Skip to main content

The Java Classpath

 

Many developers who are new to Java often have trouble with the CLASSPATH. It may seem complex, but once you understand the underlying concept, it is not difficult at all. 

If you have programmed in C++, you will probably know that the PATH environment variable is used to locate dependencies. However, Java uses the CLASSPATH environment variable to locate dependencies. Take a look at the program below.

1 public class StudentRegistrationSystem {
2   public static void main(String args[ ]) {
3     ...
4     edu.scit.studentreg.Student st =
5       new edu.scit.studentreg.Student();
6     ...
7   }
8 }
Java2html

 


Notice that this program needs a class called 'Student'. The compiler as well as the runtime will need the Student class. The question is, how do they find it? They use the CLASSPATH environment variable.

The CLASSPATH is a list of directories, zip files or jar files.

Let us now understand how a JVM locates a class using the CLASSPATH. The Java runtime iterates through all entries in the CLASSPATH and searches for the class file in every directory, zip or jar file until the class is found. If it is not found, a ClassNotFoundException is thrown. If we put a class file in a directory not listed in the CLASSPATH, then as far as the Java runtime is concerned, it does not exist.

But wait, this is not the full story. Given a particular directory in the CLASSPATH, how does the JVM locate a class that it is searching for? Does it look for the class only in that directory, or does it look for the class in all sub-directories also? To help the JVM, we must follow certain conventions. I am assuming that you are familiar with the concept of packages in Java. If you are not, then you can read a simple explanation at Jarticles.com. The convention used is to match the directory heirarchy in which a class file is put, with the package name of the class.

Let us understand this concept with an example we have used above. Where will the runtime find the class edu.scit.studentreg.Student? When the JVM needs to locate the class edu.scit.studentreg.Student, it will look at the first entry in the classpath. Suppose it is c:/scit/classes. The JVM will now try to locate the class by zeroing in to the appropriate location based on the package name of the class. First it looks for a directory 'edu' in c:/scit/classes, if it finds the directory then it looks for 'scit' inside 'edu' and 'studentreg' inside 'scit'. Once in the 'studentreg' directory it looks for a file called Student.class. If the JVM finds the file using this algorithm, it is loaded, othewise it goes to the next entry in the classpath and tries to locate the file using the same mechanism. If the file cannot be found in any of the directories specified in the classpath, then a ClassNotFoundException is thrown.

The animation below explains how the JVM locates a class. Right click on area below and select "play" to start the animation.

 

So now you know, if you get a NoClassDefFoundError, it is because the Java runtime cannot locate your class file from entries in it's classpath. If the CLASSPATH variable has not been set, the current directory is used as the only classpath entry. If it contains any entries, then the current directory will not be considered part of the classpath, unless it is explicitly added.

Recommended Books:


Comments

Popular posts from this blog

Running your own one person company

Recently there was a post on PuneTech on mom's re-entering the IT work force after a break. Two of the biggest concerns mentioned were : Coping with vast advances (changes) in the IT landscape Balancing work and family responsibilities Since I have been running a one person company for a good amount of time, I suggested that as an option. In this post I will discuss various aspects of running a one person company. Advantages: You have full control of your time. You can choose to spend as much or as little time as you would like. There is also a good chance that you will be able to decide when you want to spend that time. You get to work on something that you enjoy doing. Tremendous work satisfaction. You have the option of working from home. Disadvantages: It can take a little while for the work to get set, so you may not be able to see revenues for some time. It takes a huge amount of discipline to work without a boss, and without deadlines. You will not get the benefits (insuranc

Testing Groovy domain classes

If you are trying to test Grails domain class constraints by putting your unit test cases in the 'test/unit' directory, then your tests will fail because the domain objects will not have the 'valdate' method. This can be resolved in two ways: Place the test cases inside test/integration (which will slow things down) Use the method 'mockForConstraintsTests(Trail)' to create mock method in your domain class and continue writing your test cases in 'test/unit' What follows is some example code around this finding. I am working on a Groovy on Grails project for a website to help programmers keep up and refresh their skills. I started with some domain classes and then moved on to write some unit tests. When we create a Grails project using grails create-app , it creates several directories, one of which is a directory called 'test' for holding unit tests. This directory contains two directories, 'unit', and 'integration' for unit and

Planning a User Guide - Part 3/5 - Co-ordinate the Team

Photo by  Helloquence  on  Unsplash This is the third post in a series of five posts on how to plan a user guide. In the first post , I wrote about how to conduct an audience analysis and the second post discussed how to define the overall scope of the manual. Once the overall scope of the user guide is defined, the next step is to coordinate the team that will work on creating the manual. A typical team will consist of the following roles. Many of these roles will be fulfilled by freelancers since they are one-off or intermittent work engagements. At the end of the article, I have provided a list of websites where you can find good freelancers. Creative Artist You'll need to work with a creative artist to design the cover page and any other images for the user guide. Most small to mid-sized companies don't have a dedicated creative artist on their rolls. But that's not a problem. There are several freelancing websites where you can work with great creative ar