\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) 1998,1999,2000,2001,2002,2003 Free Software Foundation, Inc.

@end ifinfo

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

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1998,1999,2000,2001,2002,2003 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), (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.
@end ifinfo

@menu
* Introduction::                An introduction to the Classpath project
* Requirements::                Very important rules that must be followed
* Volunteering::                So you want to help out
* Project Goals::               Goals of the Classpath project
* 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
* API Compatibility::           
* Specification Sources::       Where to find the Java class library specs
* Naming Conventions::          How files and directories are named in Classpath
* Character Conversions::       Working on Character conversions
* Localization::                How Classpath handles localization/internationalization

@detailmenu
 --- The Detailed Node Listing ---

Programming Goals

* Portability::                 Writing Portable Software                
* Robustness::                  Writing Robust Software               
* Java Efficiency::             Writing Efficient Java            
* Native Efficiency::           Writing Efficient JNI          
* Security::                    Writing Secure Software

Localization

* String Collation::            Sorting strings in different locales
* Break Iteration::             Breaking up text into words, sentences, and lines
* Date Formatting and Parsing::  Locale specific date handling
* Decimal/Currency Formatting and Parsing::  Local specific number handling

@end detailmenu
@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 which offer
free software developers an alternative to Sun's proprietary
libraries.  The Classpath Project was started in the Spring of 1998 as
an official Free Software Foundation project.  Most of the volunteers
working on GNU Classpath do so in their spare time.  We appreciate
everyone's efforts in the past to improve and help the project and
look forward to future contributions by old and new members alike.

@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 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 Java 2 releases has changed, it is 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/gpl.html,GNU General Public License},
with a special exception included to allow linking with non-GPL licensed
works as long as no other license would restrict such linking.  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.  The license can be read from any
alpha release distributed from @uref{ftp://alpha.gnu.org/gnu/classpath/}
or by obtaining a copy of the current CVS tree.

@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.  GNU Classpath uses the
@uref{http://sources.redhat.com/mauve/,Mauve project} for testing the
functionality of the core class libraries.  The Classpath Project is
fast approaching the point in time where all modifications to the
source code repository will require appropriate test cases in Mauve to
ensure correctness and prevent regressions.  
@end itemize

With the exception of the AWT, writing tests and fixing bugs should be
every developer's top priority in order to reach the elusive release
of version 1.0.  

@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
or CNI support.  However, the top priority is free JVM's.  A small, and
probably incomplete list is below.

@enumerate
@item 
@uref{http://kissme.sourceforge.net/,Kissme}
@item
@uref{http://www-124.ibm.com/developerworks/oss/jikesrvm/,Jikes RVM}
@item
@uref{http://www.sablevm.org/,SableVM}
@item
@uref{http://www.intel.com/research/mrl/orp/,Intel's Open Runtime Platform}
@item
@uref{http://www.kaffe.org/,Kaffe}
@item
@uref{http://www.japhar.org/,Japhar}
@item
@uref{http://latte.snu.ac.kr/,LaTTe}
@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 other free VMs.

The initial target version for Classpath is Java 1.1.  Java 2 can be
implemented if desired, but please do not create classes that depend on
Java 2 features in other packages unless Classpath already contains
those features.

@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 1.11
@item
automake 1.6+
@item
autoconf 2.53+
@item
libtool 1.4.2+
@item
GNU m4 1.4
@end itemize

All of these tools are available from
@uref{ftp://gnudist.gnu.org/pub/gnu/,gnudist.gnu.org} via anonymous
ftp, except CVS which is available from
@uref{http://www.cvshome.org/,www.cvshome.org}.  They are fully
documented with texinfo manuals.  Texinfo can be browsed with the
Emacs editor, or with the text editor of your choice, or transformed
into nicely printable Postscript.

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 C. Brian Jones
(@email{cbj@@gnu.org}) for details.

@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 number of ways that cannot be described 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 though knowing a lot about GNU m4 is not required to
work with autoconf.

@item perl
Larry Wall's scripting language.  It is used internally by automake.
@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://www.fsf.org/prep/standards_toc.html,GNU Coding Standards},
with the exception of naming conventions.  Please follow
@uref{http://java.sun.com/docs/codeconv/html/CodeConventions.doc8.html,Sun's
naming conventions}.  There are a number of exceptions to the GNU
Coding Standards that we make for GNU Classpath and these will be
documented soon as well as hopefully providing developers with a code
formatting tool that closely matches those rules.

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, API Compatibility, 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.

@menu
* Portability::                 Writing Portable Software                
* Robustness::                  Writing Robust Software               
* Java Efficiency::             Writing Efficient Java            
* Native Efficiency::           Writing Efficient JNI          
* Security::                    Writing Secure Software
@end menu

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

The portability goal for Classpath is the following:

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

For almost all of Classpath, this is a very feasible goal, using a
combination of JNI and native interfaces.  This is what you should shoot
for.  For those few places that require knowledge of the Virtual Machine
beyond that provided by the Java standards, the VM Interface was designed.
Read the Virtual Machine Integration Guide for more information.

Right now the only supported platform is Linux.  This will change as that
version stabilizes and we begin the effort to port to many other
platforms.

@node Robustness, Java Efficiency, Portability, Programming Goals
@comment node-name, next, previous, up
@section 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/jni/classpath/jcl.h.  They
check exceptions and return values and throw appropriate exceptions.

@node Java Efficiency, Native Efficiency, Robustness, Programming Goals
@comment node-name, next, previous, up
@section 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, Security, Java Efficiency, Programming Goals
@comment node-name, next, previous, up
@section 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 empty
@emph{interpreted} Java method is typically about three and a half times
@emph{faster} than the equivalent native 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*.  Classpath provides utility libraries for caching methodIDs
and fieldIDs in native/jni/classpath/jnilink.h.  Other native data can
be cached between method calls using functions found in
native/jni/classpath/native_state.h.

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.  Japhar now supports this library, making
native state get and set calls as fast as accessing a C variable
directly.

If you are using native libraries defined outside of Classpath, then
these should be wrapped by a Classpath function instead and defined
within a library of their own.  This makes porting Classpath's native
libraries to new platforms easier in the long run.  It would be nice
to be able to use Mozilla's NSPR or Apache's APR, as these libraries
are already ported to numerous systems and provide all the necessary
system functions as well.

@node Security,  , Native Efficiency, Programming Goals
@comment  node-name,  next,  previous,  up
@section Security

Security is such a huge topic it probably deserves its own chapter.
Most of the current code needs to be audited for security to ensure
all of the proper security checks are in place within the Java
platform, but also to verify that native code is reasonably secure and
avoids common pitfalls, buffer overflows, etc.  A good source for
information on secure programming is the excellent HOWTO by David
Wheeler,
@uref{http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html,Secure
Programming for Linux and Unix HOWTO}.

@node API Compatibility, Specification Sources, Programming Goals, Top
@comment  node-name,  next,  previous,  up
@chapter API Compatibility

Sun has produced documentation concerning much of the information
needed to make Classpath serializable compatible with Sun
implementations.  Part of doing this is to make sure that every class
that is Serializable actually defines a field named serialVersionUID
with a value that matches the output of serialver on Sun's
implementation.  The reason for doing this is below.

If a class has a field (of any accessibility) named serialVersionUID
of type long, that is what serialver uses. Otherwise it computes a
value using some sort of hash function on the names of all method
signatures in the .class file.  The fact that different compilers
create different synthetic method signatures, such as access$0() if an
inner class needs access to a private member of an enclosing class,
make it impossible for two distinct compilers to reliably generate the
same serial #, because their .class files differ. However, once you
have a .class file, its serial # is unique, and the computation will
give the same result no matter what platform you execute on.

Serialzation compatibility can be tested using tools provided with
@uref{http://rainbow.netreach.net/~sballard/japi/,Japitools}.  These
tools can test binary serialization compatibility and also provide
information about unknown serialized formats by writing these in XML
instead.  Japitools is also the primary means of checking API
compatibility for GNU Classpath with Sun's Java Platform.

@node Specification Sources, Naming Conventions, API Compatibility, 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
classpath
 |
 |---->java
 |       |
 |       |-->awt
 |       |-->io
 |       |-->lang
 |       |-->util
 |       |     |
 |       |     |--->zip
 |       |     |--->jar
 |       |-->net
 |       |-->etc
 |
 |---->gnu
 |       |
 |       |-->java
 |             |
 |             |-->awt
 |             |-->lang
 |             |-->util
 |             |     |
 |             |     |-->zip
 |             |-->etc
 |
 |---->native
         |
         |-->jni
         |    |-->classpath
         |    |-->gtk-peer
         |    |-->java-io
         |    |-->java-lang
         |    |-->java-net
         |    |-->java-util
         |    |-->etc
         |-->cni
  
@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.

@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/classpath/java-util.  Classpath
actually includes an all Java version of the zip classes, so no native
code is required.

@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 hyphens removed.  For
example, the native library name specified in LoadLibrary for
java-util would be ``javautil''.

@item
Each package has its own shared library for native code (if any).

@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 JNI implementation of the native methods in
java.net.InetAddress would go in native/jni/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, Localization, 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.

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

There are many parts of the Java standard runtime library that must
be customized to the particular locale the program is being run in.
These include the parsing and display of dates, times, and numbers;
sorting words alphabetically; breaking sentences into words, etc.
In general, Classpath uses general classes for performing these tasks,
and customizes their behavior with configuration data specific to a
given locale.

@menu
* String Collation::            Sorting strings in different locales
* Break Iteration::             Breaking up text into words, sentences, and lines
* Date Formatting and Parsing::  Locale specific date handling
* Decimal/Currency Formatting and Parsing::  Local specific number handling
@end menu

In Classpath, all locale specific data is stored in a 
@code{ListResourceBundle} class in the package @code{gnu/java/locale}.
The basename of the bundle is @code{LocaleInformation}.  See the
documentation for the @code{java.util.ResourceBundle} class for details
on how the specific locale classes should be named.

@code{ListResourceBundle}'s are used instead of 
@code{PropertyResourceBundle}'s because data more complex than simple
strings need to be provided to configure certain Classpath components.
Because @code{ListResourceBundle} allows an arbitrary Java object to
be associated with a given configuration option, it provides the
needed flexibility to accomodate Classpath's needs.

Each Java library component that can be localized requires that certain
configuration options be specified in the resource bundle for it.  It is
important that each and every option be supplied for a specific 
component or a critical runtime error will most likely result.

As a standard, each option should be assigned a name that is a string.
If the value is stored in a class or instance variable, then the option
should name should have the name name as the variable.  Also, the value
associated with each option should be a Java object with the same name
as the option name (unless a simple scalar value is used).  Here is an
example:

A class loads a value for the @code{format_string} variable from the
resource bundle in the specified locale.  Here is the code in the
library class:

@example
  ListResourceBundle lrb = 
    ListResourceBundle.getBundle ("gnu/java/locale/LocaleInformation", locale);
  String format_string = lrb.getString ("format_string");
@end example

In the actual resource bundle class, here is how the configuration option
gets defined:

@example
/**
  * This is the format string used for displaying values
  */
private static final String format_string = "%s %d %i";

private static final Object[][] contents =
@{
  @{ "format_string", format_string @}
@};
@end example

Note that each variable should be @code{private}, @code{final}, and
@code{static}.  Each variable should also have a description of what it
does as a documentation comment.  The @code{getContents()} method returns
the @code{contents} array.

There are many functional areas of the standard class library that are
configured using this mechanism.  A given locale does not need to support
each functional area.  But if a functional area is supported, then all
of the specified entries for that area must be supplied.  In order to
determine which functional areas are supported, there is a special key
that is queried by the affected class or classes.  If this key exists, 
and has a value that is a @code{Boolean} object wrappering the
@code{true} value, then full support is assumed.  Otherwise it is
assumed that no support exists for this functional area.  Every class
using resources for configuration must use this scheme and define a special
scheme that indicates the functional area is supported.  Simply checking
for the resource bundle's existence is not sufficient to ensure that a
given functional area is supported.

The following sections define the functional areas that use resources
for locale specific configuration in GNU Classpath.  Please refer to the 
documentation for the classes mentioned for details on how these values 
are used.  You may also wish to look at the source file for 
@code{gnu/java/locale/LocaleInformation_en} as an example.

@node String Collation, Break Iteration, Localization, Localization
@comment node-name, next, previous, up
@section String Collation

Collation involves the sorting of strings.  The Java class library provides
a public class called @code{java.text.RuleBasedCollator} that performs
sorting based on a set of sorting rules.

@itemize @bullet
@item RuleBasedCollator - A @code{Boolean} wrappering @code{true} to indicate
that this functional area is supported.
@item collation_rules - The rules the specify how string collation is to
be performed.
@end itemize

Note that some languages might be too complex for @code{RuleBasedCollator}
to handle.  In this case an entirely new class might need to be written in
lieu of defining this rule string.

@node Break Iteration, Date Formatting and Parsing, String Collation, Localization
@comment node-name, next, previous, up
@section Break Iteration

The class @code{java.text.BreakIterator} breaks text into words, sentences,
and lines.  It is configured with the following resource bundle entries:

@itemize @bullet
@item BreakIterator - A @code{Boolean} wrappering @code{true} to indicate
that this functional area is supported.
@item word_breaks - A @code{String} array of word break character sequences.
@item sentence_breaks - A @code{String} array of sentence break character
sequences.
@item line_breaks - A @code{String} array of line break character sequences.
@end itemize

@node Date Formatting and Parsing, Decimal/Currency Formatting and Parsing, Break Iteration, Localization
@comment node-name, next, previous, up
@section Date Formatting and Parsing

Date formatting and parsing is handled by the 
@code{java.text.SimpleDateFormat} class in most locales.  This class is
configured by attaching an instance of the @code{java.text.DateFormatSymbols}
class.  That class simply reads properties from our locale specific
resource bundle.  The following items are requiered (refer to the 
documentation of the @code{java.text.DateFormatSymbols} class for details
io what the actual values should be):

@itemize @bullet
@item DateFormatSymbols - A @code{Boolean} wrappering @code{true} to indicate
that this functional area is supported.
@item months - A @code{String} array of month names.
@item shortMonths - A @code{String} array of abbreviated month names.
@item weekdays - A @code{String} array of weekday names.
@item shortWeekdays - A @code{String} array of abbreviated weekday names.
@item ampms - A @code{String} array containing AM/PM names.
@item eras - A @code{String} array containing era (ie, BC/AD) names.
@item zoneStrings - An array of information about valid timezones for this 
locale.
@item localPatternChars - A @code{String} defining date/time pattern symbols.
@item shortDateFormat - The format string for dates used by 
@code{DateFormat.SHORT}
@item mediumDateFormat - The format string for dates used by 
@code{DateFormat.MEDIUM}
@item longDateFormat - The format string for dates used by 
@code{DateFormat.LONG}
@item fullDateFormat - The format string for dates used by 
@code{DateFormat.FULL}
@item defaultDateFormat - The format string for dates used by 
@code{DateFormat.DEFAULT}
@item shortDateFormat - The format string for times used by 
@code{DateFormat.SHORT}
@item mediumDateFormat - The format string for times used by 
@code{DateFormat.MEDIUM}
@item longDateFormat - The format string for times used by 
@code{DateFormat.LONG}
@item fullDateFormat - The format string for times used by 
@code{DateFormat.FULL}
@item defaultDateFormat - The format string for times used by 
@code{DateFormat.DEFAULT}
@end itemize

Note that it may not be possible to use this mechanism for all locales.
In those cases a special purpose class may need to be written to handle 
date/time processing.

@node Decimal/Currency Formatting and Parsing,  , Date Formatting and Parsing, Localization
@comment node-name, next, previous, up
@section Decimal/Currency Formatting and Parsing

@code{NumberFormat} is an abstract class for formatting and parsing numbers.
The class @code{DecimalFormat} provides a concrete subclass that handles
this is in a locale independent manner.  As with @code{SimpleDateFormat},
this class gets information on how to format numbers from a class that
wrappers a collection of locale specific formatting values.  In this case,
the class is @code{DecimalFormatSymbols}.  That class reads its default
values for a locale from the resource bundle.  The required entries are:

@itemize @bullet
@item DecimalFormatSymbols - A @code{Boolean} wrappering @code{true} to 
indicate that this functional area is supported.
@item currencySymbol - The string representing the local currency.
@item intlCurrencySymbol - The string representing the local currency in an
international context.
@item decimalSeparator - The character to use as the decimal point as a
@code{String}.
@item digit - The character used to represent digits in a format string,
as a @code{String}.
@item exponential - The char used to represent the exponent separator of a 
number written in scientific notation, as a @code{String}.
@item groupingSeparator - The character used to separate groups of numbers
in a large number, such as the ``,'' separator for thousands in the US, as
a @code{String}.
@item infinity - The string representing infinity.
@item NaN - The string representing the Java not a number value.
@item minusSign - The character representing the negative sign, as a 
@code{String}.
@item monetarySeparator - The decimal point used in currency values, as a
@code{String}.
@item patternSeparator - The character used to separate positive and 
negative format patterns, as a @code{String}.
@item percent - The percent sign, as a @code{String}.
@item perMill - The per mille sign, as a @code{String}.
@item zeroDigit - The character representing the digit zero, as a @code{String}.
@end itemize

Note that several of these values are an individual character.  These should
be wrappered in a @code{String} at character position 0, not in a
@code{Character} object.

@bye