From：http://emma.sourceforge.net/

EMMA: a free Java code coverage tool

Until recently, the world of Java development had been plagued by an absurd discrepancy: Java developers had excellent free IDEs, free compilers, free test frameworks but had to rely on code coverage tools that charged an arm and a leg in license fees. As a Java pro, I would like to use the same free coverage tool regardless of whether it is a massive commercial project at work or a small fun project at home. I‘ve created EMMA to be that tool.

EMMA is an open-source toolkit for measuring and reporting Java code coverage. EMMA distinguishes itself from other tools by going after a unique feature combination: support for large-scale enterprise software development while keeping individual developer‘s work fast and iterative. Every developer on your team can now get code coverage for free and they can get it fast!

Chances are, you‘ve come here already knowing what coverage is all about and are, in fact, wondering what EMMA offers and why it is worth checking out. Explore the rest of this site to see why.

• EMMA can instrument classes for coverage either offline (before they are loaded) or on the fly (using an instrumenting application classloader).
• Supported coverage types: class, method, line, basic block. EMMA can detect when a single source code line is covered only partially.
• Coverage stats are aggregated at method, class, package, and "all classes" levels.
• Output report types: plain text, HTML, XML. All report types support drill-down, to a user-controlled detail depth. The HTML report supports source code linking.
• Output reports can highlight items with coverage levels below user-provided thresholds.
• Coverage data obtained in different instrumentation or test runs can be merged together.
• NEW in v2.1

  Starting with build 2.1.5320 it is possible to dump or reset coverage data remotely and without a JVM exit.

• EMMA does not require access to the source code and degrades gracefully with decreasing amount of debug information available in the input classes.
• EMMA can instrument individial .class files or entire .jars (in place, if desired). Efficient coverage subset filtering is possible, too.
• Makefile and ANT build integration are supported on equal footing.
• EMMA is quite fast: the runtime overhead of added instrumentation is small (5-20%) and the bytecode instrumentor itself is very fast (mostly limited by file I/O speed). Memory overhead is a few hundred bytes per Java class.
• EMMA is 100% pure Java, has no external library dependencies, and works in any Java 2 JVM (even 1.2.x).

More on EMMA features can be found in the FAQ.