When you submit a form to a CGI program that resides on the server, it is usually programmed to do its own check for errors.
If it finds any it sends the page back to the reader who then has to re-enter some data, before submitting again.
If you are using a free-form Java project, the IDE uses your existing Ant build script.
With standard Java projects, you can customize the build process by doing any of the following: By default, the IDE compiles the classes in a standard project when you save them.
Read the setup guide if you want to know which configuration options that are available when setting up the validation.
Here you can read more about styling of the form and error dialogs.