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{{For|the software|GNU Classpath}}
{{Multiple issues|
{{Primary sources|date=November 2020}}
{{Howto|date=November 2020}}
}}

'''Classpath''' is a parameter in the [[Java virtual machine|Java Virtual Machine]] or the [[Java compiler]] that specifies the location of user-defined [[Class (computer science)|classes]] and [[Java package|packages]]. The parameter may be set either on the [[Command line interface|command-line]], or through an [[environment variable]].

== Overview and architecture ==
{{See also|Java class loader}}
Similar to the classic [[Library (computing)#Dynamic loading|dynamic loading]] behavior, when executing [[Java (programming language)|Java]] programs, the [[Java virtual machine|Java Virtual Machine]] finds and loads classes lazily (it loads the [[Java bytecode|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:
# 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: [[Java package|packages]] that are in the ''extension'' directory of the [[Java Runtime Environment]] or [[JDK]], <code>jre/lib/ext/</code>
# user-defined packages and libraries

By default only the packages of the [[JDK]] [[Java Platform, Standard Edition|standard API]] and extension packages are accessible without needing to set where to find them. The path for all user-defined [[Java package|packages]] and libraries must be set in the command-line (or in the [[manifest file|Manifest]] associated with the [[JAR (file format)|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:
* ''HelloWorld'' (main class)
* ''SupportClass''
* ''UtilClass''

and the files defining this package are stored physically under the directory ''D:\myprogram'' (on [[Microsoft Windows|Windows]]) or ''/home/user/myprogram'' (on [[Linux]]).

The file structure looks like this:

{| class="wikitable" style="font-size:90%"
|-
! [[Microsoft Windows]]
! [[Linux]]
|-
| <pre>
D:\myprogram\
|
---> org\
|
---> mypackage\
|
---> HelloWorld.class
---> SupportClass.class
---> UtilClass.class
</pre>
| <pre>
/home/user/myprogram/
|
---> org/
|
---> mypackage/
|
---> HelloWorld.class
---> SupportClass.class
---> UtilClass.class
</pre>
|-
|}

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:
{| class="wikitable" style="font-size:90%"
|-
! [[Microsoft Windows]]
! [[Linux]]
|-
| <pre>
java -classpath D:\myprogram org.mypackage.HelloWorld
</pre>
| <pre>
java -cp /home/user/myprogram org.mypackage.HelloWorld
</pre>
|-
|}
where:

* <code>java</code> is the [[Java runtime]] launcher, a type of SDK Tool (A command-line tool, such as [[javac]], [[javadoc]], or [[APT (software)|apt]])
* ''-classpath D:\myprogram'' sets the path to the packages used in the program (on Linux, ''-cp /home/user/myprogram'') and
* ''org.mypackage.HelloWorld'' is the name of the main class

=== Setting the path through an environment variable ===
The [[environment variable]] named <code>CLASSPATH</code> may be alternatively used to set the classpath. For the above example, we could also use on Windows:

<syntaxhighlight lang="doscon">
D:\myprogram>set CLASSPATH=D:\myprogram
D:\myprogram>java org.mypackage.HelloWorld
</syntaxhighlight>

The rule is that <code>-classpath</code> option, when used to start the java application, overrides the <code>CLASSPATH</code> environment variable. If none are specified, the [[Working directory|current working directory]] is used as classpath. This means that when our working directory is <code>D:\myprogram\</code> (on Linux, <code>/home/user/myprogram/</code>), we would not need to specify the classpath explicitly. When overriding however, it is advised to include the current folder <code>"."</code> 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 format)|Jar file]] called ''supportLib.jar'', physically located in the directory ''D:\myprogram\lib\'' and the corresponding physical file structure is:
<pre>
D:\myprogram\
|
---> lib\
|
---> supportLib.jar
|
---> org\
|
---> mypackage\
|
---> HelloWorld.class
---> SupportClass.class
---> UtilClass.class
</pre>

the following [[Command-line interface#Command-line option|command-line option]] is needed:

java -classpath D:\myprogram;D:\myprogram\lib\supportLib.jar org.mypackage.HelloWorld

or alternatively:
{{sxhl|2=doscon|1=
D:\myprogram>set CLASSPATH=D:\myprogram;D:\myprogram\lib\supportLib.jar
D:\myprogram>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:
{{sxhl|2=doscon|1=
D:\myprogram>java -classpath ".;c:\mylib\*" MyApp
}}

Linux example:
{{sxhl|2=console|1=
$ java -classpath '.:/mylib/*' MyApp
}}

This works for both <code>-classpath</code> options and environment classpaths.

=== Setting the path in a manifest file ===
If a program has been enclosed in a [[JAR (file format)|Jar file]] called ''helloWorld.jar'', located directly in the directory ''D:\myprogram'', the directory structure is as follows:
<pre>
D:\myprogram\
|
---> helloWorld.jar
|
---> lib\
|
---> supportLib.jar
</pre>
The [[manifest file]] defined in ''helloWorld.jar'' has this definition:
{{sxhl|2=properties|
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 [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 {{code|java -jar}}. ''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 {{code|-classpath}} 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:
{{sxhl|2=properties|
Class-Path: lib/supportLib.jar lib/supportLib2.jar
}}

== OS specific notes ==
Being closely associated with the file system, the [[Command-line argument|command-line]] Classpath syntax depends on the operating system.<ref>{{Cite web|url=http://www.antwerkz.com/blog/the-classpath|title=The Classpath|date=6 August 2013 |access-date=2016-06-26}}</ref> For example:

* on all [[Unix|Unix-like]] operating systems (such as [[Linux]] and [[Mac OS X]]), the directory structure has a Unix syntax, with separate file paths separated by a [[Colon (punctuation)|colon]] (":").
* on [[Microsoft Windows|Windows]], the directory structure has a Windows syntax, and each file path must be separated by a [[semicolon]] (";").

This does not apply when the Classpath is defined in [[manifest file]]s, where each file path must be separated by a space (" "), regardless of the operating system.

== See also ==
* [[Java class loader]]
* [[Java Platform Module System]]
* {{Annotated link|rpath}}

== References ==
{{Reflist}}

== External links ==
* [http://docs.oracle.com/javase/6/docs/technotes/tools/findingclasses.html Note explaining how Java classes are found, on Oracle website]
* [http://docs.oracle.com/javase/1.5.0/docs/tooldocs/windows/classpath.html Specification of how to set the Classpath on Oracle site]

[[Category:Java platform|Classpath]]

Classpath - Revision history

imported>Citation bot: Added date. | Use this bot. Report bugs. | Suggested by Dominic3203 | Category:Articles containing how-to sections | #UCB_Category 233/794