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 (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).
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" (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:
Setting the path through an environment variable.
The environment variable named codice_3 may be alternatively used to set the classpath. For the above example, we could also use on Windows:
D:\myprogram ;set CLASSPATH=D:\myprogram
D:\myprogram ;java org.mypackage.HelloWorld
The rule is that codice_4 option, when used to start the java application, overrides the codice_3 environment variable. If none are specified, the current working directory is used as classpath. This means that when our working directory is codice_6 (on Linux, codice_7), we would not need to specify the classpath explicitly. When overriding however, it is advised to include the current folder codice_8 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:
the following command-line option is needed:
 java -classpath D:\myprogram;D:\myprogram\lib\supportLib.jar org.mypackage.HelloWorld
or alternatively:
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:
Linux example:
This works for both codice_4 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:
The manifest file defined in "helloWorld.jar" has this definition:
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 [app arguments]
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:
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.
References.
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