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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Java CLASSPATH explained

When you run a Java program (be it though the java virtual machine or as a servlet within a web or application container such as Tomcat), it's only the main code (class) that's initially loaded. Any other code (classes) are loaded in dynamically as needed.

Where are these classes loaded from?

They're loaded from EITHER directory structures OR jar files, and the locations of these are in part defaulted (for the standard stuff) and in part defined by the CLASSPATH - an environment variable in which you list the root of the tree in which the JVM (Java Virtual Machine) should start looking. Let's see an example:
   export CLASSPATH=/home/demo/packages:/usr/local/tomcat/shared/lib/wellhouse.jar:.
will cause the JVM to start looking in the packages subdirectory of the demo user's home directory, in a shared directory in your Tomcat installation, and in your current directory.

But that's only where it will start looking. If you're calling in a class called (say) net.wellho.tomcourse.Animal, then the extra package names will be added on to the base path and the actual files looked for on your disc will be:
   /home/demo/packages/net/wellho/tomcourse/Animal.class (an absolute path)
   /usr/local/tomcat/shared/lib/wellhouse.jar ... within in, the file ./net/wellho/tomcourse/Animal.class
   /net/wellho/tomcourse/Animal.class (i.e. below the current directory)


There are even more places that will be searched if you add some import statements to your Java class. Let's say you're loading from a class that includes:
    import net.wellho.general.*;
    import net.wellho.tomcourse.*;
    import org.apache.tomcat.*;

then you'll end up looking in no less that 12 places if you simply call up a class called Animal using the earlier CLASSPATH example:
   /home/demo/packages/Animal.class (an absolute path)
   /home/demo/packages/net/wellho/general/Animal.class (an absolute path)
   /home/demo/packages/net/wellho/tomcourse/Animal.class (an absolute path)
   /home/demo/packages/org/apache/tomcat/Animal.class (an absolute path)
   In /usr/local/tomcat/shared/lib/wellhouse.jar for Animal.class
   In /usr/local/tomcat/shared/lib/wellhouse.jar for net/wellho/general/Animal.class
   In /usr/local/tomcat/shared/lib/wellhouse.jar for net/wellho/tomcourse/Animal.class
   In /usr/local/tomcat/shared/lib/wellhouse.jar for org/apache/tomcat/Animal.class
   ./Animal.class (below the current directory)
   ./net/wellho/general/Animal.class (below the current directory)
   ./net/wellho/tomcourse/Animal.class (below the current directory)
   ./org/apache/tomcat/Animal.class (below the current directory)


Remember too that the Java compiler (known as javac) checks your code - so that it needs access to all necessary classes on the machine you're developing / compiling on to ensure that you're only calling classes - and methods within them - that actually exist. So if you're writing a Servlet, for example, you'll need to have the Servlet API on your CLASSPATH on the development machine and in the development environment, as well as on the test and production servers.
(written 2008-11-26)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
J601 - Java Introduction
  [25] Release numbers - (2004-08-23)
  [111] Training notes available under Open Distribution license - (2004-11-07)
  [124] PHP v Java - (2004-11-20)
  [317] Programming languages - a comparison - (2005-05-20)
  [792] Is Java the right language to learn? - (2006-07-04)
  [871] Java oversold? - (2006-09-19)
  [1049] Java 6, Apache Tomcat 6. - (2007-01-21)
  [1158] Private Java Course - A customer's pictures - (2007-04-22)
  [1418] A Golf Club Decision - Perl to Java - (2007-11-01)
  [1466] Effective Java training - the bootcamp approach - (2007-12-09)
  [1497] Training Season Starts again! - (2008-01-07)
  [1557] Trying out our Java examples on our web site - (2008-02-27)
  [1766] Diagrams to show you how - Tomcat, Java, PHP - (2008-08-22)
  [2115] Finding your java program - the CLASSPATH variable - (2009-04-02)
  [2423] What is a JVM, a JRE, a JDK - components of the core Java Environment - (2009-09-26)
  [2536] All the Cs ... and Java too - (2009-12-13)
  [4332] First Java Application - calculating the weight of a tablecloth - (2014-11-29)

A912 - Web Application Deployment - Upgrading and tuning Tomcat
  [1377] Load Balancing with Apache mod_jk (httpd/Tomcat) - (2007-10-02)
  [1718] Increasing Java Virtual Machine memory for Tomcat - (2008-07-24)
  [2060] Database connection Pooling, SSL, and command line deployment - httpd and Tomcat - (2009-03-01)
  [2082] Jmeter - a first test case - (2009-03-14)
  [3018] Tuning Apache httpd and Tomcat to work well together - (2010-10-27)

A503 - Web Application Deployment - Java - Sourcing, Installing, Initial Testing
  [2079] Java - Memory Allocation and garbage collection - (2009-03-14)
  [2080] Using ApacheBench and jconsole to test and monitor Tomcat - (2009-03-14)
  [2153] Class Loading and Variable Conversion in Java - (2009-05-02)
  [2184] Choosing the right version of Java and Tomcat - (2009-05-16)


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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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