Java Developer Tools

Code Coverage

    The Code Coverage tools allow you to measure how much of your code is being executed. Probably the most common reason for measuring code coverage is to evaluate the effectiveness of test code at exercising all possible paths through the code.

There are two different mechanisms provided. Deciding which one is right for your needs depends primarily on the complexity of the code being run. The first is designed for use with stand-alone Java applications that run in a single VM and do not make use of specialized class loaders.

   
The second mechanism is designed for arbitrary applications, but might require a bit more work to get it set up initially, again depending on just how complex the code is. Both mechanisms are described in separate sections below.

Simple Code Coverage

You can measure the code coverage for a particular piece of code by selecting a class that defines a main method and then selecting "Run Code Coverage" from the "CodePro Tools" submenu. The selected class will then be executed and the results will be displayed in the Coverage View. From within the Coverage View, the results may also be exported to various report formats including HTML, XML and text.

The information about how the class is to be run and what kind of coverage information to gather is captured in a special kind of launch configuration. You can also create the launch configuration manually, but the "Run Code Coverage" menu item makes that process easier.

Once a launch configuration has been created, you can measure the coverage of your code either by using the "Run Code Coverage" menu item or by directly executing the launch configuration from the "Run" menu.

Full Featured Code Coverage

Code coverage works by instrumenting the bytecodes being loaded by the VM to collect data about which bytecodes have been executed. Code run using the code coverage launch configuration is instrumented dynamically as it is loaded by using a specialized class loader. This class loader becomes the default class loader for the VM that is initially executed. If additional class loaders are created by the application, or if additional VM's are run, they will not perform the dynamic instrumentation necessary to collect code coverage information.

The alternative is to statically instrument the code before executing the application. CodePro provides support for instrumenting the code on a project-by-project basis. Each project has a Code Coverage property sheet. This property sheet allows you to control whether the class files are to be instrumented. It also allows you to control which class files are instrumented. As a convenience, the "Instrument Code" and "Uninstrument Code" menu items on the "CodePro Tools" submenu will turn instrumentation on and off while leaving the other project-related settings unchanged.

Once your projects have been instrumented, you will collect code coverage information any time the code is run, using any launch configuration that you have created. In other words, you do not need to, and should not, use the Code Coverage launch configuration to run pre-instrumented code.

There are two other implications of using this mechanism that you need to be aware of. The first is that if you want to run the code without gathering code coverage data, you will need to uninstrument all of the projects containing code that will be executed.

The second implication is similar. If you are going to distribute the class files from your local machine, you need to be careful to uninstrument the code before you package it for delivery. Otherwise the code will be collecting code coverage data whenever it is executed.

Viewing the Results

Each time you measure a particular class, a coverage report will be generated. If you measure the same class multiple times, past versions of the report will be remembered so that those versions can be compared. The number of versions to be remembered can be configured. Coverage reports are stored on disk, so they are available even after Eclipse is closed and then re-opened.

Understanding the Results

The code coverage data that is gathered measures the number of times each basic block has been executed. A basic block is a sequence of one or more bytecodes (Java Virtual Machine instructions) that contains at most one branch instruction, which must be at the end of the block. The important thing about basic blocks is that if one instruction in the block is executed, then all of the instructions in the block will be executed.

In addition, we keep track of the number of bytecodes in each basic block and enough information to be able to map each bytecode back to the class, method, and even line of code from which it was compiled.

Given this data, we can compute the code coverage at several interesting levels of granularity, resulting in the following coverage measures:

class coverage
The number of classes containing at least one basic block that was executed at least once, divided by the total number of classes.
method coverage
The number of methods containing at least one basic block that was executed at least once, divided by the total number of methods.
line coverage
The number of lines of code containing at least one basic block that was executed at least once, divided by the total number of lines of code.
block coverage
The number of basic blocks that were executed at least once, divided by the total number of basic blocks.
instruction coverage
The number of instructions in basic blocks that were executed at least once, divided by the total number of instructions.

Line coverage is probably the most easily misunderstood measure, because while all of the rest of the measured elements (classes, methods, blocks, and instructions) all map cleanly to blocks, lines do not necessarily. It all depends on how the code is formatted. Each line of code can contain either some or all of one or more basic blocks, and every basic block can map to one or more lines of code.

Acknowledgements

The Code Coverage tool is based on the EMMA code coverage engine. Detailed information on using EMMA is available from the EMMA web site:

This feature is based on technology from the EMMA Project.
http://emma.sourceforge.net
Copyright © 2001-2005 Vlad Roubtsov

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