Developing Your First Spring Boot Application

If you want to use Maven, ensure that you have Maven installed:

If you want to use Gradle, ensure that you have Gradle installed:

Most Spring Boot applications use the spring-boot-starter-parent in the parent section of the POM. The spring-boot-starter-parent is a special starter that provides useful Maven defaults. It also provides a dependency-management section so that you can omit version tags for “blessed” dependencies.

Since we are developing a web application, we add a spring-boot-starter-web dependency. Before that, we can look at what we currently have by running the following command:

The mvn dependency:tree command prints a tree representation of your project dependencies. You can see that spring-boot-starter-parent provides no dependencies by itself. To add the necessary dependencies, edit your pom.xml and add the spring-boot-starter-web dependency immediately below the parent section:

If you run mvn dependency:tree again, you see that there are now a number of additional dependencies, including the Tomcat web server and Spring Boot itself.

Most Spring Boot applications use the org.springframework.boot Gradle plugin. This plugin provides useful defaults and Gradle tasks. The io.spring.dependency-management Gradle plugin provides dependency management so that you can omit version tags for “blessed” dependencies.

Since we are developing a web application, we add a spring-boot-starter-web dependency. Before that, we can look at what we currently have by running the following command:

The gradle dependencies command prints a tree representation of your project dependencies. Right now, the project has no dependencies. To add the necessary dependencies, edit your build.gradle and add the spring-boot-starter-web dependency in the dependencies section:

If you run gradle dependencies again, you see that there are now a number of additional dependencies, including the Tomcat web server and Spring Boot itself.

The first annotation on our MyApplication class is @RestController. This is known as a stereotype annotation. It provides hints for people reading the code and for Spring that the class plays a specific role. In this case, our class is a web @Controller, so Spring considers it when handling incoming web requests.

The @RequestMapping annotation provides “routing” information. It tells Spring that any HTTP request with the / path should be mapped to the home method. The @RestController annotation tells Spring to render the resulting string directly back to the caller.

The second class-level annotation is @SpringBootApplication. This annotation is known as a meta-annotation, it combines @SpringBootConfiguration, @EnableAutoConfiguration and @ComponentScan.

Of those, the annotation we’re most interested in here is @EnableAutoConfiguration. @EnableAutoConfiguration tells Spring Boot to “guess” how you want to configure Spring, based on the jar dependencies that you have added. Since spring-boot-starter-web added Tomcat and Spring MVC, the auto-configuration assumes that you are developing a web application and sets up Spring accordingly.

The final part of our application is the main method. This is a standard method that follows the Java convention for an application entry point. Our main method delegates to Spring Boot’s SpringApplication class by calling run. SpringApplication bootstraps our application, starting Spring, which, in turn, starts the auto-configured Tomcat web server. We need to pass MyApplication.class as an argument to the run method to tell SpringApplication which is the primary Spring component. The args array is also passed through to expose any command-line arguments.

At this point, your application should work. Since you used the spring-boot-starter-parent POM, you have a useful run goal that you can use to start the application. Type mvn spring-boot:run from the root project directory to start the application. You should see output similar to the following:

If you open a web browser to localhost:8080, you should see the following output:

To gracefully exit the application, press ctrl-c.

At this point, your application should work. Since you used the org.springframework.boot Gradle plugin, you have a useful bootRun goal that you can use to start the application. Type gradle bootRun from the root project directory to start the application. You should see output similar to the following:

If you open a web browser to localhost:8080, you should see the following output:

To gracefully exit the application, press ctrl-c.

To create an executable jar, we need to add the spring-boot-maven-plugin to our pom.xml. To do so, insert the following lines just below the dependencies section:

Save your pom.xml and run mvn package from the command line, as follows:

If you look in the target directory, you should see myproject-0.0.1-SNAPSHOT.jar. The file should be around 18 MB in size. If you want to peek inside, you can use jar tvf, as follows:

You should also see a much smaller file named myproject-0.0.1-SNAPSHOT.jar.original in the target directory. This is the original jar file that Maven created before it was repackaged by Spring Boot.

To run that application, use the java -jar command, as follows:

As before, to exit the application, press ctrl-c.

To create an executable jar, we need to run gradle bootJar from the command line, as follows:

If you look in the build/libs directory, you should see myproject-0.0.1-SNAPSHOT.jar. The file should be around 18 MB in size. If you want to peek inside, you can use jar tvf, as follows:

To run that application, use the java -jar command, as follows:

As before, to exit the application, press ctrl-c.