Setting the path to execute Java programs

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 will look 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 a java application launcher, a type of sdkTool(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:

set CLASSPATH=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 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 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:

set CLASSPATH=D:\myprogram;D:\myprogram\lib\supportLib.jar
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:

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

Suppose that a program has been enclosed in a Jar file called helloWorld.jar, put directly in the D:\myprogram directory. We have the following file structure:

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

It's important that the manifest file ends with either a new line or carriage return.

To launch the program, we can use the following command:

java -jar D:\myprogram\helloWorld.jar [app arguments]

This will automatically start the org.mypackage.HelloWorld specified in the Main-Class with the arguments and user cannot replace this class name using java -jar options. The Class-Path meantime describes the location of the supportLib.jar file relative to the location of the helloWorld.jar. Neither absolute file path (which is permitted in -classpath parameter on the command line) nor jar-internal paths are supported. This particularly means that if 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

OS specific notes

Being closely associated with the file system, the command-line Classpath syntax depends on the operating system. For example:

• on all Unix-like operating systems (such as Linux and Mac OS X), the directory structure has a Unix syntax, with separate file paths separated by a colon (":").
• on Windows, the directory structure has a Windows syntax, and each file path must be separated by a semicolon (";").

This does not apply when the Classpath is defined in manifest files, where each file path must be separated by a space (" "), regardless of the operating system.

Diagnose

Application programmers may want to find out/debug the current settings under which the application is running:

System.getProperty("java.class.path")

JavaDoc

External links

• Note explaining how Java classes are found, on Oracle website
• Specification of how to set the Classpath on Oracle site
• Specification of how to install java.