ClassPath

Notes

Understanding the class path and package names

Java classes are organized into packages which are mapped to directories in the file system. But, unlike the file system, whenever you specify a package name, you specify the whole package name – never part of it. For example, the package name for java.awt.Button is always specified as java.awt.

For example, suppose you want the Java runtime to find a class named Cool.class in the package utility.myapp.

If the path to that directory is:

C:\java\MyClasses\utility\myapp

you would set the class path so that it contains:

C:\java\MyClasses

To run that app, you could use the following JVM command:

C:> java -classpath C:\java\MyClasses utility.myapp.Cool

When the app runs, the JVM uses the class path settings to find any other classes defined in the utility.myapp package that are used by the Cool class.

Note that the entire package name is specified in the command. It is not possible, for example, to set the class path so it contains C:\java\MyClasses\utility and use the command java myapp.Cool.

The class would not be found.

See: https://java.sun.com/docs/books/tutorial/getStarted/cupojava/win32.html

Sun Java 1.6

Class path entries can contain the basename wildcard character *, which is considered equivalent to specifying a list of all the files in the directory with the extension .jar or .JAR:

  • Understanding class path wildcards

  • A class path entry that contains * will not match class files.

  • For example, the class path entry foo/* specifies all JAR files in the directory named foo.

  • Class-Path Wildcards in Mustang… Linux - Note that you must quote the asterisk in order to prevent it from being interpreted by your shell…

java -classpath "*" com.sample.Search c:\temp\

Error Explanation

Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorldApp

If you receive this error, java cannot find your bytecode file, HelloWorldApp.class.

One of the places java tries to find your bytecode file is your current directory. So, if your bytecode file is in C:\java, you should change your current directory to that. To change your directory, type the following command at the prompt and press Enter:

cd c:\java

The prompt should change to C:\java. If you enter dir at the prompt, you should see your .java and .class files. Now enter java HelloWorldApp again.

If you still have problems, you might have to change your CLASSPATH variable. To see if this is necessary, try “clobbering” the classpath with the following command:

set CLASSPATH=

Now enter java HelloWorldApp again. If the program works now, you’ll have to change your CLASSPATH variable. For more information, consult the section 5. Update the PATH variable (outside of the tutorial) in the installation instructions.

Example, Trying to run lia.analysis.AnalyzerDemo in:

C:\Tools\LuceneInAction\build\classes\lia\analysis

Did not work because I was not including the current folder in the classpath. This is the working command line:

java -cp c:\tools\lucene-1.4.3\lucene-1.4.3.jar;. lia.analysis.AnalyzerDemo

Ran from:

c:\Tools\LuceneInAction\build\classes\

Printing

How to print classpath

import java.net.URL;
import java.net.URLClassLoader;

public class PrintClasspath {
    public static void main(String[] args) {

        //Get the System Classloader
        ClassLoader sysClassLoader = ClassLoader.getSystemClassLoader();

        //Get the URLs
        URL[] urls = ((URLClassLoader)sysClassLoader).getURLs();

        for(int i=0; i< urls.length; i++)
        {
            System.out.println(urls[i].getFile());
        }

    }
}