Supplying as application argument Suppose we have a package called
org.mypackage containing the classes: •
HelloWorld (main class) •
SupportClass •
UtilClass and the files defining this package are stored physically under the directory
D:\myprogram (on
Windows) or
/home/user/myprogram (on
Linux). The file structure looks like this: When we invoke Java, we specify the name of the application to run: org.mypackage.HelloWorld. However we must also tell Java where to look for the files and directories defining our package. So to launch the program, we use the following command: where: • java is the
Java runtime launcher, a type of SDK Tool (A command-line tool, such as
javac,
javadoc, or
apt) •
-classpath D:\myprogram sets the path to the packages used in the program (on Linux,
-cp /home/user/myprogram) and •
org.mypackage.HelloWorld is the name of the main class
Setting the path through an environment variable The
environment variable named CLASSPATH may be alternatively used to set the classpath. For the above example, we could also use on Windows: D:\myprogram>set CLASSPATH=D:\myprogram D:\myprogram>java org.mypackage.HelloWorld The rule is that -classpath option, when used to start the java application, overrides the CLASSPATH environment variable. If none are specified, the
current working directory is used as classpath. This means that when our working directory is D:\myprogram\ (on Linux, /home/user/myprogram/), we would not need to specify the classpath explicitly. When overriding however, it is advised to include the current folder "." into the classpath in the case when loading classes from current folder is desired. The same applies not only to java launcher but also to
javac, the java compiler.
Setting the path of a Jar file If a program uses a supporting library enclosed in a
Jar file called
supportLib.jar, physically located in the directory
D:\myprogram\lib\ and the corresponding physical file structure is: D:\myprogram\ | ---> lib\ | ---> supportLib.jar | ---> org\ | ---> mypackage\ | ---> HelloWorld.class ---> SupportClass.class ---> UtilClass.class the following
command-line option is needed: java -classpath D:\myprogram;D:\myprogram\lib\supportLib.jar org.mypackage.HelloWorld or alternatively:
Adding all JAR files in a directory In
Java 6 and higher, one can add all jar-files in a specific directory to the classpath using wildcard notation. Windows example: Linux example: This works for both -classpath options and environment classpaths.
Setting the path in a manifest file If a program has been enclosed in a
Jar file called
helloWorld.jar, located directly in the directory
D:\myprogram, the directory structure is as follows: D:\myprogram\ | ---> helloWorld.jar | ---> lib\ | ---> supportLib.jar The
manifest file defined in
helloWorld.jar has this definition: The
manifest file should end with either a new line or carriage return. The program is launched with the following command: java -jar D:\myprogram\helloWorld.jar [app arguments] This automatically starts
org.mypackage.HelloWorld specified in class
Main-Class with the arguments. The user cannot replace this class name using the invocation .
Class-Path describes the location of
supportLib.jar relative to the location of the library
helloWorld.jar. Neither absolute file path, which is permitted in parameter on the command line, nor jar-internal paths are supported. This means that if the main class file is contained in a jar,
org/mypackage/HelloWorld.class must be a valid path on the root within the jar. Multiple classpath entries are separated with spaces: == OS specific notes ==