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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Running straight from the jar, but not from a tar

Why can you run applications in Java straight from the jar, but you'll never find mainstream applications extracting their code from a tar file? It all comes down to the different indexing structure.

A tar file was originally designed as a "tape archive". So it's a sequential file that contains file name, file length, file data for the first file, then the same thing for the second file, for the third file, and so on. It means that when a tape is written, you have a high data integrity even on a device that doesn't have random access facilities, as there's no need to store statistics about a file once it's been added, nor to look them up right at the start or end of writing the file which may - on a multiuser system - have changed. But it does mean that if you want to read back something from a "tar" file you have to look at each header block in turn until you find what you need.

By contrast, a jar file [same format as zip] contains all the data at the start of the jar, then an index of where each object starts and ends at the end of the file. Which means you can very quickly read the list of contents ("Manifest") and jump to what you need even in a huge file - but it does require random access.

You may ask why the list of contents for a jar is at the end. That's so that a jar file can be updated / have things added to it without having to rewrite the whole file - adding a new file is as simple as reading the old manifest, adding the new file's data starting where the manifest used to start, and then writing an updated manifest.

(written 2009-05-26)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
A165 - Web Application Deployment - Installing Software
  [809] What to do during a Linux build - (2006-07-20)
  [1700] FTP server on Fedora Linux - (2008-07-06)
  [2139] OS Commerce install made simple - (2009-04-24)
  [3053] Make - automating the commands for building and installing - (2010-11-16)
  [4259] Upgrading our training systems to all the current stable versions - (2014-04-07)

A101 - Web Application Deployment - Linux -An Introduction For Users
  [73] vi - full circle - (2004-10-04)
  [74] pushd and popd - (2004-10-05)
  [152] Aladdin, or careful what you wish. - (2004-12-15)
  [249] An easy way out - (2005-03-17)
  [430] Linux commands - some basics - (2005-08-31)
  [431] File permissions of Linux and Unix systems - (2005-08-31)
  [593] Finding where the disc space has gone - (2006-02-06)
  [659] Web Application Components - (2006-03-28)
  [679] More or less on the edge of the page - (2006-04-11)
  [703] Copying files and preserving ownership - (2006-04-28)
  [710] Linux training Glasgow, Python programming course Dundee - (2006-05-05)
  [711] THE home directory or MY home directory - (2006-05-06)
  [749] Cottage industry or production line data handling methods - (2006-06-07)
  [984] Cardinal numbers and magic numbers - (2006-12-14)
  [1012] Moving files between Windows / DOS and Linux / Unix - (2006-12-30)
  [1013] Copy multiple files - confusing error message from cp - (2006-12-30)
  [1068] ls -l report, Linux / Unix - types and permssions - (2007-02-06)
  [1259] Where am I and how did I get here? - (2007-07-05)
  [1287] Work and play at Well House Manor - Football and Shell Shortcuts - (2007-08-02)
  [1288] Linux run states, shell special commands, and directory structures - (2007-08-03)
  [1366] awk - a powerful data extraction and manipulation tool - (2007-09-25)
  [1408] Wireless hotel tips - FTP and Skype connections failing - (2007-10-26)
  [1438] Copy and paste / cut and paste and other vi techniques - (2007-11-20)
  [1527] Selecting file names in a shell - one word or another - (2008-02-02)
  [1651] ls command - favourite options - (2008-05-23)
  [1764] Yank and Push - copy and move in vi - (2008-08-21)
  [1803] FTP passive mode - a sometimes cure for upload hangs - (2008-09-20)
  [1893] Some Linux and Unix tips - (2008-11-18)
  [1897] Keeping on an even keel - (2008-11-21)
  [1902] sstrwxrwxrwx - Unix and Linux file permissions - (2008-11-23)
  [1904] Ruby, Perl, Linux, MySQL - some training notes - (2008-11-23)
  [2203] Always use su with minus. And where do programs come from? - (2009-05-27)
  [2299] How much space does my directory take - Linux - (2009-07-20)
  [2300] What does x on a linux directory mean? - (2009-07-21)
  [2479] Accidentally typed ci rather than vi? - (2009-10-27)
  [2494] Making Linux Politically correct - (2009-11-06)
  [2636] Linux - useful tips including history and file name completion - (2010-02-15)
  [2831] Recording (a macro) in vi - (2010-06-27)
  [3179] Oops - I typed ci not vi, and have lost my file ... - (2011-02-21)
  [3256] Displaying a directory or file system tree - Linux - (2011-04-22)
  [3791] The Kernel, Shells and Daemons. Greek Gods in computing - (2012-07-01)
  [3819] Packing a tar, jar or war file - best practise - (2012-07-26)


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Some other Articles
An evening walk in Carrickfergus
Images in a database? How big is a database? (MySQL)
Bright Colours
Running straight from the jar, but not from a tar
England, Scotland, Northern Ireland
Improving the structure of your early PHP programs
On the road, at a Service area thats a cut above the others
New Example - cacheing results in PHP for faster loading
A two day gap - first time in five years?
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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