java urlclassloader usage.Is it needed only in some rare cases?

In which case one would use a URLClassLoader to load a class from a specific jar in a specified path?

E.g.

URL url = new URL("file:///path/to/customClasses.jar");
URLClassLoader pluginLoader = new URLClassLoader(new URL[] { url });
Class<?> cl = pluginLoad开发者_Python百科er.loadClass("apackage.MyCustomClass");

If I wanted to use classes from customClasses.jar, I always thought that placing this jar in a path accesible from CLASSPATH is enough.

Then in my code just use apackage.MyCustomClass.

I think I have something missunderstood or missing here, so could someone please explain and give an example of when the above snippet of loading class this way is useful?

Thanks!

I would say that depending on the type of programming you are doing, the use of the URLClassLoader should be a very rare occurrence.

Typically you will use the class loader for loading in classes at runtime that you couldn't anticipate in advance.

A good example is if you build a tool that can be extended with plugins, and the plugins are loaded at runtime. For example, Eclipse.

If you have the jar available at compile time and are on a command line, add the needed jar file to your compile statement. For example,

javac -cp /path/to/lib/customClasses.jar MyClassThatReferencesCustomClasses

If you're using Eclipse, add the jar to your project, and right click on it and select add to build path.

Regards,

Will