What is a classpath and how do I set it?

Question

I was just reading this line:

The first thing the format() method does is load a Velocity template from the classpath named output.vm

Please explain what was meant by classpath in this context, and how I should set the classpath.

Answer 1

When programming in Java, you make other classes available to the class you are writing by putting something like this at the top of your source file:

import org.javaguy.coolframework.MyClass; 

Or sometimes you 'bulk import' stuff by saying:

import org.javaguy.coolframework.*; 

So later in your program when you say:

MyClass mine = new MyClass(); 

The Java Virtual Machine will know where to find your compiled class.

It would be impractical to have the VM look through every folder on your machine, so you have to provide the VM a list of places to look. This is done by putting folder and jar files on your classpath.

Before we talk about how the classpath is set, let's talk about .class files, packages, and .jar files.

First, let's suppose that MyClass is something you built as part of your project, and it is in a directory in your project called output. The .class file would be at output/org/javaguy/coolframework/MyClass.class (along with every other file in that package). In order to get to that file, your path would simply need to contain the folder 'output', not the whole package structure, since your import statement provides all that information to the VM.

Now let's suppose that you bundle CoolFramework up into a .jar file, and put that CoolFramework.jar into a lib directory in your project. You would now need to put lib/CoolFramework.jar into your classpath. The VM will look inside the jar file for the org/javaguy/coolframework part, and find your class.

So, classpaths contain:

• JAR files, and
• Paths to the top of package hierarchies.

How do you set your classpath?

The first way everyone seems to learn is with environment variables. On a unix machine, you can say something like:

export CLASSPATH=/home/myaccount/myproject/lib/CoolFramework.jar:/home/myaccount/myproject/output/ 

On a Windows machine you have to go to your environment settings and either add or modify the value that is already there.

The second way is to use the -cp parameter when starting Java, like this:

java -cp "/home/myaccount/myproject/lib/CoolFramework.jar:/home/myaccount/myproject/output/"  MyMainClass 

A variant of this is the third way which is often done with a .sh or .bat file that calculates the classpath and passes it to Java via the -cp parameter.

There is a "gotcha" with all of the above. On most systems (Linux, Mac OS, UNIX, etc) the colon character (':') is the classpath separator. In windowsm the separator is the semicolon (';')

So what's the best way to do it?

Setting stuff globally via environment variables is bad, generally for the same kinds of reasons that global variables are bad. You change the CLASSPATH environment variable so one program works, and you end up breaking another program.

The -cp is the way to go. I generally make sure my CLASSPATH environment variable is an empty string where I develop, whenever possible, so that I avoid global classpath issues (some tools aren't happy when the global classpath is empty though - I know of two common, mega-thousand dollar licensed J2EE and Java servers that have this kind of issue with their command-line tools).