What is White Box Testing ?

White box testing strategy deals with the internal logic and structure of the code. White box testing is also called as glass, structural, open box or clear box testing. The tests written based on the white box testing strategy incorporate coverage of the code written, branches, paths, statements and internal logic of the code etc. White box testing is testing from the inside--tests that go in and test the actual program structure.
In order to implement white box testing, the tester has to deal with the code and hence is needed to possess knowledge of coding and logic i.e. internal working of the code. White box testing also needs the tester to look into the code and find out which unit/statement/chunk of the code is malfunctioning. The tester chooses test case inputs to exercise paths through the code and determines the appropriate outputs.
While white box testing is applicable at the unit, integration and system levels of the software testing process, it's typically applied to the unit. So while it normally tests paths within a unit, it can also test paths between units during integration, and between subsystems during a system level test. Though this method of test design can uncover an overwhelming number of test cases, it might not detect unimplemented parts of the specification or missing requirements. But you can be sure that all paths through the test object are executed.
For a further definition, White box testing is a test case design method that uses the control structure of the procedural design to derive test cases. Test cases can be derived that
1. guarantee that all independent paths within a module have been exercised at least once,
2. exercise all logical decisions on their true and false sides,
3. execute all loops at their boundaries and within their operational bounds, and
4. exercise internal data structures to ensure their validity.

Typical white box test design techniques include:
* Control flow testing
* Data flow testing