There are many approaches to code coverage measurement. Broadly there are three approaches, which may be used in combination:
 

http://www.onestoptesting.com/code-coverage/working.asp#">  Instrumentation	This approach adds instrumentation statements to the source code and compiles the code with the normal compile tool chain to produce an instrumented assembly.
Intermediate code Instrumentation	Here the compiled class files are instrumented by adding new bytecodes and a new instrumented class generated.
Runtime Information collection	This approach collects information from the runtime environment as the code executes to determine coverage information

 

Clover uses source code instrumentation, because although it requires developers to perform an instrumented build, source code instrumentation produces the most accurate coverage measurement for the least runtime performance overhead.

As the code under test executes, code coverage systems collect information about which statements have been executed. This information is then used as the basis of reports. In addition to these basic mechanisms, coverage approaches vary on what forms of coverage information they collect. There are many forms of coverage beyond basic statement coverage including conditional coverage, method entry and path coverage.