What is code coverage and how do YOU measure it?

问题:

What is code coverage and how do YOU measure it?

I was asked this question regarding our automating testing code coverage. It seems to be that, outside of automated tools, it is more art than science. Are there any real-world examples of how to use code coverage?

回答1:

Code coverage is a measurement of how many lines/blocks/arcs of your code are executed while the automated tests are running.

Code coverage is collected by using a specialized tool to instrument the binaries to add tracing calls and run a full set of automated tests against the instrumented product. A good tool will give you not only the percentage of the code that is executed, but also will allow you to drill into the data and see exactly which lines of code were executed during a particular test.

Our team uses Magellan - an in-house set of code coverage tools. If you are a .NET shop, Visual Studio has integrated tools to collect code coverage. You can also roll some custom tools, like this article describes.

If you are a C++ shop, Intel has some tools that run for Windows and Linux, though I haven't used them. I've also heard there's the gcov tool for GCC, but I don't know anything about it and can't give you a link.

As to how we use it - code coverage is one of our exit criteria for each milestone. We have actually three code coverage metrics - coverage from unit tests (from the development team), scenario tests (from the test team) and combined coverage.

BTW, while code coverage is a good metric of how much testing you are doing, it is not necessarily a good metric of how well you are testing your product. There are other metrics you should use along with code coverage to ensure the quality.

回答2:

Code coverage basically tests that how much of your code is covered under tests. So, if you have 90% code coverage than it means there is 10% of code that is not covered under tests. I know you might be thinking that 90% of the code is covered but you have to look from a different angle. What is stopping you to get 100% code coverage?

A good example will be this:

if(customer.IsOldCustomer()) 
{
}
else 
{
}

Now, in the above code there are two paths/branches. If you are always hitting the "YES" branch then you are not covering the else part and it will be shown in the Code Coverage results. This is good because now you know that what is not covered and you can write a test to cover the else part. If there was no code coverage then you are just sitting on a time bomb to explode.

NCover is a good tool to measure code coverage.

回答3:

Just remember, having "100% code-coverage" doesn't mean everything is tested completely - while it means every line of code is tested, it doesn't mean they are tested under every (common) situation..

I would use code-coverage to highlight bits of code that I should probably write tests for. For example, if whatever code-coverage tool shows myImportantFunction() isn't executed while running my current unit-tests, they should probably be improved.

Basically, 100% code-coverage doesn't mean your code is perfect. Use it as a guide to write more comprehensive (unit-)tests.

回答4:

Complementing a few points to many of the previous answers:

Code coverage means, how well your test set is covering your source code. i.e. to what extent is the source code covered by the set of test cases.

As mentioned in above answers, there are various coverage criteria, like paths, conditions, functions, statements, etc. But additional criteria to be covered are

1. Condition coverage: All boolean expressions to be evaluated for true and false.
2. Decision coverage: Not just boolean expressions to be evaluated for true and false once, but to cover all subsequent if-elseif-else body.
3. Loop Coverage: means, has every possible loop been executed one time, more than once and zero time. Also, if we have assumption on max limit, then, if feasible, test maximum limit times and, one more than maximum limit times.
4. Entry and Exit Coverage: Test for all possible call and its return value.
5. Parameter Value Coverage (PVC). To check if all possible values for a parameter are tested. For example, a string could be any of these commonly: a) null, b) empty, c) whitespace (space, tabs, new line), d) valid string, e) invalid string, f) single-byte string, g) double-byte string. Failure to test each possible parameter value may leave a bug. Testing only one of these could result in 100% code coverage as each line is covered, but as only one of seven options are tested, means, only 14.2% coverage of parameter value.
6. Inheritance Coverage: In case of object oriented source, when returning a derived object referred by base class, coverage to evaluate, if sibling object is returned, should be tested.

Note: Static code analysis will find if there are any unreachable code or hanging code, i.e. code not covered by any other function call. And also other static coverage. Even if static code analysis reports that 100% code is covered, it does not give reports about your testing set if all possible code coverage is tested.

回答5:

Code coverage has been explained well in the previous answers. So this is more of an answer to the second part of the question.

We've used three tools to determine code coverage.

1. JTest - a proprietary tool built over JUnit. (It generates unit tests as well.)
2. Cobertura - an open source code coverage tool that can easily be coupled with JUnit tests to generate reports.
3. Emma - another - this one we've used for a slightly different purpose than unit testing. It has been used to generate coverage reports when the web application is accessed by end-users. This coupled with web testing tools (example: Canoo) can give you very useful coverage reports which tell you how much code is covered during typical end user usage.

We use these tools to

• Review that developers have written good unit tests
• Ensure that all code is traversed during black-box testing

回答6:

Code coverage is simply a measure of the code that is tested. There are a variety of coverage criteria that can be measured, but typically it is the various paths, conditions, functions, and statements within a program that makeup the total coverage. The code coverage metric is the just a percentage of tests that execute each of these coverage criteria.

As far as how I go about tracking unit test coverage on my projects, I use static code analysis tools to keep track.

回答7:

For Perl there's the excellent Devel::Cover module which I regularly use on my modules.

If the build and installation is managed by Module::Build you can simply run ./Build testcover to get a nice HTML site that tells you the coverage per sub, line and condition, with nice colors making it easy to see which code path has not been covered.