Rules about classpath

Source : http://mindprod.com/jgloss/classpath.html

Rules About Classpaths

1. Simply copying or moving your jar files to the ext directory pointed to by the system property java.ext.dirs =C:\Program Files\java\jre6\lib\ext automatically puts them on the classpath without having to mention them explicitly. This is a great way to prune back an overblown classpath. It is safest to put your jars in all the ext directories:
   where to look for ext directories :
   

   • in C:\Program Files\java\jre6\lib\ext in JRE 1.6.0 on your local hard disk.
   • in J:\Program Files\java\jdk1.6.0_22\jre\lib\ext in JDK 1.6.0 on your local hard disk.
   • in C:\Program Files\java\jre1.5.0_22\lib\ext in the older JRE 1.5.0 on your local hard disk.
   • in J:\Program Files\java\jdk1.5.0_22\jre\lib\ext in the older JDK 1.5.0 on your local hard disk.
   You never know for sure where your javac.exe or java.exe is going to look. Grrr. You can change the location of the

   java.exe -Djava.ext.dirs=C:\mylibs mypackage.MyClass

   See the ext dirs entry for details.
2. Class names are always fully qualified with the complete package name. There is no way to ever abbreviate the higher levels.
3. Each element of the CLASSPATH provides a starting point to look for a fully qualified package and class name, identical to the way it appeared in the package or import statement.
4. If the element of the CLASSPATH is the name of a directory, Java will look in that tree for directory names matching the package name structure. It looks at one place only. It does not search. The class file sought must be filed under the one and only precise fully qualified pathname, or it won’t be found. It wants to find a *.class file (or*.java file), at the precisely correct spot in the directory tree. You must get an exact match on the fully qualified name. The name of the directory specified in the CLASSPATH itself is totally immaterial in determining the package name.
5. If the element of the CLASSPATH is a jar, Java will look in the internal directory structure of the jar for an exact match on the fully qualified name. It looks; it does not search. The class file sought must be filed under the one and only precise fully qualified pathname, or it won’t be found. The location of the jar is totally immaterial in determining the name of the package. If you peek inside the jar with WinZip you should see pathnames on each class file matching the package structure. Download and examine any of my jars and the corresponding source to see how it works.
6. An alternative, ultimately more confusing way of looking at it, is that you specify part of the operating system’s name for a class file in the CLASSPATH and part in the package name. Java source imports and package statements, javac.exe and java.exe command lines specify only the fully qualified package name, not the higher order part handled by the CLASSPATH. The higher levels that are handled by the CLASSPATH (which could appear on the command line via -classpath option), are effectively invisible to your Java programs. However, you are not at liberty to shuffle levels of qualification between your import and classpath unless you adjust all your package statements and recompile as well.