Classpath


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.

Overview and architecture

Similar to the classic dynamic loading behavior, when executing Java programs, the Java Virtual Machine finds and loads classes lazily. 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:
  1. bootstrap classes: the classes that are fundamental to the Java Platform.
  2. extension classes: packages that are in the extension directory of the JRE or JDK, jre/lib/ext/
  3. 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.

Setting the path to execute Java programs

Supplying as application argument

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 or /home/user/myprogram.
The file structure looks like this:
Microsoft WindowsLinux

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:
Microsoft WindowsLinux

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

 java -cp /home/user/myprogram org.mypackage.HelloWorld

where:
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\, we would not need to specify the classpath explicitly. When overriding however, it is advised to include the 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 located 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

If a program has been enclosed in a Jar file called helloWorld.jar, located directly in the directory D:\myprogram, the directory structure is as follows:

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

The manifest file should end with either a new line or carriage return.
The program is launched with the following command:
java -jar D:\myprogram\helloWorld.jar
This automatically starts org.mypackage.HelloWorld specified in class Main-Class with the arguments. The user cannot replace this class name using the invocation. Class-Path describes the location of supportLib.jar relative to the location of the library helloWorld.jar. Neither absolute file path, which is permitted in parameter on the command line, nor jar-internal paths are supported. This means that if the 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:
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.