path: root/doc/hacking.texinfo

\input texinfo @c -*-texinfo-*-

@c %**start of header
@setfilename hacking.info
@settitle GNU Classpath Hacker's Guide
@c %**end of header

@setchapternewpage none

@ifinfo
This file contains important information you will need to know if you
are going to hack on the GNU Classpath project code.

Copyright (C) 1998, 1999 Free Software Foundation, Inc.

@end ifinfo

@titlepage
@title GNU Classpath Hacker's Guide
@author Aaron M. Renn
@author Paul N. Fisher
@author John Keiser

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1998, 1999 Free Software Foundation, Inc.
@sp 2
Permission is granted to make and distribute verbatim copies of
this document provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
document under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation
approved by the Free Software Foundation.

@end titlepage

@ifinfo
@node Top, Introduction, , (dir)
@top GNU Classpath Hacker's Guide

This document contains important information you'll want to know if you
want to hack on GNU Classpath, the free implementation of the Java standard
class libraries.

This document is definitely a work in progress.
@end ifinfo

@menu 
* Introduction::            An introduction to the Classpath project
* Requirements::            Very important rules that must be followed
* Project Goals::           Goals of the Classpath project
* Volunteering::            So you want to help out
* Programming Tools::       A list of tools you will need for hacking
* Programming Standards::   Standards to use when writing code for Classpath
* Programming Goals::       What to consider when writing code for Classpath
* Portability::             How to ensure your code is portable
* Robustness::              How to make native code robust
* Java Efficiency::         Tips for making Java code faster
* Native Efficiency::       Tips for making native Java code faster
* Specification Sources::   Where to find the Java class library specs
* Naming Conventions::      How files and directories are named in Classpath
* Character Encodings::     How byte to char conversions work in Classpath
@end menu

@node Introduction, Requirements, Top, Top
@comment node-name, next, previous, up
@chapter Introduction

The Classpath Project is a dedicated to providing a 100% free, clean
room implementation of the standard Java class libraries.  Because there
is currently no free implementation of the Java environment, no free
operating system can ship with Java included.  Parts of a free Java
implementation have already been written, including free Java virtual
machines (JVM's) such as @uref{http://www.kaffe.org/,Kaffe} and
@uref{http://www.hungry.com/products/japhar/,Japhar}, and Java compilers
such as @uref{http://www.cs.berkeley.edu/~engberg/guavac/,Guavac}.
However, there is currently no free replacement for Sun's proprietary
libraries.  This Classpath project aims to correct this problem by
supplying a free class library implementation that will allow a 100%
free Java platform to be distributed.  Note that Kaffe now ships with a
partial class library that is also free, so there is more than one group
working towards a common goal.

@node Requirements, Volunteering, Introduction, Top
@comment node-name, next, previous, up
@chapter Requirements

Although Classpath is following an open development model where input
from developers is welcome, there are certain base requirements that
need to be met by anyone who wants to contribute code to this project.
They are mostly unfortunately dictated by legal requirements and are not
arbitrary restrictions chosen by the Classpath team.

You will need to adhere to the following things if you want to donate
code to the Classpath project:

@itemize @bullet
@item
@b{Never under any circumstances refer to Sun's code while working on
Classpath.}  It is best if you have never looked at Sun's code at all.
To reduce temptation, it would be best if you deleted the @samp{src.zip}
file from your JDK distribution.  If you have signed Sun's
non-disclosure statement, then you unfortunately cannot work on
Classpath code at all.  If you have any reason to believe that your code
might be ``tainted'', please say something on the mailing list before
writing anything.  If it turns out that your code was not developed in a
clean room environment, we could be very embarrassed someday in court.
Please don't let that happen.

@item
@b{Never decompile Sun's class libraries.}  While the wording of the
license in Sun's JDK version 1.2 has changed, it not acceptable, under
any circumstances, for a person working on Classpath to decompile Sun's
class libraries.  Allowing the use of decompilation in the Classpath
project would open up a giant can of legal worms, which we wish to
avoid.

@item
Classpath is licensed under the terms of the
@uref{http://www.fsf.org/copyleft/lgpl.html,GNU Library General Public
License}.  To preserve freedom for all users and to maintain uniform
licensing of Classpath, we will not accept code into the main
distribution that is not licensed under these terms.

@item
Classpath is GNU software and this project is being officially sponsored
by the @uref{http://www.fsf.org/,Free Software Foundation}.  Because of
this, the FSF will hold copyright to all code developed as part of
Classpath.  This will allow them to pursue copyright violators in court,
something an individual developer may neither have the time nor
resources to do.  Everyone contributing code to Classpath will need to
sign a copyright assignment statement.  Additionally, if you are
employed as a programmer, your employer may need to sign a copyright
waiver disclaiming all interest in the software.  This may sound harsh,
but unfortunately, it is the only way to ensure that the code you write
is legally yours to distribute.
@end itemize

@node Volunteering, Project Goals, Requirements, Top
@comment node-name, next, previous, up
@chapter Volunteering to Help

The Classpath project needs volunteers to help us out.  People are
needed to write unimplemented Java packages, to test Classpath on
various platforms, and to port it to platforms that are currently
unsupported.

While pretty much all contributions are welcome (but see 
@pxref{Requirements}) it is always preferable that volunteers do the
whole job when volunteering for a task.  So when you volunteer to write
a Java package, please be willing to do the following:

@itemize @bullet
@item
Implement a complete drop-in replacement for the particular package.
That means implementing any ``internal'' classes.  For example, in the
java.net package, there are non-public classes for implementing sockets.
Without those classes, the public socket interface is useless.  But do
not feel obligated to completely replace all of Sun's functionality at
once.  For example, in the java.net package, there are different types
of protocol handlers for different types of URL's.  Not all of these
need to be written at once.

@item
Please write complete and thorough javadoc comments for every public and
protected method and variable.  These should be superior to Sun's and
cover everything about the item being documented.

@item
Please write a regression test package that can be used to run tests of
your package's functionality.
@end itemize

Nobody likes to write documentation and test cases, but they are vital
to a complete and robust product.  Writing them as you go is much easier
than going back at the end and adding them.

@node Project Goals, Programming Tools, Volunteering, Top
@comment node-name, next, previous, up
@chapter Project Goals

The goal of the Classpath project is to produce a
@uref{http://www.fsf.org/philosophy/free-sw.html,free} implementation of
the standard class library for Java.  However, there are other more
specific goals as to which platforms should be supported.

Classpath is targeted to support the following operating systems:

@enumerate
@item
Free operating systems.  This includes GNU/Linux, GNU/Hurd, and the free
BSDs.

@item
Other Unix like operating systems.

@item
Platforms which currently have no Java support at all.

@item 
Other platforms such as MS-Windows.
@end enumerate

While free operating systems are the top priority, the other priorities
can shift depending on whether or not there is a volunteer to port
Classpath to those platforms and to test releases.

Eventually we hope the Classpath will support all JVM's that provide JNI
support.  However, the top priority is free JVM's.  The JVM support
priority list is:

@enumerate
@item
Japhar
@item
Kaffe
@item
Sun's JDK
@item
Other JNI Compliant JVM's.
@end enumerate

As with OS platform support, this priority list could change if a
volunteer comes forward to port, maintain, and test releases for a
particular JVM.  Kaffe is now developing its own class library, so the
priority of supporting that platform is not as high as for Japhar.

The initial target version for Classpath is Java 1.1.  Java 1.2 can be
implemented if desired, but please do not create classes that depend on
1.2 features in other packages.

@node Programming Tools, Programming Standards, Project Goals, Top
@comment node-name, next, previous, up
@chapter Programming Tools

If you want to hack on Classpath, you should download, install, and
familiarize yourself with the following tools:

@itemize @bullet
@item
CVS 1.9
@item
automake 1.3
@item
autoconf 2.12
@item
dejagnu 1.3
@item
libtool 1.2
@item
GNU m4 1.4
@item
perl 5.X
@item
GNU MP 2.0.2
@end itemize

All of these tools are available from
@uref{ftp://prep.ai.mit.edu/pub/gnu/,prep.ai.mit.edu} via anonymous ftp.
With the exception of perl, they are fully documented with texinfo
manuals.  Texinfo can be browsed with the Emacs editor, or with the text
editor of your choice.

Here is a brief description of the purpose of those tools.

@table @b

@item CVS  
A version control system that maintains a centralized Internet
repository of all code in the Classpath system.  Access to the
repository requires an account.  Contact Paul Fisher
(@email{rao@@gnu.org}) for details.

@item dejagnu  
A package for automating regression test suites for programs.  Your
regression test package should work with this.

@item automake  
This tool automatically creates Makefile.in files from Makefile.am
files.  The Makefile.in is turned into a Makefile by autoconf.  Why use
this?  Because it automatically generates every makefile target you
would ever want (clean, install, dist, etc) in full compliance with the
GNU coding standards.  It also simplifies Makefile creation in a ton of
different ways I can't describe here.  Read the docs for more info.

@item autoconf  
Automatically configures a package for the platform on which it is being
built and generates the Makefile for that platform.

@item libtool  
Handles all of the zillions of hairy platform specific options needed to
build shared libraries.

@item m4
The free GNU replacement for the standard Unix macro processor.
Proprietary m4 programs are broken and so GNU m4 is required for
autoconf to work.

@item perl
Larry Wall's scripting language.  It is used internally by automake.

@item MP
Required for java.lang.Float, java.lang.Double, java.math.BigInteger,
and java.math.BigDecimal.

@end table

@node Programming Standards, Programming Goals, Programming Tools, Top
@comment node-name, next, previous, up
@chapter Programming Standards

For C code, follow the
@uref{http://www.fsf.org/prep/standards_toc.html,GNU Coding Standards}.
The standards also specify various things like the install directory
structure.  These should be followed if possible.

For Java code, please follow the
@uref{http://java.sun.com/docs/codeconv/html/CodeConventionsTOC.doc.html,Sun
programming standards}.  As an exception, do not feel obligated to
following their bracket and indentation style if you consider it to be
wrong.

For documentation comments, please follow
@uref{http://java.sun.com/products/jdk/javadoc/writingdoccomments.html,How
to Write Doc Comments for Javadoc}.

@node Programming Goals, Portability, Programming Standards, Top
@comment node-name, next, previous, up
@chapter Programming Goals

When you write code for Classpath, write with three things in mind, and
in the following order: portability, robustness, and efficiency.

If efficiency breaks portability or robustness, then don't do it the
efficient way.  If robustness breaks portability, then bye-bye robust
code.  Of course, as a programmer you would probably like to find sneaky
ways to get around the issue so that your code can be all three ... the
following chapters will give some hints on how to do this.

@node Portability, Robustness, Programming Goals, Top
@comment node-name, next, previous, up
@chapter Portability

The ultimate portability goal would be to create:

@enumerate
@item
a binary set for each platform that works across all VMs on that
platform
@item
a single classfile set that work across all VMs on all platforms that
support the binary set.
@end enumerate

With Java code, this is no problem.  You end up delegating VM- or
platform-specific stuff to native code anyway.  Unfortunately, it is
impossible to write native code that works out of the box on multiple
VMs, even on a given platform.  The APIs Sun has created just do not
give the flexibility required to do that if you are implementing the
java.* hierarchy.

The native libraries we use in Classpath are JNI, JVMDI, and VMI.  JNI
you should be familiar with; introduced in Java 1.1, it is the basis for
our native code.  JVMDI is a 1.2 concoction, and thus we can only
support it on 1.1 VMs that we have source access to (i.e. Japhar and
Kaffe).  VMI is our own invention entirely; it is where we push the
VM-specific functionality that JNI and JVMDI do not support.

However, using JVMDI and VMI in 1.1 breaks the "ideal goal" for the
project.  With these two in the mix, Classpath's portability becomes:

@enumerate
@item
a JVMDI and VMI lib per VM per platform;
@item
a binary set for each platform that works across all VM/platform combos
supporting the JVMDI/VMI;
@item
a single classfile set that works across all VM / platform combos that
support both of the above.
@end enumerate

When writing, therefore, you should always keep shy of anything
VM-specific yourself, and talk to the VM using the VMI.  If you need a
VM-specific function that is not supported by JNI or JVMDI, write a new
VMI function!

Note that the preferred method is not to use the JVMDI or VMI at
@emph{all}, but to use only JNI.  If you can write it without the VMI,
your code will be quicker to port to new VMs.

There is another issue, however, and that is the efficiency issue.  Some
things can be done using JNI only, but they are a ton slower than they
would be if you used the VMI or talked directly to the VM.

In these cases, the preferred method in Classpath is to create a
@emph{library}.  Hide the implementation of the functions from the
caller.  If the library can be written using only JNI, no matter how
inefficient, you should write a version of it for that.  Portability,
remember, is the ultimate goal, and the closer we are to it, the better
off we are.

@node Robustness, Java Efficiency, Portability, Top
@comment node-name, next, previous, up
@chapter Robustness

Native code is very easy to make non-robust.  (That's one reason Java is
so much better!)  Here are a few hints to make your native code more
robust.

Always check return values for standard functions.  It's sometimes easy
to forget to check that malloc() return for an error.  Don't make that
mistake.  (In fact, use JCL_malloc() in the jcl library instead--it will
check the return value and throw an exception if necessary.)

Always check the return values of JNI functions, or call
@code{ExceptionOccurred} to check whether an error occurred.  You must
do this after @emph{every} JNI call.  JNI does not work well when an
exception has been raised, and can have unpredictable behavior.

Throw exceptions using JCL_ThrowException.  This guarantees that if
something is seriously wrong, the exception text will at least get out
somewhere (even if it is stderr).

Check for null values of jclasses before you send them to JNI functions.
JNI does not behave nicely when you pass a null class to it: it
terminates Java with a "JNI Panic."

In general, try to use functions in native/lib/jcl.h.  They check
exceptions and return values and throw appropriate exceptions.

@node Java Efficiency, Native Efficiency, Robustness, Top
@comment node-name, next, previous, up
@chapter Java Efficiency

For methods which explicitly throw a NullPointerException when an
argument is passed which is null, per a Sun specification, do not write
code like:

@example
int strlen(String foo) throws NullPointerException @{
  if (foo == null)
    throw new NullPointerException("foo is null");
  return foo.length();
@}
@end example

Instead, the code should be written as:

@example
int strlen(String foo) throws NullPointerException @{
  return foo.length();
@}
@end example

Explicitly comparing foo to null is unnecessary, as the virtual machine
will throw a NullPointerException when length() is invoked.  Classpath
is designed to be as fast as possible -- every optimization, no matter
how small, is important.

@node Native Efficiency, Specification Sources, Java Efficiency, Top
@comment node-name, next, previous, up
@chapter Native Efficiency

You might think that using native methods all over the place would give
our implementation of Java speed, speed, blinding speed.  You'd be
thinking wrong.  Would you believe me if I told you that an
@emph{interpreted} Java method is typically about three and a half times
@emph{faster} than the equivalent native method?  This is true even for
a totally blank method.

Bottom line: JNI is overhead incarnate.  In Sun's implementation, even
the JNI functions you use once you get into Java are slow.

A final problem is efficiency of native code when it comes to things
like method calls, fields, finding classes, etc.  Generally you should
cache things like that in static C variables if you're going to use them
over and over again.  GetMethodID(), GetFieldID(), and FindClass() are
*slow*.

Here are a few tips on writing native code efficiently:

Make as few native method calls as possible.  Note that this is not the
same thing as doing less in native method calls; it just means that, if
given the choice between calling two native methods and writing a single
native method that does the job of both, it will usually be better to
write the single native method.  You can even call the other two native
methods directly from your native code and not incur the overhead of a
method call from Java to C.

Cache methodIDs and fieldIDs wherever you can.  String lookups are
expensive.  The best way to do this is to use the native/lib/jnilink.h
library.  It will ensure that jmethodIDs are always valid, even if the
class is unloaded at some point.  In 1.1, jnilink simply caches a
NewGlobalRef() to the method's underlying class; however, when 1.2 comes
along, it will use a weak reference to allow the class to be unloaded
and then re-resolve the jmethodID the next time it is used.

Cache classes that you need to access often.  jnilink will help with
this as well.  The issue here is the same as the methodID and fieldID
issue--how to make certain the class reference remains valid.

If you need to associate native C data with your class, use Paul
Fisher's native_state library (NSA).  It will allow you to get and set
state fairly efficiently.  Note that there is no built-in mechanism to
associate C data with instances of a class.  This is a library Paul
built from scratch.

@node Specification Sources, Naming Conventions, Native Efficiency, Top
@comment node-name, next, previous, up
@chapter Specification Sources

There are a number of specification sources to use when working on
Classpath.  In general, the only place you'll find your classes
specified is in the JavaDoc documentation or possibly in the
corresponding white paper.  In the case of java.lang, java.io and
java.util, you should look at the Java Language Specification.

Here, however, is a list of specs, in order of canonicality:

@enumerate
@item
@uref{http://java.sun.com/docs/books/jls/clarify.html,Clarifications and Amendments to the JLS - 1.1}
@item
@uref{http://java.sun.com/docs/books/jls/html/1.1Update.html,JLS Updates
- 1.1}
@item
@uref{http://java.sun.com/docs/books/jls/html/index.html,The 1.0 JLS}
@item
@uref{http://java.sun.com/docs/books/vmspec/index.html,JVM spec - 1.1}
@item
@uref{http://java.sun.com/products/jdk/1.1/docs/guide/jni/spec/jniTOC.doc.html,JNI spec - 1.1}
@item
@uref{http://java.sun.com/products/jdk/1.1/docs/api/packages.html,Sun's javadoc - 1.1}
(since Sun's is the reference implementation, the javadoc is
documentation for the Java platform itself.)
@item
@uref{http://java.sun.com/products/jdk/1.2/docs/guide/jvmdi/jvmdi.html,JVMDI spec - 1.2},
@uref{http://java.sun.com/products/jdk/1.2/docs/guide/jni/jni-12.html,JNI spec - 1.2}
(sometimes gives clues about unspecified things in 1.1; if
it was not specified accurately in 1.1, then use the spec
for 1.2; also, we are using JVMDI in this project.)
@item
@uref{http://java.sun.com/products/jdk/1.2/docs/api/frame.html,Sun's javadoc - 1.2}
(sometimes gives clues about unspecified things in 1.1; if
it was not specified accurately in 1.1, then use the spec
for 1.2)
@item
@uref{http://developer.java.sun.com/developer/bugParade/index.html,The
Bug Parade}: I have obtained a ton of useful information about how
things do work and how they *should* work from the Bug Parade just by
searching for related bugs.  The submitters are very careful about their
use of the spec.  And if something is unspecified, usually you can find
a request for specification or a response indicating how Sun thinks it
should be specified here.
@end enumerate

You'll notice that in this document, white papers and specification
papers are more canonical than the JavaDoc documentation.  This is true
in general.


@node Naming Conventions, Character Conversions, Specification Sources, Top
@comment node-name, next, previous, up
@chapter Directory and File Naming Conventions

The Classpath directory structure is laid out in the following manner:

@example
jcl
 |
 |---->java
 |       |
 |       |-->awt
 |       |-->io
 |       |-->lang
 |       |-->util
 |       |     |
 |       |     |--->zip
 |       |     |--->jar
 |       |-->net
 |       |-->etc
 |
 |---->gnu
 |       |
 |       |-->java
 |             |
 |             |-->awt
 |             |-->lang
 |             |-->util
 |             |     |
 |             |     |-->zip
 |             |-->etc
 |
 |---->native
 |       |
 |       |-->java.io
 |       |-->java.lang
 |       |-->java.net
 |       |-->java.util.jar
 |       |-->etc
 |
 |---->test
 |       |
 |       |-->java.io
 |       |-->java.lang
 |       |-->etc
 |
 |---->compat
         |
         |-->java.io
         |-->java.lang
         |-->etc

@end example

Here is a brief description of the toplevel directories and their contents.

@table @b

@item java
Contains the source code to the Java packages that make up the core
class library.  Because this is the public interface to Java, it is
important that the public classes, interfaces, methods, and variables
are exactly the same as specified in Sun's documentation.  The directory
structure is laid out just like the java package names.  For example,
the class java.util.zip would be in the directory java/util/zip.

@item gnu/java
Internal classes (roughly analogous to Sun's sun.* classes) should go
under the gnu/java directory.  Classes related to a particular public
Java package should go in a directory named like that package.  For
example, classes related to java.util.zip should go under a directory
gnu/java/util/zip.  Sub-packages under the main package name are
allowed.  For classes spanning multiple public Java packages, pick an
appropriate name and see what everybody else thinks.

@item native
This directory holds native code needed by the public Java packages.
Each package has its own subdirectory, which is the ``flattened'' name
of the package.  For example, native method implementations for
java.util.zip should go in native/java.util.zip.

@item test
This directory contains test packages written for DejaGnu used to test
releases of Classpath.  The test scripts for a given package go in the
subdirectory that is the same as the ``flattened'' name of the package.
For example, test scripts for java.util.zip should go in
test/java.util.zip

@item compat
This directory contains misc scripts designed not to test an
implementation, but to determine various things about Sun's reference
implementation that are needed in order to write a compatible package.
Each package has its own directory which is the ``flattened'' package
name.  For example, compatibility scripts for java.util.zip go in
compat/java.util.zip
@end table

Each person working on a package get's his or her own ``directory
space'' underneath each of the toplevel directories.  In addition to the
general guidelines above, the following standards should be followed:

@itemize @bullet

@item
Classes that need to load native code should load a library with the
same name as the flattened package name, with all periods removed.  For
example, the native library name specified in LoadLibrary for
java.util.zip would be ``javautilzip''.

@item
Each package has its own shared library for native code (if any).  The
actual library name to be built will depend on the target JVM.

@item
The main native method implementation for a given method in class should
go in a file with the same name as the class with a ``.c'' extension.
For example, the implementation of the native methods in
java.util.InetAddress would go in native/java.net/InetAddress.c.
``Internal'' native functions called from the main native method can
reside in files of any name.
@end itemize

@node Character Conversions, , Naming Conventions, Top
@comment node-name, next, previous, up
@chapter Character Conversions

Java uses the Unicode character encoding system internally.  This is a
sixteen bit (two byte) collection of characters encompassing most of the
world's written languages.  However, Java programs must often deal with
outside interfaces that are byte (eight bit) oriented.  For example, a
Unix file, a stream of data from a network socket, etc.  Beginning with
Java 1.1, the @code{Reader} and @code{Writer} classes provide functionality
for dealing with character oriented streams.  The classes 
@code{InputStreamReader} and @code{OutputStreamWriter} bridge the gap
between byte streams and character streams by converting bytes to 
Unicode characters and vice versa.

In Classpath, @code{InputStreamReader} and @code{OutputStreamWriter}
rely on an internal class called @code{gnu.java.io.EncodingManager} to load
translaters that perform the actual conversion.  There are two types of
converters, encoders and decoders.  Encoders are subclasses of
@code{gnu.java.io.encoder.Encoder}.  This type of converter takes a Java
(Unicode) character stream or buffer and converts it to bytes using
a specified encoding scheme.  Decoders are a subclass of 
@code{gnu.java.io.decoder.Decoder}.  This type of converter takes a 
byte stream or buffer and converts it to Unicode characters.  The
@code{Encoder} and @code{Decoder} classes are subclasses of
@code{Writer} and @code{Reader} respectively, and so can be used in
contexts that require character streams, but the Classpath implementation
currently does not make use of them in this fashion.

The @code{EncodingManager} class searches for requested encoders and
decoders by name.  Since encoders and decoders are separate in Classpath,
it is possible to have a decoder without an encoder for a particular 
encoding scheme, or vice versa.  @code{EncodingManager} searches the
package path specified by the @code{file.encoding.pkg} property.  The
name of the encoder or decoder is appended to the search path to
produce the required class name.  Note that @code{EncodingManager} knows
about the default system encoding scheme, which it retrieves from the
system property @code{file.encoding}, and it will return the proper
translator for the default encoding if no scheme is specified.  Also, the 
Classpath standard translator library, which is the @code{gnu.java.io} package, 
is automatically appended to the end of the path.

For efficiency, @code{EncodingManager} maintains a cache of translators
that it has loaded.  This eliminates the need to search for a commonly
used translator each time it is requested.

Finally, @code{EncodingManager} supports aliasing of encoding scheme names.
For example, the ISO Latin-1 encoding scheme can be referred to as
''8859_1'' or ''ISO-8859-1''.  @code{EncodingManager} searches for 
aliases by looking for the existence of a system property called
@code{gnu.java.io.encoding_scheme_alias.<encoding name>}.  If such a
property exists.  The value of that property is assumed to be the
canonical name of the encoding scheme, and a translator with that name is 
looked up instead of one with the original name.

Here is an example of how @code{EncodingManager} works.  A class requests
a decoder for the ''UTF-8'' encoding scheme by calling
@code{EncodingManager.getDecoder("UTF-8")}.  First, an alias is searched
for by looking for the system property 
@code{gnu.java.io.encoding_scheme_alias.UTF-8}.  In our example, this
property exists and has the value ''UTF8''.  That is the actual
decoder that will be searched for.  Next, @code{EncodingManager} looks
in its cache for this translator.  Assuming it does not find it, it
searches the translator path, which is this example consists only of
the default @code{gnu.java.io}.  The ''decoder'' package name is 
appended since we are looking for a decoder.  (''encoder'' would be 
used if we were looking for an encoder).  Then name name of the translator
is appended.  So @code{EncodingManager} attempts to load a translator
class called @code{gnu.java.io.decoder.UTF8}.  If that class is found,
an instance of it is returned.  If it is not found, a
@code{UnsupportedEncodingException}.

To write a new translator, it is only necessary to subclass 
@code{Encoder} and/or @code{Decoder}.  Only a handful of abstract
methods need to be implemented.  In general, no methods need to be
overridden.  The needed methods calculate the number of bytes/chars
that the translation will generate, convert buffers to/from bytes,
and read/write a requested number of characters to/from a stream.

Many common encoding schemes use only eight bits to encode characters.
Writing a translator for these encodings is very easy.  There are 
abstract translator classes @code{gnu.java.io.decode.DecoderEightBitLookup}
and @code{gnu.java.io.encode.EncoderEightBitLookup}.  These classes
implement all of the necessary methods.  All that is necessary to
create a lookup table array that maps bytes to Unicode characters and
set the class variable @code{lookup_table} equal to it in a static
initializer.  Also, a single constructor that takes an appropriate
stream as an argument must be supplied.  These translators are
exceptionally easy to create and there are several of them supplied
in the Classpath distribution.

Writing multi-byte or variable-byte encodings is more difficult, but
often not especially challenging.  The Classpath distribution ships with
translators for the UTF8 encoding scheme which uses from one to three
bytes to encode Unicode characters.  This can serve as an example of
how to write such a translator.

Many more translators are needed.  All major character encodings should
eventually be supported.

@bye