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Built-in Fake Objects

tdd java

While mock objects are perfect instruments for unit testing, mocking through mock frameworks may turn your unit tests into an unmaintainable mess. Thanks to them we often hear that "mocking is bad" and "mocking is evil".

The root cause of this complexity is that our objects are too big. They have many methods and these methods return other objects, which also have methods. When we pass a mock version of such an object as a parameter, we should make sure that all of its methods return valid objects.

This leads to inevitable complexity, which turns unit tests to waste almost impossible to maintain.

Object Hierarchy

Take the Region interface from jcabi-dynamo as an example (this snippet and all others in this article are simplified, for the sake of brevity):

public interface Region {
  Table table(String name);

Its table() method returns an instance of the Table interface, which has its own methods:

public interface Table {
  Frame frame();
  Item put(Attributes attrs);
  Region region();

Interface Frame, returned by the frame() method, also has its own methods. And so on. In order to create a properly mocked instance of interface Region, one would normally create a dozen other mock objects. With Mockito it will look like this:

public void testMe() {
  // many more lines here...
  Frame frame = Mockito.mock(Frame.class);
  Table table = Mockito.mock(Table.class);
  Region region = Mockito.mock(Region.class);

And all of this is just a scaffolding before the actual testing.

Sample Use Case

Let's say, you're developing a project that uses jcabi-dynamo for managing data in DynamoDB. Your class may look similar to this:

public class Employee {
  private final String name;
  private final Region region;
  public Employee(String empl, Region dynamo) {
    this.name = empl;
    this.region = dynamo;
  public Integer salary() {
    return Integer.parseInt(
        .where("name", this.name)

You can imagine how difficult it will be to unit test this class, using Mockito, for example. First, we have to mock the Region interface. Then, we have to mock a Table interface and make sure it is returned by the table() method. Then, we have to mock a Frame interface, etc.

The unit test will be much longer than the class itself. Besides that, its real purpose, which is to test the retrieval of an employee's salary, will not be obvious to the reader.

Moreover, when we need to test a similar method of a similar class, we will need to restart this mocking from scratch. Again, multiple lines of code, which will look very similar to what we have already written.

Fake Classes

The solution is to create fake classes and ship them together with real classes. This is what jcabi-dynamo is doing. Just look at its JavaDoc. There is a package called com.jcabi.dynamo.mock that contains only fake classes, suitable only for unit testing.

Even though their sole purpose is to optimize unit testing, we ship them together with production code, in the same JAR package.

This is what a test will look like, when a fake class MkRegion is used:

public class EmployeeTest {
  public void canFetchSalaryFromDynamoDb() {
    Region region = new MkRegion(
      new H2Data().with(
        "employees", new String[] {"name"},
        new String[] {"salary"}
      new Attributes()
        .with("name", "Jeff")
        .with("salary", new AttributeValue().withN(50000))
    Employee emp = new Employee("Jeff", region);
    assertThat(emp.salary(), equalTo(50000))

This test looks obvious to me. First, we create a fake DynamoDB region, which works on top of H2Data storage (in-memory H2 database). The storage will be ready for a single employees table with a hash key name and a single salary attribute.

Then, we put a record into the table, with a hash Jeff and a salary 50000.

Finally, we create an instance of class Employee and check how it fetches the salary from DynamoDB.

I'm currently doing the same thing in almost every open source library I'm working with. I'm creating a collection of fake classes, that simplify testing inside the library and for its users.

BTW, a great article on the same subject: tl;dw: Stop mocking, start testing by Ned Batchelder.

PS. Check this out, on a very similar subject: Mocking of HTTP Server in Java.

  • What Fake Objects Are For? (webinar #8); 4 November 2015; 278 views; 11 likes