Classpath (Java)

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For the Free Software Foundation's implementation of the Java standard library, see GNU Classpath.

The Classpath is an argument set on the command-line that tells the Java Virtual Machine where to look for user-defined classes and packages in Java programs.

By default only the packages of the JDK standard API are accessible without needing to set where to find them. The path for all user-defined packages and libraries must be set in the command-line (or in the Manifest associated with the Jar file containing the classes).

1 Examples
2 Specificities
3 See also
4 References

[edit] Examples

[edit] Basic example

Suppose we have a package structure called org.mypackage containing the following classes : HelloWorld (main class), SupportClass, and UtilClass, the package being physically under the directory D:\myprogram (on Windows).

The corresponding physical file structure is :

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

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

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

where :

-classpath D:\myprogram set the path to the packages used in the program
org.mypackage.HelloWorld is the path of the main class

[edit] Setting the path of a Jar file

Now, suppose the program uses a supporting library enclosed in a Jar file called supportLib.jar, physically in the directory D:\myprogram\lib\.

The corresponding physical file structure is :

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

We should use the following command-line option :

java -classpath D:\myprogram;D:\myprogram\lib\supportLib.jar 
     org.mypackage.HelloWorld

[edit] Setting the path in a Manifest file

Suppose that our 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 this Jar file have this definition :

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

Note: It is important that the manifest file end with either a new line or carriage return.

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

java -jar D:\myprogram\helloWorld.jar

It is not necessary to define the Main class, the Classpath to the program classes, and the support library classes, because they are already defined in the manifest file.

The syntax for specifying multiple library JAR files in the manifest file is to put a space between each file, as so:

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

[edit] Specificities

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

on Windows, the directory structure has a Windows syntax, and each filepath must be separated by a semicolon (";").
on Linux, the directory structure has a Linux syntax, and each filepath must be separated by a colon (":").

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