Groovy Mocking a Method That Exists on Object

In Groovy, the find method exists on Object. The find method is also an instance method on EntityManager which is commonly used in JPA to get an instance based on a database id. I was trying to create a Mock of EntityManager like:

def emMock = new MockFor(EntityManager)
emMock.demand.find { Class clazz, Object rquid -> return new Request(rqUID: rquid) }
def em = emMock.proxyDelegateInstance()

This gave me an error though:

groovy.lang.MissingMethodException: No signature of method:$_2_get_closure1.doCall() is applicable for argument types: (groovy.mock.interceptor.Demand) values: [groovy.mock.interceptor.Demand@a27ebd9]
Possible solutions: doCall(java.lang.Class, java.lang.Object), findAll(), findAll(), isCase(java.lang.Object), isCase(java.lang.Object)
at org.codehaus.groovy.runtime.metaclass.ClosureMetaClass.invokeMethod( ~[groovy-all-1.8.6.jar:1.8.6]
at groovy.lang.MetaClassImpl.invokeMethod( ~[groovy-all-1.8.6.jar:1.8.6]
at ~[groovy-all-1.8.6.jar:1.8.6]
at ~[groovy-all-1.8.6.jar:1.8.6]

It ends up that find is a DefaultGroovyMethod that’s added to all Objects and this was causing the mock.demand to get confused because it thought I was trying to call the Object.find.

Luckily there is a workaround and that’s to use the ordinal argument for find.

emMock.demand.find(1) { Class clazz, Object rquid -> return new Request(rqUID: rquid) }

That’s enough of a change to invoke mock version instead of the DefaultGroovyMethod version.
Now if I only knew why….

Grails Testing Acegi Security

Almost every web application I create needs some form of user authentication and authorization. Grails provides a great plugin system that allows you to extend the base framework to provide these kinds of features. My current favorite security plugin for Grails is the Acegi Security Plugin. It’s built using the Spring Security framework and the robust and widely used Acegi framework. I like the fact that it is built on top of existing, well-known frameworks which is the philosophy of Grails itself. It is also a flexible plugin that has a good default object model.

One of the situations that inevitably arises is how to unit test code that depends on your security framework. Either you will want to test that basic authorization is working, or you might have code that makes decisions based upon who is logged in or what roles they might have. To run those test you have to setup your test cases so that the code that works when the system is running normally is tested.

Unit Testing Setup for Testing Acegi Security Code

This situation arose for me the other day and so I thought I would share the solution that I came up with.

import as SCH

class AuthenticationBaseTests extends GroovyTestCase {
def setUp() {
def user = User.get(1) // or create a new one if one doesn't exist
authenticate(user, "abc123", [new GrantedAuthorityImpl('ROLE_ADMIN')])

protected Authentication authenticate(user, credentials, authorities) {
def principal = new Expando()
principal.domainClass = user

Authentication authentication = new TestingAuthenticationToken(principal, null, authorities as GrantedAuthority[])
authentication.authenticated = true
SCH.context.authentication = authentication
return authentication


The setUp() method will get executed prior to your tests being run. So if you want to do different things for each test you would want to move this code to a more appropriate place. In the setup method we are creating our Principle class, in this case, the User. Next we pass the user along with all of the roles we want the user to have for the given test case.

authenticate(user, credentials, authorities)

The real setup work is done in the authenticate() method. This method does the real work of setting up the Authentication Token and setting it in the static context, which is a thread local variable I think, that is used by the framework to determine who is making the request and what rights they have.

The fun thing about this message is the use of the Expando class. Expando allows you to create a class on the fly. This is a really interesting way to do a Mock object on-the-fly in Groovy. In this case we’re creating a Mock authentication principle. We know, by looking at the Acegi Security code, that the security framework expects an object that will call domainClass to get the underlying implementation of the User (or Principle) implemented in the application. That will return us the User object that we are setting.

This simple method can be added to a base class and then inherited from any tests that need user authentication to run successfully.

On Singletons

The Singleton Pattern is one of the most widely used patterns from the Gang of Four (GoF) Desgin Patterns Book. One the reasons that it’s so widely used, I think, is because it’s also very easy to understand. The basic idea is that you control the creation of an object so that it will only ever have one instance. This is achieved by having a private constructor and a static instance that is accessed through a static method on the Class.

Singleton Example:

public class FirstSingleton {

private static final FirstSingleton instance = new FirstSingleton();

public static FirstSingleton instance() {
return instance;

private FirstSingleton() {

Reasons to Avoid the Singleton Pattern

Practical Limitations

There are some practical limitations of the Singleton pattern though. With modern VM based languages like Java there can be multiple classloaders in a given application. A Singleton then is no longer a Singleton as there will be an instance-per-classloader. So if you can’t enforce an actual single instance why use the Singleton Pattern? You might answer that the classloader case is a rare one and that in a “normal” application your not going to run into that. And you would be right. But I still think that you should avoid the Singleton

Multiple Concerns

Fundamentally a Class in Object Oriented Programming should have one and only one responsibility. The Single Responsibility Principle often leads to good design and the benefits of a highly-cohesive system such as being easily understood and being easier to change and maintain over time.

The Singleton, by its very definition, has multiple responsibilities. The primary responsibility defined in the pattern is controlling constructions of Objects of this Class. Of course from an application point of view, this is pretty weak. What you really want from a class is a useful service. So if you have a Singleton Data Access class which is more important? The Singularity of the instance or the ability to get at data in your data store? I really hope you picked the latter.

There are a number of ways that you can control access to the construction of a class that do not rely on the Singleton pattern. A Factory or a Container for example can be used to get at an instance in such a way that the calling code doesn’t care about the implementation of the construction of the class. Whether a new instance is created or a single instance returned should be immaterial to the user of that instance. This also does a good job of dealing with our Single Responsibility concern raised earlier. Now the inforcement of a single instance is handled by another class whose sole responsibility is that singleton enforcement.


One consideration that must be made when you are designing software is: How are you going to test the code? If no consideration is given, then often the code will be very difficult to test and quality will suffer.

The example of a Singleton above is relatively simple. But what happens if you introduce a dependency to your Singleton? Now you have a tight-coupling between two classes. If the dependent class does something like access the database or call a web service or itself has a bunch of dependencies then all of a sudden the code becomes very hard to test.

public class SecondSingleton {

private static final SecondSingleton instance = new SecondSingleton();

private SomeDependency myDependency;

public static SecondSingleton instance() {
return instance;

private SecondSingleton() {
myDependency = new SomeDependency();

I’ve written previously about Mock Objects for testing. Mock Objects allow you to create dummy implementations of dependencies to make testing a Class much easier. In the Singleton example here, you can see that there would be no way to inject a Mock instance into the SecondSingleton class.

Factories or Containers to Enforce Single Instances

One of the best ways to access dependencies in a class is through Constructor based Dependency Injection. This can be done through a container like Spring or Pico Container.

You can of course do this yourself as well. If you are using a Factory for object creation, then the singleton enforcement and the dependency injection can happen in the same class (whose sole responsibility is to manage the construction of objects).

public class NotSingleton {

private SomeDependency myDependency;

private NotSingleton(SomeDependency depends) {
myDependency = depends;

public class Factory {

private NotSingleton notsingleInstance
= new NotSingleton(
(SomeDependency) lookupType(SomeDependency.class));

public NotSingleton getNotSingleton() {
return notsingleInstance;

* Get an instance for a given Interface
* @param interface The interface for which you want the instance
public Object lookupType(Class clazz) {
// ...

With this structure you can now independently test all of your classes without worrying about the interactions of dependent objects. You also have not structured the code in such a way that having multiple instances should cause any problems, so your code won’t break if the classloader case ever comes up.

Mocking .NET Objects with NUnit

NUnit is my Unit Testing tool of choice for .NET development. Microsoft provides a unit testing framework but it only works with some higher-end versions of Visual Studio. They’re so similar that it’s almost ridiculous that Microsoft created their own version.
(See one of my previous posts for more information on Automating NUnit with MSBuild.) In the Java world it’s fairly common to do Mocking to help make unit testing easier. I’ve written about using JMock for Unit Tesing in Java. In this post, I’d like to talk about a relatively new feature of NUnit which now supports Mocks out of the box.

What Are Mock Objects

Mock Objects are a technique that allow you to isolate classes from their dependencies for testing purposes. This isolation allows for fine-grained testing of single methods in a single class, possibly even before the dependent classes are fully implemented. This isolation allows your tests to run quickly and it makes testing small pieces of functionality much easier. When you’ve tested individual pieces of code in isolation you can have much higher confidence in larger-grained tests. This isolation becomes even more interesting when you are dealing with dependencies such as a data layer or a web service layer. External calls like that can be very time consuming or could fail if the remote system is down for maintenance.

One of the great things about using Mock Objects is that they force you to think about the dependencies that your classes and methods have. It forces you to think about the coupling between your classes. If you have high coupling then your code is often harder to test. If you have a loosely coupled design then testing and using Mock Objects is very much easier. Thinking about those design notions early can help you more easily manage change over time.


  • Good design is better than bad design
  • Loosely coupled objects are usually a better design than tightly coupled objects
  • Testing improves code quality and developer efficiency over time
  • Testing is easier with a loosely coupled designs

A Sample Project

We’re going to start with some simple code. We create a Domain object called Person and an interface for a Data Access object called IPersonRepository. Pretty simple at this point.

public class Person
public string Id;
public string FirstName;
public string LastName;
public Person(string newId, string fn, string ln)
Id = newId;
FirstName = fn;
LastName = ln;

public interface IPersonRepository
List GetPeople();
Person GetPersonById(string id);

Next we create a PersonService object. This would represent all of the business logic in our application. It would interact with the Data Access tier and return information to the UI layer for display.

We wire together our objects using Constructor based Dependency Injection. All of the dependent Objects are sent in through the constructor. This allows for the loose coupling since the PersonService doesn’t know about the Implementing class, but only the interface. Since it’s done in the constructor we can also never have an invalid PersonService as would be the case if there was a setter for the IPersonRepository implementation.

This is again a fairly straightforward implementation, but I hope enough to display the issue at hand.

public class PersonService
private IPersonRepository personRepos;
public PersonService(IPersonRepository repos)
personRepos = repos;
public List GetAllPeople()
return personRepos.GetPeople();
public List GetAllPeopleSorted()
List people = personRepos.GetPeople();
people.Sort(delegate(Person lhp, Person rhp) {
return lhp.LastName.CompareTo(rhp.LastName);
return people;
public Person GetPerson(string id)
return personRepos.GetPersonById(id);
catch (ArgumentException)
return null; // no person with that id was found

Using Mocks with NUnit

Now we can start testing our PersonService. Notice that we haven’t even implemented the IPersonRepository yet. That way we can make sure that everything in our PersonService class works as expected without having to think about other layers of the application.

using System;
using System.Collections.Generic;
using NUnit.Framework;
using NUnit.Mocks;
public class PersonServiceTest
// The dynamic mock proxy that we will use to implement IPersonRepository
private DynamicMock personRepositoryMock;
// Set up some testing data
private Person onePerson = new Person("1", "Wendy", "Whiner");
private Person secondPerson = new Person("2", "Aaron", "Adams");
private List peopleList;
public void TestInit()
peopleList = new List();
// Construct a Mock Object of the IPersonRepository Interface
personRepositoryMock = new DynamicMock(typeof (IPersonRepository));
public void TestGetAllPeople()
// Tell that mock object when the "GetPeople" method is
// called to return a predefined list of people
personRepositoryMock.ExpectAndReturn("GetPeople", peopleList);
// Construct a Person service with the Mock IPersonRepository
PersonService service = new PersonService(
(IPersonRepository) personRepositoryMock.MockInstance);
// Call methods and assert tests
Assert.AreEqual(2, service.GetAllPeople().Count);
public void TestGetAllPeopleSorted()
// Tell that mock object when the "GetPeople" method is called to
// return a predefined list of people
personRepositoryMock.ExpectAndReturn("GetPeople", peopleList);
PersonService service = new PersonService(
(IPersonRepository) personRepositoryMock.MockInstance);
// This method really has "business logic" in it - the sorting of people
List people = service.GetAllPeopleSorted();
Assert.AreEqual(2, people.Count);
// Make sure the first person returned is the correct one
Person p = people[0];
Assert.AreEqual("Adams", p.LastName);
public void TestGetSinglePersonWithValidId()
// Tell that mock object when the "GetPerson" method is called to
// return a predefined Person
personRepositoryMock.ExpectAndReturn("GetPersonById", onePerson, "1");
PersonService service = new PersonService(
(IPersonRepository) personRepositoryMock.MockInstance);
Person p = service.GetPerson("1");
Assert.AreEqual(p.Id, "1");
public void TestGetSinglePersonWithInalidId()
// Tell that mock object when the "GetPersonById" is called with a null
// value to throw an ArgumentException
new ArgumentException("Invalid person id."), null);
PersonService service = new PersonService(
(IPersonRepository) personRepositoryMock.MockInstance);
// The only way to get null is if the underlying IPersonRepository
// threw an ArgumentException

The PersonService doesn’t have a lot of logic in it, but I hope this illustrates how you easily can test various conditions using Mock objects. It also illustrates the idea of testing early by allowing you to test some code before all of the dependent objects are implemented.

While the Mocks built into NUnit might not be the most powerful or complete Mocking library out there, it should be sufficient for most uses. I’m sure they will continue to improve them over time as well, so I look forward to them becoming more powerful (and having better documentation) in the future.

Download Code Example:
NMock C# Example Project

For more on NUnit you might like to check out:

jMock For Mocking Unit Tests

Ok, so I might not exactly be on the cutting edge with this post, but I just started playing with jMock, a framework for creating Mock objects.

Mock objects are used to isolate various layers of an application by providing a fake implementation that will mimic the behavior of the real implementation but offers a deterministic behavior. It’s very useful for isolating database layers as well because DB access can slow down unit tests dramatically. And if unit tests take to long then they won’t get run. Like Inversion of Control (IoC), Mock Objects, can be done by hand without the use of a framework, but frameworks can make the job a lot easier.

jMock alows you to setup the methods that you want to call and define the return values for a given set of parameters. You basically script the expected calls with the return values you want. This lets you isolate classes so that you can test just a single method or class at a time thus simplifying the tests.

public class MockTester extends MockObjectTestCase {
Mock mockOrderService;
OrderService orderService;

public void setUp() throws Exception {
mockOrderService = new Mock(OrderService.class);
orderService = (OrderService) mockOrderService.proxy();

public void testSomeServiceMethod() throws Exception {
String orderId = "orderId";

// The "doesOrderExist" method will be called once with the orderId parameter
// and will return a true boolean value
// If the method isn't called, then the mock will complain

FullfillmentService fullfillment = new FullfillmentServiceImpl(orderService);

One thing to realize is that by default jMock will only create proxies for interfaces. If you want to mock concrete classes, you’ll need the jmock-cglib extension and the asm library so that it can create proxies of the concrete classes.

I find this sort of scripting of Mock objects very compelling. It allows you to focus on testing the behavior of a very isolated piece of code. It even allows you to test code without having written all of the dependent objects. I encourage you to check it out for yourself.