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 set either on the command-line, or through an environment variable
Setting the path to execute Java programs
Supplying as application argumentSuppose we have a package called org.mypackage containing the classes:
- HelloWorld (main class)
The file structure looks like this:
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:
java -classpath D:\myprogram org.mypackage.HelloWorld
java -cp /home/user/myprogram org.mypackage.HelloWorld
javais a java application launcher, a type of sdkTool(A command-line tool, such as javac, javadoc, or 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 variableThe environment variable named
CLASSPATHmay 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
-classpathoption, when used to start the java application, overrides the
CLASSPATHenvironment variable. If none are specified, the current working directory is used as classpath. This means that when our working directory is
/home/user/myprogram/), we would not need to specify the classpath explicitly. When overriding however, it is advised to include 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 fileIf a program uses a supporting library enclosed in a Jar file called supportLib.jar, physically in the directory D:\myprogram\lib\ and the corresponding physical file structure is:
| ---> 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.HelloWorldor alternatively:
set CLASSPATH=D:\myprogram;D:\myprogram\lib\supportLib.jar java org.mypackage.HelloWorld
Adding all JAR files in a directoryIn Java 6 and higher, one can add all jar-files in a specific directory to the classpath using wildcard notation.
java -classpath ".;c:\mylib\*" MyApp
java -classpath '.:/mylib/*' MyApp
This works for both
-classpathoptions and environment classpaths.