Software Testing - Unit, Integration and System testing

The first type of testing that can be conducted in any development phase is unit testing.

In this, discrete components of the final product are tested independently before being assembled into larger units. Units are typically tested through the use of ‘test harnesses’ which simulate the context into which the unit will be integrated. The test harness provides a number of known inputs and measures the outputs of the unit under test, which are then compared with expected values to determine if any issues exist.

In integration testing smaller units are integrated into larger units and larger units into the overall system. This differs from unit testing in that units are no longer tested independently but in groups, the focus shifting from the individual units to the interaction between them.

At this point “stubs” and “drivers” take over from test harnesses.

A stub is a simulation of a particular sub-unit which can be used to simulate that unit in a larger assembly. For example if units A, B and C constitute the major parts of unit D then the overall assembly could be tested by assembling units A and B and a simulation of C, if C were not complete. Similarly if unit D itself was not complete it could be represented by a “driver” or a simulation of the super-unit.