How it works...

If we run our application again, by executing $ ./gradlew clean bootRun, we will get an output that is similar to the previous one. However, we will see our Hello message in the logs as well, which is as follows:

2017-12-16 21:57:51.048  INFO --- 
com.example.bookpub.StartupRunner         : Hello
  

Even though the program will get terminated on execution, at least we made it do something!

Command-line runners are a useful functionality to execute the various types of code that only have to be run once, after startup. Some also use this as a place to start various executor threads, but Spring Boot provides a better solution for this task, which will be discussed at the end of this chapter. The command-line runner interface is used by Spring Boot to scan all of its implementations and invoke each instance's run method with the startup arguments. We can also use an @Order annotation or implement an Ordered interface so as to define the exact order in which we want Spring Boot to execute them. For example, Spring Batch relies on the runners to trigger the execution of the jobs.

As the command-line runners are instantiated and executed after the application has started, we can use the dependency injection to our advantage to wire in whatever dependencies we need, such as datasources, services, and other components. These can be utilized later while implementing run.