Classpath (Java)

Classpath is a parameter in the Java Virtual Machine or the Java compiler that specifies the location of user-defined classes and packages. The parameter may be set either on the command-line, or through an environment variable.

Similar to the classic dynamic loading behavior, when executing Java programs, the Java Virtual Machine finds and loads classes lazily (it loads the bytecode of a class only when the class is first used). The classpath tells Java where to look in the filesystem for files defining these classes.

The virtual machine searches for and loads classes in this order:

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).

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

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:

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:

where:

The environment variable named CLASSPATH may be alternatively used to set the classpath. For the above example, we could also use on Windows:

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