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TCP/IPThreads

Using TCP/IP with Java

Claus Traulsen

14. November 2007

Claus Traulsen Using TCP/IP with Java Slide 1

TCP/IPThreads

TCP/IPSome BackgroundUsing Java

Threads


TCP/IPThreads

Some BackgroundUsing Java

Network Protocols

7 Application6 Presentation5 Session

4 Transport3 Network2 Data Link1 Physical

OSI reference model

Ordered in layers

Each layer offers some services

Each layer only uses the servicesoffered by the layer below.

Applications only care aboutapplication layer


TCP/IPThreads


TCP/IP

7 Application654 Transport

3 Network2 Host-to network1

TCP/IPreference model

We will use TCP/IP

Cares about routing, failuredetection, . . .

Clients use sockets to connect toserver

Independent of programminglanguage, OS

Read and write on networkresources

Have to define a protocol




TCP/IPThreads


Sockets

socket

bind

listen

accept connect

socket

writeread

write read

close close

Server

Client

Abstraction for networkend-point

Specified by host-addressplus port

Server waits for clients toconnect

Multiple clients can connect

to one server


TCP/IPThreads


Further Information

A. Tannenbaum, Computer Networks

Java ist auch eine Insel, Chap 9, 16

Lectures by Prof. Luttenberger Internet Communications Computer Networks


TCP/IPThreads


TCP/IP and Java

TCP/IP is directly supported by Java

Classes java.net.socket and java.net.ServerSocket Only need host-address and port

Offer input- and output-stream→ just like file IO

Still have to interpret the data


TCP/IPThreads


Example

Data

+inc() : void

+dec() : void

+toString() : String

-sleep(sleeptime : int) : void

-x : int = 0

-y : int = 0

Server

+main(args : String[]) : void

-port : int

-d : Data

1

1

Client

+main(args : String[]) : void

-host : String

-port : int

Server stores data (x,y)

Invariant: y = 2x

Client sends command to

server:+ increment

- decrement

Server returns current values




TCP/IPThreads


Common Problems

Stream Encoding

Make sure sender and receiver use sameencoding

Otherwise, data might look the same, but is notequal.

Line Ending

Java/TCP/IP is independent of OS Network IO is the same as file IO Be aware of \n vs. \r\n


TCP/IPThreads

Multiple Clients

Want to connect multiple clients to the server

Might want to listen to socket while computing

Cannot send and receive at the same time


TCP/IPThreads

Multiple Clients

Want to connect multiple clients to the server

Might want to listen to socket while computing Cannot send and receive at the same time

Use threads


TCP/IPThreads

Threads in Java

Create subclass C of java.lang.Thread

Overwrite method run()

Start new thread with C.start()

The thread terminates, when its run method terminates




TCP/IPThreads

Example

Data

+inc(): void

+dec(): void

+toString(): String

-sleep(sleeptime :int) :void

-x : in t= 0

-y : in t= 0

Server

+main(args:String[]): void

-port:int

-d :Data

1

1

Connection

+Connection(client:void,Data :d) :void+run(): void

-client: java.net.Socket

-d :Data

java.lang.Thread

java.net.Socket

11

11

0..n

1

1

Multiple clients canconnect to the serversimultaneously

A new thread is startedfor each of them

Multiple inc, deccommands coming at thesame time


TCP/IPThreads

Race Condition

Multiple threads access same data

Thread T1 and T2 want to increment x : T1 reads x and increments local register: r 0 := x + 1 T2 reads x and increments local register: r 1 := x + 1

T1 writes x := r 0 T2 writes x := r 1

Ups x = x + 1, not the value we expect!

Might be unlikely, but it occurs

→ Synchronize threads, protect critical sections


TCP/IPThreads

Synchronize

Synchronization easy with java

Declare a method synchronize:

Only one thread can execute it at the same time Other threads are blocked

Can declare a code region synchronized as well

Sycnhronization lock is always the object

To much synchronization is bad for performance!


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