Java classpath

Learn how to set classpath as environment variable and pass as command-line argument. During runtime of any Java application, the CLASSPATH is a parameter that tells the JVM where to look for classes and packages, and it can be set using an environment variable or command-line argument.

Class Path Separator :

Windows – ; [Semicolon]
Linux/Unix – : [Colon]

1. Set java classpath as environment variable

When you have set of jar files which are always required during your application runtime, then it’s probably best to add them in machine’s environment variable 'CLASSPATH'. During application runtime, application class loader will always scan the jar files and classes at specified paths in this variable.

To set classpath environment variable, find the location of user variables in your machine and add all paths where Jar files are stored. Use the separator between different two folders, jar files or classes.

For example, you can find the environment variables by –

1. From the desktop, right click the Computer icon.
2. Choose Properties from the context menu.
3. Click the Advanced system settings link.
4. Click Environment Variables. In the section System Variables, find the CLASSPATH environment variable and select it. Click Edit. If the CLASSPATH environment variable does not exist, click New.
5. Add all folders separated with separator. Click OK. Close all remaining windows by clicking OK.

If you are creating CLASSPATH for the first time, you need to specify the name for Variable Name in the window. Use '.' (dot) to denote current directory.

2. Set java classpath from command line

Use -classpath argument to set classpath from command prompt/console. Use below given commands to set classpath for different requirements. Let’s say we have a folder named dependency where JAR files and other classes are placed.

2.1. Add single jar file in classpath

Below syntax examples will add single jar file in classpath.

//WINDOWS
$ set CLASSPATH=.;C:\dependency\framework.jar

//Linux/Unix
$ export CLASSPATH=.:/dependency/framework.jar

2.2. Add multiple jar files in classpath

Below syntax examples will add more than one jar file in classpath. To do so, simply use the delimiter for your operating system (either ; or :) as a separator between the locations specified for the CLASSPATH.

To add all JAR files present in a directory, use wildcard character ('*').

//WINDOWS
$ set CLASSPATH=C:\dependency\framework.jar;C:\location\otherFramework.jar 				
$ set CLASSPATH=C:\dependency\framework.jar;C:\location\*.jar

//Linux/Unix
$ export CLASSPATH=/dependency/framework.jar:/location/otherFramework.jar  	
$ export CLASSPATH=/dependency\framework.jar:/location/*.jar

2.3. Add classes to classpath

Many times, you may need to add individual classes in classpath as well. To do so, simply add the folder where classfile is present. e.g. let’s say there are five .class files are present in location folder which you want to include in classpath.

//WINDOWS
$ set CLASSPATH=C:\dependency\*;C:\location

//Linux/Unix
$ export CLASSPATH=/dependency/*:/location

As a best practice, always organize all JAR files and application classes inside one root folder. This may be the workspace for the application.

3. Execute Java programs with ‘-classpath’ argument

Apart from setting classpath to the environment variable, you can pass additional classpath to Java runtime while launching the application using –classpath argument.

$ javac –classpath C:\dependency\framework.jar MyApp.Java
$ java –classpath C:\dependency\framework.jar MyApp

4. How to check classpath

Anytime you wish to verify all path entries in CLASSPATH variable, you can verify using echo command.

//Windows
c:/> echo %CLASSPATH%

//Linux/Unix
$ echo $CLASSPATH

If CLASSPATH is not set you will get a CLASSPATH: Undefined variable error (Solaris or Linux) console or simply %CLASSPATH% printed in windows command prompt.

Happy Learning !!

Read More:

Oracle Java Doc