Enter Mock Objects
The concept behind mock objects is that we want to create an object that will take the place of the real object. This mock object will expect a certain method to be called with certain parameters and when that happens, it will return an expected result. Using the above code as an example, let's say that when I pass in 1234 for my key to the
service.lookupByKeycall, I should get back a
Listwith 4 values in it. Our mock object should expect
lookupByKeyto be called with the parameter "1234" and when that occurs, it will return a
Listwith four objects in it.
How Mock Objects Work
There are many different mocking frameworks in the Java space. I am not going to discuss any of them in great detail here. However, I will discuss how they work and design considerations you need to take in mind because of their implementations.
There are essentially two main types of mock object frameworks, ones that are implemented via proxy and ones that are implemented via class remapping. Let's take a look at the first (and by far more popular) option, proxy.
A proxy object is an object that is used to take the place of a real object. In the case of mock objects, a proxy object is used to imitate the real object your code is dependent on. You create a proxy object with the mocking framework, and then set it on the object using either a setter or constructor. This points out an inherent issue with mocking using proxy objects. You have to be able to set the dependency up thru an external means. In other words, you can't create the dependency by calling
new MyObject()since there is no way to mock that with a proxy object. This is one of the reasons Dependency Injection frameworks like Spring have taken off. They allow you to inject your proxy objects without modifying any code.
The second form of mocking is to remap the class file in the class loader. The mocking framework jmockit is the only framework I am aware of that currently exploits this ability for mock objects. The concept is relatively new (since JDK 1.5 although jmockit supports jdk1.4 thru other means as well) and is provided by the new
java.lang.Insturmentclass. What happens is that you tell the class loader to remap the reference to the class file it will load. So let's say that I have a class MyDependency with the corresponding .class file called MyDependency.class and I want to mock it to use MyMock instead. By using this type of mock objects, you will actually remap in the classloader the reference from MyDependency to MyMock.class. This allows you to be able to mock objects that are created by using thew new operator. Although this approach provides more power than the proxy object approach, it is also harder/more confusing to get going given the knowledge of classloaders you need to really be able to use all its features.
Mock objects are a very valuable tool in testing. They provide you with the ability to test what you write without having to address dependency concerns.
Ref : http://www.michaelminella.com/testing/the-concept-of-mocking.html