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A path specifies the name and location of a file on the
file system. It starts with the name of the disk or the root of the
filesystem and works its way down through various directories until
reaches the file. File, directory, and path naming conventions are
platform specific. For example a Unix path looks like
/home/users/elharo/html/javafaq.html. A DOS/Windows path
looks like C:\html\javafaq.htm. A Macintosh path looks like
My Hard Drive:html:Java FAQ List v1.1. All three of these
examples point to a file. Paths can also point to a directory. For
example, /home/users/elharo/html, C:\html, or
My Hard Drive:html:.
The character that separates one directory from the next in a path
is called the separator character. It is a slash (/)
on Unix, a backslash (\) in Windows and a colon (:) on the Mac. You can
get its value on a particular platform by looking at the static
variable java.io.File.separatorCharacter.
If you actually check this on the Mac, you'll note something funny.
java.io.File.separatorCharacter appears to be a slash (/)
like on Unix, not a colon like a Mac programmer would expect. Why Java had to
be different from every other Mac program in the universe
I don't know. This is problematic because
Mac file names can include slashes.
The CLASSPATH is an environment variable that contains a list of
directories where Java looks for classes referenced in a program. If
the CLASSPATH isn't set properly no program written in Java will be
able to run, and the compiler won't be able to compile. Each entry
in this list is separated from the other entries by the
java.io.File.pathSeparatorChar. This is semicolon (;) on
Windows and a colon (:) on Unix and the Mac. For example
Unix: ~/classes:/usr/local/netscape/classes
Windows: C:\java\classes;C:\netscape\classes
Mac: My Hard Drive/JDK/classes:My Hard Drive/My Project:My Hard Drive/classes
On most platforms, the JDK's java interpreter appends some
directories to the CLASSPATH you set manually. These are
set relative to where the java interpreter itself is. For example,
if the java program is installed in /usr/local/java/bin, then
it will append /usr/local/java/classes and /usr/local/java/lib/classes.zip
to the CLASSPATH. Another way of thinking about it: if the directory
where the java interpreter is installed is $JAVA, then
$JAVA/../classes and $JAVA/../lib/classes.zip are automatically
in your CLASSPATH.
Java applets and applications aren't self-contained. They need
access to other classes to do their work. For instance when you call
System.out.println() Java needs to know where to look
to find the file that includes the System class.
The directories in the CLASSPATH are where Java starts searching for
classes. To find a class Java first changes the periods in the full
package-qualified name of the class (e.g. java.util.Date and not
just Date) into directory separators (/ on Unix, \ on Windows, : on
the Mac). Thus if it wants the java.awt.GridBagLayout class, it
looks for the file java/awt/GridBagLayout.class in each of the root
directories listed in the CLASSPATH variable from left to right
until it finds the file. With the Unix CLASSPATH listed above, Java
first looks for ~/classes/java/awt/GridBagLayout.class
Then for,
/usr/local/netscape/classes/java/awt/GridBagLayout.class.
The specification of the CLASSPATH is somewhat platform dependent.
For instance ~ means the home directory on Unix but has no meaning
on the Mac.
Under Unix you set CLASSPATH variables like this:
csh: % setenv CLASSPATH my_class_path
sh: % CLASSPATH=my_class_path
You'll probably want to add one of these lines to your .login or .cshrc file
so it will be automatically set every time.
Under Windows you set
the CLASSPATH environment variable with a DOS command like
C:\> SET CLASSPATH=C:\JDK\JAVA\CLASSES;c:\java\lib\classes.zip
You can also add this to your autoexec.bat file. You should of course point it at
whichever directories actually contain your classes.
The CLASSPATH variable is also important when you run Java applets,
not just when you compile them. It tells the web browser or applet
viewer where it should look to find the referenced .class files. If
the CLASSPATH is set improperly, you'll probably see messages like
"Applet could not start."
Since large packages can contain many, many .class files Sun has
built the capability to read zip archives into Java. Therefore an
entire directory structure of class files can be zipped to save
space. If you want to see what's inside the zip file, unzip it. Java
doesn't care whether or not a directory has been zipped. You just
need to make sure that the .zip file is named the same as the
directory it replaces plus the .zip extension and that it is in the
same location.
In Netscape you should make sure that the first directory in the
CLASSPATH is the directory that contains Netscape's class files (The
defaults are /usr/local/netscape/java/classes on Unix and
C:\NETSCAPE\NAVIGATOR\Program\java\classes in Windows.)
Finally note that if you install additional packages such as Jeeves
or any third party package, you need to add the directory
where the package is installed to your CLASSPATH. For example let's
say you buy a package of statistics classes from SPSS, and you put
those classes in /opt/classes/stats. Then you you need to add
/opt/classes/stats to the end of your CLASSPATH.
You can
temporarily add a directory to the CLASSPATH by giving the
-classpath option to the java interpreter or the javac
compiler. For example,
javac -classpath $CLASSPATH:/opt/classes/stats
To use just the classes in /opt/classes/stats and not the classes
normally found in your CLASSPATH, omit $CLASSPATH like this:
javac -classpath /opt/classes/stats
Finally if the CLASSPATH environment variable has not been set, and
you do not specify one on the command line, then Java sets the
CLASSPATH to the default:
Unix: .:$JAVA/classes:$JAVA/lib/classes.zip
Windows: .:$JAVA\classes:$JAVA\lib\classes.zip
Mac: ./$JAVA:classes/$JAVA:lib:classes.zip
Here . is the current directory and $JAVA is the main Java
directory where the different tools like javac were installed. |
