Keep 'em laughing as you go, Just remember that the last laugh is on you, And always look on the bright side of life..., Always look on the right side of life... (Always Look on the Bright Side of Life - Mony Python)

Integration tests are kind of tests which individual modules are combined and tested as a whole. Moreover integration tests might use system dependent values, accessing external systems like file system, database, web services, ..., and testing multiple aspects of one test case. We can say it is a high-level test.

This differs from unit test where only a single component is tested. Unit tests runs in isolation, mocking-out external components or using in-memory database in case of DAO layers. A unit test might be:

• Repeatable.
• Consistent.
• In Memory.
• Fast.
• Self-validating.
• Testing single concept

The problem when we are writing tests, is how to test rare (or untypical) conditions like "No disk space" in case of accessing file system, or "Connection lost" when executing a database query.

In unit testing this is not a problem you can mock up that component (database connection or filesystem access), generating required output like throwing IOException.

The problem becomes "harder" with integration tests. It would be strange to mock a component, when what you really want to do is validate the real system. So arrived at this point I see two possibilities:

• Creating a partial mock.
• Using fault injection.

In this post I am going to show you how to use fault injection approach to test unusual erroneous situations. 

Fault injection is a technique which involves changing application code under test at specific locations. This modifications will introduce faults on error handling code paths which otherwise would rarely be followed.

I am going to talk about how to use fault injection using Byteman in a JUnit test, and run it with Maven.

Let's start coding. Imagine you need to write a backup module, which shall save a string into a local file, but if hard disk is full (IOException is thrown), content shall be sent to remote server.

First we are going to code a class that writes content into file.

Next class, would be the one that sends data through socket but will not be shown, because it is not necessary for this example.

And finally the backup service responsible of managing described behavior.

And now testing time. First of all a brief introduction to Byteman.

Byteman is a tool which allows you to insert/modify code into an application at runtime. These modifications can be used to inject code on your compiled application causing unusual or unexpected operations (aka Fault Injection).

Byteman uses a clear, simple scripting language, based on a formalism called Event Condition Action (ECA) rules to specify where, when and how the original Java code should be transformed.

An example of ECA script is:

But Byteman also supports annotations. And in my opinion, annotations are a better approach than script file, because only watching your test case you can understand what you are exactly testing. If not you should switch context from unit class to script file to understand what are you testing.

So let's create an integration test that that validates that when IOException is thrown while writing content into disk, data is sent to a server.

See that BMUnitRunner (a special jUnit runner that comes with Byteman) is required.

First test called aFileWithContentShouldBeCreated is a standard test that writes Hello world into backup file.

But the second one dataShouldBeSentToServerInCaseOfIOException, has BMRule annotation which will contain when, where and what code should be injected. First parameter is the name of the rule, in this case a description of what we are going to do (throwing an IOException). Next attributes, targetClass and targetMethod configure when injected code should be added. In this case when FileUtils.createFileWithContent method is called. Next attribute targetLocation is location where code is inserted, and in our case is where createFileWithContent method calls write method of BufferedWriter. And finally what to do that obviously in this test is throwing an IOException.

So now you can go to your IDE and run them, and all tests should pass, but if you run through Maven using Surefire plugin, test will not work. To use Byteman with Maven, Surefire plugin should be configured in a specific way.

First important thing is adding tools jar as dependency. This jar provides classes needed in order to dynamically install the Byteman agent.

In Surefire plugin configuration is important to set useManifestOnlyJar to false to ensure that the Byteman jar appears in the classpath of the test JVM.  Also see that we are defining empty environment variables (BYTEMAN_HOME and org.jboss.byteman.home). This is because when it loads the agent the BMUnit package will use environment variable BYTEMAN_HOME or System property org.jboss.byteman.home to locate byteman.jar but only if it is a non-empty string. Otherwise it scans the classpath to locate the jar. Because we want to ensure that jar added on dependency section is used, we are overriding any other configuration present on system.

And now you can run mvn clean test and two tests are successful too.

See that Byteman opens a new world into how we are writing our integration tests, now we can test in an easy way unusual exceptions like Communications Error, Input/Output Exceptions or Out Of Memory Error. Moreover because we are not mocking FileUtils, we are executing real code; for example in our second test, we are running a few lines of FileUtils object until write method is reached. If we had mocked-up FileUtils class, these lines would not be executed. Thanks of using fault injection our code coverage is improved.

Byteman is more than what I have shown you, it also has built-ins designed for testing in multithreaded environments, parameter binding, and an amount of location specifiers, to cite a few things.

I wish you have found this post useful and help you testing rare conditions of your classes.

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