket Start Listening Interacting with User Listen thread starts here start listening Waiting stop listening Waiting for Connections Closing Close Socket Stop Listening accept connection Connection handling threads start here For each connection (n possible threads): Handling a Connection do: react to messages from client handle disconnection terminate 22

23 Concurrency Requirements of the Client (Thread 1: User Interaction) One thread is required to interact with the client-user. Many client programs, such as a WWW browser, an client, or an internet chat client, have a Graphical User Interface (GUI) to a user. While other client programs have a basic command line interface to a user. This client thread essentially monitors the interface to determine if the user is requesting anything, and if so, responds appropriately. Without this thread, the program may not be able to interact with the user satisfactorily. For example, if the client program spends most of its time sending/receiving messages to/from the server, there would be very little time left to interact with the user, such as to accept user commands or display program status. 23

24 Concurrency Requirements of the Client (Thread 2: Handling Server Events) One thread is required to handle events that are received at the socket that is connected to the server. When a connection to a server is established, a thread must be started, whose responsibility is to handle server and network events. A server-event-handler thread essentially monitors incoming events at the socket: looking for server messages or responding to network disconnections. Whenever the server sends a message to the client, the stack captures the message and gives it to the thread, which will then hand off the message to another client program object, accordingly. When an unexpected network disconnection occurs, the stack will detect it and notify the thread, which will handle it, appropriately. Without this thread, the client may not be able to receive server messages or handle network disconnections. For example, if the client spends most of its time interacting with the client-user, messages coming from the server may be lost, or their processing delayed. 24

25 1. Initializes itself 2. Open a connection to a Server 3. Handles events concurrently Thread: Looks for and responds to user events Gets and responds to user input Send a message to the server, as required Thread: Looks for and responds to server events Receives and responds to server messages Handles server disconnection 4. Must cleanly terminate Thread: Looks for and responds to user events. Initialize Open a connection to a Server Fork Join Terminate Thread: Looks for and responds to server events. 25