Handling command line arguments with args4j

From time to time, we need to write a tool that is using command line arguments as input.
Having an interface similar to unix command line tools is not a trivial task but with args4j it becomes quite easy.

We're going to write a fake tool called FileCompressor which process of invocation would look like:

where -i is input file name, -o output file name and -p the priority of the process.

Let's start with defining the dependencies of our project:

We'll need args4j, the rest is for testing.

Our fake FileCompressor requires a configuration that will be populated by args4j.

The configuration object has fields that will be mapped to command line arguments with args4j annotations.

And here is the class that will be invoked:

First of all, the arguments are parsed and configuration object is populated with them.
If there is a parsing problem, the exception is printed along with the usage (args4j can automatically print the usage of our application based on used annotations).

Testing our application requires some effort.
To begin with, we would like to mock completely the FileCompressor. There is no setter for it so we'll use Spring's ReflectionTestUtils together with Mockito.

We've also prepared some test data.

Let's check if the configuration object is populated properly and if the collabolator was invoked:

We would also like to know if the usage was printed when invalid arguments were passed.
To do that we need to use a spy, as we want to have the original behavior of populating argument, but we want to verify if printUsage method was invoked.

The last thing to check is if the exception was printed to the output stream when invalid arguments were passed.
Similarly we need to use a spy.

The whole project can be found at github.