Hi ooo.stephen
This is a good question, because it worth to understand the "classpath"
philosophy in the beginning, once for all.
In brief, Java CLASSPATH is a list of directories representing the place
where Java looks for the (fully qualified with the package name) .class
files, in about the same way Unix/Linux looks for its programs in $PATH
and Windows in %PATH%. The difference is that Java classes use a package.
A .java file use a "package" statement in the beginning of the code to
specify its package. After compiling, the ".class" file "knows" that it
can be called only by specifying the package. For e.g. "java -cp C:\foo
bar.test" will execute the "main" static method of the file
"C:\foo\bar\test.class".
But "java -cp C:\ foo.bar.test" will not work and "java -cp
C:\foo\bar test" neither because "test.class" "knows" that it should be
called as "bar.test" only (as the file "test.java" contains "package
bar" at the beginning).
Now, where should "java bar.test" look for a subdirectory "bar"
containing a file "test.class"? The answer is in the CLASSPATH.
The CLASSPATH is a list of directories separated by ":" under Unix/Linux
(and OS-X I think) and by ";" under Windows. The Java loader will scan
each of the directories in the CLASSPATH, in sequence, to find the
requested files.
You can set the CLASSPATH by defining an environment variable containing
the list of directories. Such as:
-- Unix/Linux/OS-X
export CLASSPATH=/usr/tst:/var/lib:/var/tst
java bar.test
-- Windows
set CLASSPATH=C:\libs;D:\Users\myself\Documents\tests
java bar.test
You can override the value of the currently defined CLASSPATH variable
on the command line. Such as:
-- Unix/Linux/OS-X
export CLASSPATH=/usr/tst:/var/lib:/var/tst
java -cp /home/myself/tst:/home/myself/lib bar.test
-- Windows
set CLASSPATH=C:\libs;D:\Users\myself\Documents\tests
java -cp
D:\Users\myself\Documents\proj;D:\Users\myself\Documents\proj\lib bar.test
(No need to say that "cp" stands for CLASSPATH).
If no CLASSPATH variable is used and no -cp command line option, then
Java use some default classpath (that can be modified too, but that is
another matter) that is usually ".".
The classpath (the variable or the -cp option and so on) may also
contain a .jar file too (or more), allowing to look for the requested
package/file.class in the .jar file(s) too.
Hope it helps
Mihai
ooo.stephen a écrit :
my grasp of this is a little shaky. please correct me if i'm wrong,
but my understanding (and maybe my articulation will help others), is
the following:
1- -cp statement essentially counter acts/accommodates the package
path statement. this is because the package statement is a directive
to look for .class files in the path stated in the package, starting
in the current dir the file is being executed from. in order to
compile from the command line, you need to set the -cp path to a dir
that would allow the package path to be true, i.e.:
E:\Programming\Java Passion\basic\10_classpath\homework\foodpackage
\fruitpackage>
java -cp ../../ foodpackage.fruitpackage.FoodMain
2- -cp is not necessary when running the 'java' command with a true
package path. that is, if you run 'java' from two dirs above, the
following is true;
E:\Programming\Java Passion\basic\10_classpath\homework>
java foodpackage.fruitpackage.FoodMain
same as:
E:\Programming\Java Passion\basic\10_classpath\homework>
java -cp . foodpackage.fruitpackage.FoodMain
i'm having a hard time conceptualizing a real world example or need
for dealing with the class path. because the package statement must
match the dir structure, changing the class path seems undesirable,
and would require recompiling the .class file.
thanks kindly,
ooo.stephen
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