Concept: Stubs |
Create a placeholder for components that still need to be developed. The stub contains just enough code to allow it to be compiled and linked with the rest of the components.
Relationships
Related Elements | Developer Developer Testing Implement the Solution Implement Unit Tests Run Unit Tests Stubs
Main Description
A stub is a component that doesn't actually do anything other than declare itself and the parameters it accepts. Encapsulated software (components, subsystems, classes, etc) can be is tested by sending inputs to its interface, waiting for the software to process them, then checking the results. In the course of its processing, a sotware component very likely uses other components by sending inputs to them and using their results: Fig1: Testing a Component you've implemented The components you use may cause problems for your testing. They: May not be implemented yet. May have defects that prevent your tests from working or make you spend a lot of time discovering that a test failure was not caused by your component. May make it hard to run tests when you need to. For instance, if a component is a commercial database, your company might not have enough floating licenses for everyone. Or one of the components may be hardware that's available only at scheduled times in a separate lab. May make testing so slow that tests aren't run often enough. For example, initializing the database might take five minutes per test. May be difficult to provoke into produciong certain results. For example, you may want each of your methods that writes to disk to handle "disk full" errors. How do you make sure the disk fills at just the moment that method is called? To avoid these problems, you may choose to use stub components (also called mock objects). Stub components behave like the real components, at least for the values that your component sends them while responding to its tests. They may go beyond that: they may be general-purpose emulators that seek to faithfully mimic most or all the component's behaviors. For example, it's often a good strategy to build software emulators for hardware. They behave just like the hardware, only slower. They're useful because they support better debugging, more copies of them are available, and they can be used before the hardware is finished. Fig2: Testing a Component you've implemented by stubbing out a dependent component Stubs have two disadvantages. Stubs can be expensive to build - this is especially true for emulators. Stubs need to be maintained since they're also software. Stubs may mask errors. For example, suppose your component uses trigonometric functions, but no math library is available yet. You could construct a stub for sine that returns preset values for specific inputs. All is fine until you integrate your component with the real trigonometric library, and you discover that the sine function takes arguments in radians and the expected results are now different from what the test expects. The good news is that a code defect was found - your code needs to use radians. The bad news is that the error was discovered later in the development cycle than desirable. ### Further information For further information related to stubs, see the following: Endo-Testing: Unit testing with Mock Objects, "eXtreme Programming and Flexible Processes in Software Engineering - XP2000". © 2000 Tim Mackinnon, Steve Freeman, Philip Craig. (Get Adobe Reader)
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Examples | Stubs
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