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:

D:\myprogram\
      |
      ---> org\  
            |
            ---> mypackage\
                     |
                     ---> HelloWorld.class       
                     ---> SupportClass.class   
                     ---> UtilClass.class     

/home/user/myprogram/
            |
            ---> org/  
                  |
                  ---> mypackage/
                           |
                           ---> HelloWorld.class       
                           ---> SupportClass.class   
                           ---> UtilClass.class     

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:

 java -classpath D:\myprogram org.mypackage.HelloWorld

 java -cp /home/user/myprogram org.mypackage.HelloWorld 

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:

D:\myprogram>set CLASSPATH=D:\myprogram;D:\myprogram\lib\supportLib.jar
D:\myprogram>java org.mypackage.HelloWorld

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:

D:\myprogram>java -classpath ".;c:\mylib\*" MyApp

Linux example:

$ java -classpath '.:/mylib/*' MyApp

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:

Main-Class: org.mypackage.HelloWorld
Class-Path: lib/supportLib.jar

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 java -jar. 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 -classpath 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:

Class-Path: lib/supportLib.jar lib/supportLib2.jar