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

Overview and Architecture

Similar to the classic dynamic loading behavior, when executing Java programs, the Java Virtual Machine finds and loads classes lazily (the bytecode of a class is loaded only when this class is first used). The virtual machine searches and loads classes in this order:

Bootstrap classes: the classes that are fundamental to the Java Platform (comprising the public classes of the Java Class Library, and the private classes that are necessary for this library to be functional).
Extension classes: packages that are in the extension directory of the JRE or JDK.
User-defined packages and libraries

By default only the packages of the JDK standard API and extension packages 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).

Setting the path to execute Java programs

Basic usage

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

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

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

or alternatively :

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

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 has this definition:

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

Note: 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

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 separate the entries with a space:

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 Linux, Mac OS X, and more generally on all Unix-like operating systems, the directory structure has a Unix syntax, and each filepath must be separated by a colon (":").
on Windows, the directory structure has a Windows syntax, and each filepath must be separated by a semicolon (";").

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.

References

External links

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 == OS specific notes ==
 Being closely associated with the file system, the command-line Classpath syntax depends on the operating system. For example :
-* on [[Microsoft Windows|Windows]], the directory structure has a [[Microsoft Windows|Windows]] syntax, and each filepath must be separated by a [[semicolon]] (";").
 * on [[Linux]], [[Mac OS X]], and more generally on all [[Unix|Unix-like]] operating systems, the directory structure has a [[Unix]] syntax, and each filepath must be separated by a [[Colon (punctuation)|colon]] (":").
+* on [[Microsoft Windows|Windows]], the directory structure has a [[Microsoft Windows|Windows]] syntax, and each filepath must be separated by a [[semicolon]] (";").
 This does not apply when the Classpath is defined in [[manifest file|Manifest files]], where each filepath must be separated by a space (" "), regardless of the operating system.