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From: Apache Wiki <wikidiffs@apache.org>
To: dev@commons.apache.org
Date: Fri, 05 Oct 2007 12:43:14 -0000
Message-ID: <20071005124314.27412.93587@eos.apache.org>
Subject: [Commons Wiki] Update of "CompressImplementationDetails" by
 ChristianGrobmeier

Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Commons Wiki" for change notification.

The following page has been changed by ChristianGrobmeier:
http://wiki.apache.org/jakarta-commons/CompressImplementationDetails

New page:
##language:en

= Compress Implementation Details =

== About this page ==

There has be some discussion in the past about improving compress and the need to 
get it out of the sandbox. Some of this has been discussed starting with this message:
http://mail-archives.apache.org/mod_mbox/commons-dev/200708.mbox/%3c46CE592C.50200@wso2.com%3e

Compress is considered to be complex in its implementation; it seems to be difficult to 
contribute new Compressors/Archivers. This page shall give a basis for further design
discussions and help people to jump into the implementation.

== "Modules" ==

Compress started with implementations to create zip-, tar- and bzip2-files (called providers from nowon). In 2006 a common interface for these implementations has been added. The common interface can be found in org.apache.commons.compress and the providers in 
org.apache.commons.compress.archivers 
and
org.apache.commons.compress.compressors.

As you can see, the providers has been splitted up into compressor (f.e. BZip2) and archivers (tar and zip, altough zip provides compression too). At the moment there are redundant provider packages in the root package, which is by fault (TODO: jira issue).

== Providers ==
The providers are quite long part of these project; they have been extracted from other apache projects and didn't change for a long time. At the moment everything works quite well with a few Jira issues. The code should be more tidy and could have some more documentation.   Sometimes code can be improved.

== Common Interface == 
The Common Interface constists of a factory, namely CompressorFactory and ArchiveFactory, which return Compressor or Archives. In fact with the introduction of the common interface each provider has become a new class which implements one of these interfaces, Compressor or Archive. For example, there is now a BZip2Compressor.

There were many common operations for Compressors and Archives, so AbstractArchive and AbstractCompressor has been written, which provide these operations. One can now simply extend f.e. AbstractCompressor and gain these functionality.
With doing so, a new provider has just to implement a few methods; in case of a compressor,
there should be the following:

- compressTo(FileInputStream inputStream, FileOutputStream outputStream) throws CompressException

- decompressTo(FileInputStream input, FileOutputStream outputStream) throws CompressException 

and also some Methods like getHeader, which is used to indentify an file by its byte-header,  getName or the getDefaultExtension, which is used for creating new files (in case of a Zip-File, it "zip").

== Other classes ==
There are just a few other classes, like ArchiveException or CrompessException (selfspeaking). CompressUtils provides soem kind of copy-function. PackableObject,
the mother of AbstractArchive and AbstractCompressor provides the "Identify by Header" Functionity.
ArchivEntry is just a representation of an Entry in an Archive.

= Ideas for enhancing the design =
TODO


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