46. Introduction

Groovy takes on Java’s approach to raising, handling and reporting errors and exceptions:

• Errors are serious problems that usually cause an application to halt (“crash”). In most cases you won’t handle these - they’ll happen and the program will fall over.
• Exceptions are less serious and applications, with good exception handling set up, may be able to keep going.

At the heart of this model is the java.lang.Throwable¹ class. Exceptions and Errors are are two sides to the Throwable family tree:

• java.lang.Exception is the superclass (parent) of all exceptions
• java.lang.Error is the superclass (parent) of all errors

When writing Groovy applications you’re likely to interact with exceptions in the following ways:

• Catching exceptions (using try-catch) thrown by various classes, be they written by you, part of the Java/Groovy libraries or from a third-party.
• Throwing your own exceptions using (throw) to signal that something’s gone wrong in your code.
• Being surprised and frustrated when exceptions “just happen”

Your goal should be to use defensive coding and testing that seeks to handle what you think is necessary but be prepared for issues to arise and use facilities such as logging to help you determine the root-cause.