We can run the Hello World application in multiple ways. Let's start running it with the simplest option--running as a Java application. In your IDE, right-click on the application class and run it as Java Application. The following screenshot shows some of the log from running our Hello World application:

The following are the key things to note:

• Tomcat server is launched on port 8080--Tomcat started on port(s): 8080 (http).
• DispatcherServlet is configured. This means that Spring MVC Framework is ready to accept requests--Mapping servlet: 'dispatcherServlet' to [/].

• Four filters--characterEncodingFilter, hiddenHttpMethodFilter, httpPutFormContentFilter and requestContextFilter--are enabled by default
• The default error page is configured--Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
• WebJars are autoconfigured. As we discussed in Chapter 3, Building a Web Application with Spring MVC, WebJars enable dependency management for static dependencies such as Bootstrap and query--Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]

The following screenshot shows the application layout as of now. We have just two files, pom.xml and Application.java:

With a simple pom.xml file and one Java class, we were able to get to launch the Spring MVC application, with all the preceding functionality described. The most important thing about Spring Boot is to understand what happens in the background. Understanding the preceding start up log is the first. Let's look at the Maven dependencies to get a deeper picture.

The following screenshot shows some of the dependencies that are configured with the basic configuration in the pom.xml file that we created:

Spring Boot does a lot of magic. Once you have the application configured and running, I recommend that you play around with it to gain a deeper understanding that will be useful when you are debugging problems.

As Spiderman says, with great power, comes great responsibility. This is absolutely true in the case of Spring Boot. In the time to come, the best developers with Spring Boot would be the ones who understand what happens in the background--dependencies and auto-configuration.