IBM DeveloperWorks Java Sockets.pdf

Java sockets 101

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Table of Contents

If you're viewing this document online, you can click any of the topics below to link directly to that section.

1. Tutorial tips .............................................................. 2

2. Socket basics ........................................................... 3

3. An undercover socke t ................................................. 7

4. A simple example ...................................................... 11

5. A multithreaded example ............................................. 19

6. A pooled example ...................................................... 23

7. Sockets in real life ...................................................... 29

8. Summary ................................................................ 33

9. Appendix ................................................................. 35

Java sockets 101

Page 1 of 40

Presented by developerWorks, your source for great tutorials ibm.com/developerWorks

Section 1. Tutorial tips

Should I take this tutorial?

Sockets, which provide a mechanism for communication between two computers, have been

around since long before the Java language was a glimmer in James Gosling's eye. The

language simply lets you use sockets effectively without having to know the details of the

underlying operating system. Most books that focus on Java coding either fail to cover the

topic, or leave a lot to the imagination. This tutorial will tell you what you really need to know to

start using sockets effectively in your Java code. Specifically, we'll cover:

* What sockets are

* Where they fit into the structure of programs you're likely to write

* The simplest sockets implementation that could possibly work -- to help you understand

the basics

* A detailed walkthrough of two additional examples that explore sockets in multithreaded

and pooled environments

* A brief discussion of an application for sockets in the real world

If you can describe how to use the classes in the java.net package, this tutorial is probably a

little basic for you, although it might be a good refresher. If you have been working with

sockets on PCs and other platforms for years, the initial sections might bore you. But if you are

new to sockets, and simply want to know what they are and how to use them effectively in your

Java code, this tutorial is a great place to start.

Getting help

For questions about the content of this tutorial, contact the authors, Roy Miller (at

rmiller@rolemodelsoft.com ) or Adam Williams (at awilliams@rolemodelsoft.com ).

Roy Miller and Adam Williams are Software Developers at RoleModel Software, Inc. They

have worked jointly to prototype a socket-based application for the TINI Java platform from

Dallas Semiconductor. Roy and Adam are currently working on porting a COBOL financial

transaction system to the Java platform, using sockets.

Prior to joining RoleModel, Roy spent six years with Andersen Consulting (now Accenture)

developing software and managing projects. He co-authored Extreme Programming Applied:

Playing to Win (Addison-Wesley XP Series) scheduled for publication in October 2001.

Java sockets 101

Page 2 of 40

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Section 2. Socket basics

Introduction

Most programmers, whether they're coding in the Java language or not, don't want to know

much about low-level details of how applications on different computers communicate with

each other. Programmers want to deal with higher-level abstractions that are easier to

understand. Java programmers want objects that they can interact with via an intuitive

interface, using the Java constructs with which they are familiar.

Sockets live in both worlds -- the low-level details that we'd rather avoid and the abstract layer

we'd rather deal with. This section will explore just enough of the low-level details to make the

abstract application understandable.

Computer networking 101

Computers operate and communicate with one

another in a very simple way. Computer chips are

a collection of on-off switches that store and

transmit data in the form of 1s and 0s. When

computers want to share data, all they need to do

is stream a few million of these bits and bytes back

and forth, while agreeing on speed, sequence,

timing, and such. How would you like to worry

about those details every time you wanted to

communicate information between two

applications?

To avoid that, we need a set of packaged protocols

that can do the job the same way every time. That

would allow us to handle our application-level work

without having to worry about the low-level

networking details. These sets of packaged

protocols are called stacks . The most common

Java sockets 101

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stack these days is TCP/IP. Most stacks (including

TCP/IP) adhere roughly to the International

Standards Organization (ISO) Open Systems

Interconnect Reference Model (OSIRM). The

OSIRM says that there are seven logical layers in

a reliable framework for computer networking (see

the diagram). Companies all over have contributed

something that implements some of the layers in

this model, from generating the electrical signals

(pulses of light, radio frequency, and so on) to

presenting the data to applications. TCP/IP maps

to two layers in the OSI model, as shown in the

diagram.

We won't go into the details of the layers too much,

but we want you to be aware of where sockets fit.

Where sockets fit

Sockets reside roughly at the Session Layer of the

OSI model (see the diagram). The Session Layer

is sandwiched between the application-oriented

upper layers and the real-time data communication

lower layers. The Session Layer provides services

for managing and controlling data flow between

two computers. As part of this layer, sockets

provide an abstraction that hides the complexities

of getting the bits and bytes on the wire for

transmission. In other words, sockets allow us to

transmit data by having our application indicate

that it wants to send some bytes. Sockets mask

the nuts and bolts of getting the job done.

When you pick up your telephone, you provide

sound waves to a sensor that converts your voice

Java sockets 101

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into electrically transmittable data. The phone is a

human's interface to the telecommunications

network. You aren't required to know the details of

how your voice is transported, only the party to

whom you would like to connect. In the same

sense, a socket acts as a high-level interface that

hides the complexities of transmitting 1s and 0s

across unknown channels.

Exposing sockets to an

application

When you write code that uses sockets, that code

does work at the Presentation Layer . The

Presentation Layer provides a common

representation of information that the Application

Layer can use. Say you are planning to connect

your application to a legacy banking system that

understands only EBCDIC. Your application

domain objects store information in ASCII format.

In this case, you are responsible for writing code at

the Presentation Layer to convert data from

EBCDIC to ASCII, and then (for example) to

provide a domain object to your Application Layer.

Your Application Layer can then do whatever it

wants with the domain object.

The socket-handling code you write lives only at

the Presentation Layer. Your Application Layer

doesn't have to know anything about how sockets

work.

What are sockets?

Now that we know the role sockets play, the question remains: What is a socket? Bruce Eckel

describes a socket this way in his book Thinking in Java :

The socket is the software abstraction used to represent the "terminals" of a connection

between two machines. For a given connection, there's a socket on each machine, and you

can imagine a hypothetical "cable" running between the two machines with each end of the

"cable" plugged into a socket. Of course, the physical hardware and cabling between

machines is completely unknown. The whole point of the abstraction is that we don't have to

know more than is necessary.

In a nutshell, a socket on one computer that talks to a socket on another computer creates a

Java sockets 101

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