[Flex] How to call classes outside flex.war

Problem Summary
You want to have Java classes called by Blaze DS/LCDS but they reside outside the flex.war/WEB-INF/classes folder and Flex does not find them.
Solution Summary
Use the java.class.path system variable in your JVM configuration in order to point to the folder or JAR containing your classes.
Explanation
Usually, you call from Flex some Java classes that reside within flex.war/WEB-INF/classes.
But what if you need to have them outside the flex.war folder, especially for deployment reasons ?
Then, you have to specify a path to the Flex servlet so it can find them. The way to do this is to add to the JVM configuration file the java.class.path system variable.
This apply to all J2EE servers.