A server in Java

From now on, we will be using the Java language and the Spring Boot framework to run real web servers, as you might do in a software engineering project.

Compile and run

Make sure you have java and maven installed as we did earlier in the unit. Clone the repository git@github.com:cs-uob/COMSM0085 if you have not done so already, and navigate to the folder code/server01. In this folder, run mvn spring-boot:run to compile and run the sample application. The first time you do this, it might download lots of files.

This runs a web server on port 8000.

Explore the application

The pom.xml file tells maven that this is a spring boot project, and what the project is called (softwaretools.server01).

Under src/main/resources you find two files. First, application.properties which is a spring file configured to run on port 8000 (the default would otherwise be 8080). Secondly, a HTML file that the application can serve.

Under src/main/java is the source code. This is only two files, but of course the whole spring framework is active when the application runs. Server01Application is the main class, but this just contains boilerplate code for now.

Controller.java is the interesting one: in application development, Model - View - Controller is one of several patterns to structure an application, where a Controller is a piece of code that does the heavy lifting part, for example replying to a HTTP request.

Spring works with annotations, special classes whose name begins with an @ sign. When the server starts, spring scans all files for annotations and sets up code to deal with them. Here we can see:

  • @SpringBootApplication (on the Server01Application class) tells spring that this is the main file to run.
  • @RestController tells spring that this class contains methods to deal with HTTP requests. It is so named because spring has libraries to make implementing the REST principles particularly easy.
  • @AutoWired on a field tells spring that this field is spring's responsibility to set up when the application starts. The ResourceLoader is part of spring and lets you access files in src/main/resources.
  • @GetMapping(PATH) tells spring that this method should be called to reply to a HTTP GET request for the provided path (there is of course also a @PostMapping and so on).

The mainPage method, which is called when you load localhost:8000, shows the basic way to reply to a HTTP request: set up any headers you need - in this case the content type - and return a ResponseEntity taking three parameters: the HTTP body of the response to return (this will be shown in your browser), the HTTP headers to set, and the response code which is 200 (OK) here.

The htmlPage method replies to requests for localhost:8000/html. Here we want to serve a file, so we use the resource loader to get it from the classpath which includes the src/main/resources folder (or rather, the version of it that ends up in the compiled JAR file). It also shows a second way to create a ResponseEntity using the builder pattern.

Exercises

  • Compile and run the application, and test both localhost:8000 an localhost:8000/html in your browser. Observe both the headers in the developer tools, and the log messages that spring prints out for each request.
  • Add a method that replies to requests for localhost:8000/bad with a HTTP 404 NOT FOUND error. The body of the page can be a simple string with a message of your choice. Stop and restart the application, and check that you get the correct error message when you try and open this page.