CLASSPATH in JAVA

CLASSPATH: CLASSPATH describes the location where the required “.class files” are available. 
 Then JVM uses CLASSPATH to locate required .class files.

 We can set the CLASSPATH in the following 3 ways.

  1. By using environment variable CLASSPATH (in WINDOWS),by updating .bashrc (in LINUX/UNIX).

     This CLASSPATH will be preserved even after system restart aslo. Hence this is permanent way of setting CLASSPATH.

2.  Temporary for a particular command prompt level by using “set” command ( in Windows OS),

        "export" command (in Linux/Unix OS).

               

          C:\>set CLASSPATH = %CLASSPATH%;C:\ME

          me# export CLASSPATH= ${CLASSPATH}:/home

 

·         Once if you close the command prompt/terminal automatically this class path will be lost.

      

      3. We can set the CLASSPATH for a particular command level by using “–cp” (or) “–classpath” option

        

                C:\>java -cp D:\ME WordCount

      By using -cp or -classpath option is conveying to the JVM that the required classes are available in D:\ME

        Once command is executed this classpath will be lost.

     Examples:

     

     C:\java Student   (valid) but Runtime error :java.lang.NoSuchMethodError: main Exception in thread “main” as Student.java doesn't have main(String[] args) method)

     C:\java College   //Runtime error : NoClassDefFoundError: College

     C:\java -cp D:\ME College  (valid)

    C:\java -cp D:\ME Student    // Runtime error : NoClassDefFoundError: Student (here JVM checks Student.class inside D:\ME, as it couldn't found the Student.class then thrown error).

• If we are not setting CLASSPATH then the JVM always search in the current working directory for the required .class files.
• if we are setting CLASSPATH explicitly then JVM won't search in current working directory.It always search in the specified CLASSPATH only.

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