AbstractMethodError when using traits in Groovy/Grails

Introduction

The 2.3 version of groovy introduced traits, sets of fields and methods that can be implemented by classes like superpowered interfaces or abstract classes; they not only contain the declaration, but also the definition of methods and can contain attributes. A class can implement multiple traits at once, providing some kind of multiple inheritance for groovy that classes could not provide in java.

AbstractMethodError

This feature being quite new, the compiler still has some glitches when dealing with traits. The most notable error is that when you change the source of a trait, it won't recompile the classes implementing it, causing sometimes an AbstractMethodError at runtime when the class doesn't find a recently added or modified method of the trait.

Resolution

To force the recompilation of the sources files, you can either do a full clean and rebuild of your project - but it can be a huge loss of time for a large project - or add a trivial modification to each source file implementing the trait to trigger their recompilation instead of using the cache; adding a comment for example, then compile - but for a trait that is widely used, like an helper trait used in every unit test, adding a comment to a large number of file can be tedious, and you must remember to remove them before committing...

A better solution

A better solution is to use a script add these comments in all the files of a folder to trigger their rebuilding and another one to remove the comments. This way you avoid the manual modification of the files while still avoiding the rebuild the full project.

The workflow after getting an AbstractMethodError:

1. Run the mark_files_as_changed.py script on the source folder that contains the classes implementing the trait. For a test helper trait use it on the test/ folder, for a controller helper trait run it on the controllers/ folder.
2. Recompile you project. Only the 'changed' classes, that have been marked by your script will be recompiled.
3. Run the unmark_files.py on the same folder, removing the comments added by the first script  to clean the files.

Implementation in Python:

mark_files_as_changed.py:

"""
This script appends comments to all files in the folder whose path is given as parameter, to ensure that
Intellij IDEA rebuilds them. This is useful to ensure that classes that implements a trait are recompiled when that trait is edited.
After compilation, please run the unmark files to remove the added comments.
Created on Tue Jul 19 14:04:51 2016
@author: hschoonjans
Python 3
"""

import os
import sys

walk_dir = sys.argv[1]

print('walk_dir = ' + walk_dir)

print('walk_dir (absolute) = ' + os.path.abspath(walk_dir))

APPEND = 'a'

for root, subdirs, files in os.walk(walk_dir):
    print('--\nroot = ' + root)

    for filename in files:
        file_path = os.path.join(root, filename)

        print('\t- file %s (full path: %s)' % (filename, file_path))

        with open(file_path, APPEND) as file:
            file.write("//CHANGED")

unmark_files.py:

"""
See mark_file_as_changed.py.
Removes the "//CHANGED" comments in all the files of the folder.
Created on Tue Jul 19 14:04:51 2016
@author: hschoonjans
Python 3
"""

import os
import sys

walk_dir = sys.argv[1]

print('walk_dir = ' + walk_dir)

READ_MODE = 'r'
WRITE_MODE = 'w'

for root, subdirs, files in os.walk(walk_dir):
    print('--\nroot = ' + root)

    for filename in files:
        file_path = os.path.join(root, filename)

        print('\t- file %s (full path: %s)' % (filename, file_path))

        f = open(file_path, READ_MODE)
        file_data = f.read()
        f.close()

        new_data = file_data.replace("//CHANGED", "")

        f = open(file_path, WRITE_MODE)
        f.write(new_data)
        f.close()

You can also find the implementation on my GitHub.