Build your own Java library
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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. Introduction to libraries 3
3. Design issue: Encapsulation 5
4. Design issue: Extensibility 8
5. Design issue: Debugging information 10
6. The source code 11
7. Wrapup 15
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Section 1. Tutorial tips
Should I take this tutorial?
Code reuse is one of the Holy Grails of computer programming. Writing code that can be
easily reused is a difficult skill, but certainly one that can be mastered.
In this tutorial, you will learn:
* How the Java language can help you make a good, reusable library
* A few of the key principles of good library design
* The most efficient way to implement each of these ideas in the Java language
To illustrate these ideas, we'll walk through the design of a simple library.
To use this tutorial successfully, you'll need to have a basic understanding of Java
programming, including the ability to create, compile, and execute simple command-line
Java programs.
Objectives
When you complete this tutorial, you will:
* Know the major obstacles to code reuse
* Have a working knowledge of the ideas behind library design
* Know how to implement these ideas in the Java language
* Possess an excellent design of a Java library as a sample for future projects
Getting help
For questions about the content of this tutorial, please contact the author, Greg Travis, at
mito@panix.com .
Greg Travis is a freelance programmer living in New York City. His interest in computers can
be traced back to that episode of "The Bionic Woman" where Jamie is trying to escape a
building whose lights and doors are controlled by an evil artificial intelligence, which mocks
her through loudspeakers. Greg is a firm believer that, when a computer program works, it's
a complete coincidence.
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Section 2. Introduction to libraries
What is a library?
A library is a reusable software component that saves developers time by providing access
to the code that performs a programming task. Libraries exist to assist with many different
types of tasks.
Library design is difficult. It's easy to take an algorithm you've written and call it a library, but
it's harder to structure it in such a way that it can fit into someone else's program and still
perform the tasks it is designed to do without interfering with the original program's operation.
Most modern languages try to help the programmer create good libraries, and the Java
language is no exception. In this tutorial, you'll learn how the Java language can help you
make a good, reusable library.
Why reuse code?
There is no sense in reinventing the wheel. If there is code that can be reused, it should be.
Also, modern applications have become so large that it's inconceivable to even consider
writing them without the use of libraries.
But that's not the only reason. Even if you have no intention of distributing a piece of code,
creating it as a library can help you think more clearly about what it's supposed to do, thus
helping refine the design process.
Why aren't libraries used more often?
The answer to this question is simple: if you don't write a library yourself, you have no control
over the decisions that go into the crafting of it. And if the library behaves in a certain way
that is not acceptable, you'll be less likely to use it. You could modify the library to suit your
needs, but that might break compatibility. And, sometimes, rewriting something yourself is
faster (and less stressful) than trying to understand someone else's code.
It is possible to make libraries simpler and more flexible, and therefore more useful for a
wider range of programs. But no library is going to be perfect for every possible application.
Still, it's possible to design and code libraries in such a way as to maximize their ease of use.
Put yourself in the user's shoes
When designing a library, it's important to put yourself in the place of the person who is going
to be using the library. Ask these questions:
* What problem will the library solve? (For example, what should the end result be?)
* What details do the users care about? (For example, how do they want to get to the end
results?)
* What details would the users rather forget about? (For example, what elements and
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operations do they want other parts of the program to handle?)
In the final analysis, good library design involves a bit of psychology. Sure, your code works
well, but does it seem like it works well? When people hear or read about it, will they say to
themselves, "Wow, that's exactly what I need and exactly how I would have done it myself!"
A sample library
Our objective is to learn how the Java language can help to build an effective library. To
achieve this goal, we'll discuss the design of a simple library that facilitates implementing
network servers.
When creating a network server, there are a number of issues to consider:
* Listening on a socket
* Accepting connections
* Getting access to the streams represented by a connection
* Processing the incoming data in some way, and sending back a response
Our sample library is going to take care of the first three issues, leaving the fourth
consideration for the library user to implement.
The main class in the library is called Server, and the test class that uses it is called
EchoServer. The latter implements a simple, serial-connection server that takes data from
the client and sends it right back. We'll talk about EchoServer on page 11in Section 6.
Design issues and practical implementations
The following sections cover the design issues of encapsulation, extensibility, and
debugging. Each section deals with a single design issue and then discusses how the issue
can be handled using the Java language.
The design principles themselves are not language dependent -- they can be applied to the
design of libraries in any language.
Nevertheless, pay special attention to the implementation details; sometimes, they can
illustrate a theoretical point better than a conceptual description can.
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Section 3. Design issue: Encapsulation
What is encapsulation?
A library should function as a tight, self-contained unit rather than a scattered collection of
objects whose functions and relationships are unclear.
The practice of making a library self-contained is called encapsulation.
What is a package?
The Java language provides an explicit mechanism for class-file-level encapsulation:
packages. A package is a group of Java class files that are stored in a single directory; a
package has a namespace to itself.
The advantage of giving a set of classes its own namespace is that you don't have to worry
about namespace conflicts.
In our example, the main class has a fairly common name -- Server. It wouldn't be
surprising if we eventually ran into a class in another library that had the same name. Putting
the class in its own namespace takes care of any conflicts this might cause.
In the next few panels, I'll illustrate how to put a class into a package.
Packages have names
Every package has a name that consists of a set of strings separated by periods, such as
java.lang or javax.swing.plaf.basic.
In fact, the full name of any class consists of the name of its package, followed by its own
name, as in java.lang.Object or javax.swing.plaf.basic.BasicMenuBarUI.
Note that there is a special package called the default package. If you don't put your class
into a specific package, then it is assumed to be in the default package.
Package names correspond to directories
Each package maps directly onto a subdirectory in the filesystem. This correspondence
allows the Java virtual machine (JVM) to find classes at run time.
You can convert a package name into a subdirectory path by replacing the periods with "/"
(or whatever symbol your operating system uses to separate directory names). For example,
java.lang.Object is stored in the file java/lang/Object.java.
A class in the default package is placed in the current directory.
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Classes declare their packages
To make sure a class is in the right package and in the right directory, a class must declare
he package that it is in.
The declaration looks like this:
// Server.java
package mylib;
public class Server implements Runnable
{
// ...
Other classes can import packages
To use a class that lives in another package, you can reference it by its full name, such as
mylib.Server. For example:
mylib.Server server = new mylib.Server( portNum );
It can be tedious to type out the full package name for a class, so you can take a shortcut
and import the package like this:
import mylib.*;
// ...
Server server = new Server( portNum );
It's also possible to import a single class:
import mylib.Server;
// ...
Server server = new Server( portNum );
Choosing public classes
The Java language also allows you to decide which classes in a package are visible to the
outside.
A public class can be accessed by code in any other package, while a private class can only
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be used within its own package.
It's important to make public only those classes you want people to use directly.
Designing the interface to a public class takes more care than designing one for a private
class because the interface for a public class must be crafted to be as clear as possible for
the user. A private class doesn't need to have as tidy an interface.
Making a class public requires using the public keyword on the first line of the class
declaration:
// Server.java
package mylib;
import java.io.*;
import java.net.*;
public class Server implements Runnable
{
To make a class private, you simply leave the public keyword off. In our Server example,
there's a private class in the same package called Reporter, which reports periodically on
the condition of the Server object. Here is how it is declared:
// Reporter.java
package mylib;
class Reporter implements Runnable
{
Encapsulation summary
As we've seen, the Java language provides a few features that were created specifically for
defining the boundaries of your code.
By dividing code into packages and by defining classes as public or private, you can make
precise decisions about what the user of the library must deal with when they are using your
library.
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Section 4. Design issue: Extensibility
Inheritance
Encapsulation defines boundaries around a piece of code. All object-oriented programming
languages provide a mechanism for extending a piece of code without violating these
boundaries. In the Java language, this mechanism is provided by inheritance.
Customization through inheritance
The main class in our example library is called Server. If you look at the source code for this
class, you'll see that, by itself, it does nothing.
The main loop (which runs in a separate thread) listens for incoming connections. When one
comes in, it hands it off to a method called handleConnection(), as shown here:
// subclass must supply an implementation
abstract public void handleConnection( Socket s );
There is no default implementation, and so the class is declared abstract, which signals
the user to supply an implementation. EchoServer implements this method:
// This is called by the Server class when a connection
// comes in. "in" and "out" come from the incoming socket
// connection
public void handleConnection( Socket socket ) {
try {
InputStream in = socket.getInputStream();
OutputStream out = socket.getOutputStream();
// just copy the input to the output
while (true)
out.write( in.read() );
} catch( IOException ie ) {
System.out.println( ie );
}
}
You can think of this process as a type of customization: the class that does most of the work
(Server) is incomplete; the subclass (EchoServer) completes it by adding an
implementation of the method handleConnection().
Plan for hooks
It's important to think ahead to the kinds of customizations a user of your library is going to
want to make. The previous example was obvious -- you have to provide a way for the
subclass to actually use the incoming connections or your program won't do anything useful.
But if you don't think ahead, some other customizations might not occur to you. Instead,
they'll occur to the user, who will wish that you had thought of them in the first place.
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In our sample library, there is a method in Server called cleanUp(). This method is called
by the server's background thread just before it exits. The base server doesn't need to do
anything in this method, so it is empty:
// Put any last-minute clean-up stuff in here
protected void cleanUp() {
}
Note: Do not declare this method abstract, as it would require the user to implement this
method, which, in some cases, isn't necessary.
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Section 5. Design issue: Debugging information
Why should you plan for debugging?
A scenario: Your library works perfectly. The user needs to do nothing more than learn the
API and use the code. Right?
Wrong. This scenario never (or rarely) happens. Bugs are unavoidable; debugging is
inevitable.
At some point, a user is going to have a problem and will need to know exactly what is going
on inside the library. The bug could be in the library itself, or it could be a bug in the user's
code that is only triggered inside the library.
A little text goes a long way
If you supply the source with your library, a user might consider using a debugger, but you
shouldn't count on that.
A better way to counter this inevitable problem is to add debugging println() statements
to your code to force each piece of the code to report on what it is doing. Watching this
information can often help the user know where something is going wrong.
The following example demonstrates this technique. The user's code can install a
PrintStream object by calling the static method Server.setDebugStream(). Once
implemented, the debug information will be sent to the provided stream.
// set this to a print stream if you want debug info
// sent to it; otherwise, leave it null
static private PrintStream debugStream;
// call this to send the debugging output somewhere
static public void setDebugStream( PrintStream ps ) {
debugStream = ps;
}
You can then sprinkle your library with calls to the debug() method:
// send debug info to the print stream, if there is one
static public void debug( String s ) {
if (debugStream != null)
debugStream.println( s );
}
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Section 6. The source code
Our sample library
Before we wrap up, take a look at the complete source code, which includes EchoServer,
mylib.Server, and mylib.Reporter.
EchoServer
// $Id$
import java.io.*;
import java.net.*;
import mylib.*;
public class EchoServer extends Server
{
public EchoServer( int port ) {
// The superclass knows what to do with the port number, we
// don't have to care about it
super( port );
}
// This is called by the Server class when a connection
// comes in. "in" and "out" come from the incoming socket
// connection
public void handleConnection( Socket socket ) {
try {
InputStream in = socket.getInputStream();
OutputStream out = socket.getOutputStream();
// just copy the input to the output
while (true)
out.write( in.read() );
} catch( IOException ie ) {
System.out.println( ie );
}
}
protected void cleanUp() {
System.out.println( "Cleaning up" );
}
static public void main( String args[] ) throws Exception {
// Grab the port number from the command-line
int port = Integer.parseInt( args[0] );
// Have debugging info sent to standard error stream
Server.setDebugStream( System.err );
// Create the server, and it's up and running
new EchoServer( port );
}
}
mylib.Server
// $Id$
package mylib;
import java.io.*;
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import java.net.*;
abstract public class Server implements Runnable
{
// the port we'll be listening on
private int port;
// how many connections we've handled
int numConnections;
// the Reporter that's reporting on this Server
private Reporter reporter;
// set this to true to tell the thread to stop accepting
// connections
private boolean mustQuit = false;
public Server( int port ) {
// remember the port number so the thread can
// listen on it
this.port = port;
// the constructor starts a background thread
new Thread( this ).start();
// and start a reporter
reporter = new Reporter( this );
}
// this is our background thread
public void run() {
ServerSocket ss = null;
try {
// get ready to listen
ss = new ServerSocket( port );
while( !mustQuit ) {
// give out some debugging info
debug( "Listening on "+port );
// wait for an incoming connection
Socket s = ss.accept();
// record that we got another connection
numConnections++;
// more debugging info
debug( "Got connection on "+s );
// process the connection -- this is implemented
// by the subclass
handleConnection( s );
}
} catch( IOException ie ) {
debug( ie.toString() );
}
debug( "Shutting down "+ss );
cleanUp();
}
// the default implementation does nothing
abstract public void handleConnection( Socket s );
// tell the thread to stop accepting connections
public void close() {
mustQuit = true;
reporter.close();
}
// Put any last-minute clean-up stuff in here
protected void cleanUp() {
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}
// everything below provides a simple debug system for
// this package
// set this to a print stream if you want debug info
// sent to it; otherwise, leave it null
static private PrintStream debugStream;
// we have two versions of this ...
static public void setDebugStream( PrintStream ps ) {
debugStream = ps;
}
// ... just for convenience
static public void setDebugStream( OutputStream out ) {
debugStream = new PrintStream( out );
}
// send debug info to the print stream, if there is one
static public void debug( String s ) {
if (debugStream != null)
debugStream.println( s );
}
}
mylib.Reporter
// $Id$
package mylib;
class Reporter implements Runnable
{
// the Server we are reporting on
private Server server;
// our background thread
private Thread thread;
// set this to true to tell the thread to stop accepting
// connections
private boolean mustQuit = false;
Reporter( Server server ) {
this.server = server;
// create a background thread
thread = new Thread( this );
thread.start();
}
public void run() {
while (!mustQuit) {
// do the reporting
Server.debug( "server has had "+server.numConnections+" connections" );
// then pause a while
try {
Thread.sleep( 5000 );
} catch( InterruptedException ie ) {}
}
}
// tell the background thread to quit
public void close() {
mustQuit = true;
}
}
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Section 7. Wrapup
Revisiting the design issues
In this tutorial, we've looked at some concrete ways in which the Java language eases the
creation of a good, reusable library. We've also explored some of the theory upon which
these language features are based.
A good library is one that conveys its internal structure clearly so that the user knows how to
use it without knowing how it works internally.
Encapsulation serves the purpose of tightening and clarifying the interface to your code,
which makes it more reliable and easier to understand.
Extensibility allows you to create a library that does a single thing well, leaving the user to
fill in the missing parts and to re-purpose the library to meet user-specific needs. Because
the Java language is object oriented, the method is clear: inheritance provides a way to
customize code without having to know all the details of the code you are customizing.
Finally, providing a way to get debugging information from your code helps users find their
own bugs (or yours!), again without having to know every detail of the library's
implementation.
Resources
Following is a listing of additional readings to help solidify your understanding of libraries:
* Take a look at " Techniques for adding trace statements to your Java application "
(developerWorks, January 2001) for some more advanced ideas about adding
extensive debugging output to your library.
* Peter Haggar offers advice for how to use the Java libraries judiciously (IBM
PartnerWorld for Developers).
* Frameworks are an excellent conceptual tool for designing reusable code. "
Frameworks save the day " (developerWorks, October 2000) explores frameworks using
Java technology.
* The October 1998 issue of Web Techniques is dedicated to Java frameworks.
* Brush up on import and packages in the The Java Language Specification .
Your feedback
Please let us know whether this tutorial was helpful to you and how we could make it better.
We'd also like to hear about other tutorial topics you'd like to see covered. Thanks!
For questions about the content of this tutorial, contact the author, Greg Travis, at
mito@panix.com .
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