Spring boot

What is spring boot?

Spring boot is an opinionated setup of different frameworks (including the spring framework) to give you a simple environment that just runs.

Start a simple project

Create a project by configuring and downloading it from start.spring.io.

The main class that will be executed looks like this:

package javacasts;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class BootApplication {

    public static void main(String[] args) {
        SpringApplication.run(BootApplication.class, args);
    }
}

When this BootApplication is started, it calls SpringApplication.run with the class itself and the call-arguments. Spring now decides what to do. As the BootApplication is annotated with SpringBootApplication, spring now makes some “magic”.
Take a look at the SpringBootApplication-annotation, it itself is annotated with @Configuration, @EnableAutoConfiguration and @ComponentScan.

Simple Console Application

The console application is like the easiest way to write an application. As it’s inevitable to write a hello world-application, this will be the first application we create.
There are multiple ways to have a commandline application run by spring, here’s one solution to this.

package javacasts;

import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class BootApplication implements ApplicationRunner
{

    public static void main(String[] args) {
        SpringApplication.run(BootApplication.class, args);
    }

     @Override
     public void run(ApplicationArguments arg0) throws Exception
     {
         System.out.println("hello world");
     }
}

We have the class implement a ApplicationRunner. You can also implement a CommandLineRunner, they are nearly the same, the difference is the treatment of the arguments.
Now the run-method needs to be implemented. In here, we simply write hello world to the systems output.

Simple Rest-API

The next step is to change the application to be a Rest-API. The first thing that we need to change is to add the dependencies to have all in place what we need. So we change the dependency of spring-boot-starter to spring-boot-starter-web. The BootApplication now does not need any more to implement an ApplicationRunner, but we mark it that it’s a @RestController. Now we need to tell spring-boot which URLs can be answered by this @RestController. We create a new method that we mark with the annotation @RequestMapping, and include a value "/" what means that this controller-method will handle the root-URL.
The response will be mapped to JSON automaticly. For this simple example, we just return the string hello world.

package javacasts;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@SpringBootApplication
public class BootApplication
{

   public static void main(String[] args)
   {
       SpringApplication.run(BootApplication.class, args);
   }

   @RequestMapping("/")
   public String index()
   {
       return "hello world";
   }
}

Simple Web Application

But a user usually does not want to receive just strings or JSON. So this is how to change the application to return html pages.
First we need to decide od the template engine. Spring-boot provides different engines you can use. I now pick the thymeleaf template engine. For this the first thing I need is the dependency. As spring-boot-starter-web is a dependency of the spring-boot-starter-thymeleaf-package, it’s fine to just use the thymeleaf-dependency.

The templates can be provided in the src/main/resources/templates-directory. You can change all these directories, but it’s a good idea to stick with the default unless you really need to change something. We create a simple index.html-template. It’s almost a simple html-page, there’s only one non-standard html-tag (th:text). Thymeleaf uses this tag to replace the contents of the html-tag.

<!DOCTYPE html>

<html xmlns="http://www.w3.org/1999/xhtml"
      xmlns:th="http://www.thymeleaf.org">

  <head>
    <title>spring-boot - hello world</title>
  </head>

  <body>
    <p th:text="'hello ' + ${target}">hello world</p>
  </body>

</html>

The @RestController also needs some change. First we only mark it as @Controller. Then we change the index-method to take a parameter marked with @RequestPara, and we return a ModelAndView. The ModelAndView contains the name of the view (index.html) and some variables that can be used by the template (target in this example).

package javacasts;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.servlet.ModelAndView;


@Controller
@SpringBootApplication
public class BootApplication
{

    public static void main(String[] args)
    {
        SpringApplication.run(BootApplication.class, args);
    }

    @RequestMapping(value = "/", method = RequestMethod.GET)
    public ModelAndView index(@RequestParam(name = "to", defaultValue = "world") String recipient)
    {
        ModelAndView result = new ModelAndView("index");
        result.getModel().put("target", recipient);
        return result;
    }
}

Now you can call the application and receive a hello world. You even can give a parameter to that will translate the message to hello <to>.