Although EMMA's instrumentation is very fast (it is usually fast enough so that the overall processing time is dominated by file I/O), the key to making EMMA into an even faster tool for individual development is to make EMMA do just the right amount of work, i.e. define the right instrumentation set of classes.
Understanding what gets instrumented is also important for another reason: EMMA coverage reports are based exclusively on the classes in the instrumentation set as implied by coverage metadata.
Instrumentation set. The set of classes that get instrumented in a given invocation of a tool like <instr>/instr or <emmajava>/emmarun is determined by the following rules:
What this means in practice is that you choose the right set of class directories and archives via the instrumentation path option and then narrow it further down via a number of coverage filters, described next.
 Strictly speaking, Java interfaces can contain executable bytecode, but it usually corresponds to field initializer expressions that execute unconditionally when the interface is loaded.