Inheriting constructors in Groovy classes

In Java, class inheritance doesn't support the inheriting of constructors for a number of good reasons (leaving the details of constructing an object to the programmer is generally a good idea). There are times when automatically inheriting the constructors of a class would be really useful and make the code less verbose. One of these cases is when inheriting from a class that extends Exception, where all the constructors are just calling super. Groovy has a convenient annotation for doing just that, @InheritConstructors.

In this recipe, we will explore how to use this annotation.

How to do it...

Let's demonstrate the features that the @InheritConstructors annotation gives:

  1. Create an Exception class: one of the classes that are used to communicate that something is horribly wrong with the code or the data:
    class BusinessException extends Exception {

}
  2. Try to instantiate the class using one of the default constructors of the Exception class, for instance:
    def be = new BusinessException('missing resource')

    The code fails at runtime with the following error:

    Could not find matching constructor for:
BusinessException(java.lang.String)
  3. Add the groovy.transform.InheritConstructors annotation to the BusinessException and try to instantiate the class again with a String message. This time, the code will execute without errors:
    @groovy.transform.InheritConstructors
class BusinessException extends Exception {

}

How it works...

The @InheritConstructors annotation removes the boilerplate of writing matching constructors for a superclass. By adding the @InheritConstructors annotation to the class, we can create the BusinessException by using any of the constructors exposed by java.lang.Exception:

import groovy.transform.InheritConstructors

@InheritConstructors
class BusinessException extends Exception { }

assert new BusinessException('hello').message == 'hello'

def b1 = new BusinessException('missing resource')
def b2 = new BusinessException('catastrophic failure', b1)
assert b2.cause.message == 'missing resource'

The annotation is smart enough to detect your existing constructors, if any, and avoid overriding them during the constructor generation phase.

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