Classpath (Java)

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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.


[edit] 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 classpath tells java where to look on the filesystem for files defining these classes.

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).

[edit] Setting the path to execute Java programs

[edit] Basic usage

Suppose we have a package called org.mypackage containing the classes:

  • HelloWorld (main class),
  • SupportClass,
  • UtilClass.

and the files defining this package is stored physically under the directory D:\myprogram (on Windows).

The file structure will look like this:

      ---> org\  
            ---> mypackage\
                     ---> HelloWorld.class       
                     ---> SupportClass.class   
                     ---> UtilClass.class     

When we invoke java, we specifiy 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

where :

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

Note that if we ran java in D:\myprogram\ then we would not need to specify the classpath since java implicitly looks in the current working directory for files containing classes.

[edit] 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

[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 :

      ---> 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 

or alternatively :

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

      ---> 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.

Also, note that the classpath string in this case describes the location of the supportLib.jar file relative to the location of the helloWorld.jar file, and not as an absolute file path (as it might be when setting the -classpath parameter on the command line, for example). Thus, the actual locations of the jar file and its support library are irrelevant so long as the relative directory structure between the two is preserved.

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 at launch, 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

[edit] 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.

[edit] See also

[edit] References

[edit] External links

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