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Building an Application with Spring Boot

This guide provides a sampling of how Spring Boot helps you accelerate application development. As you read more Spring Getting Started guides, you will see more use cases for Spring Boot. This guide is meant to give you a quick taste of Spring Boot. If you want to create your own Spring Boot-based project, visit Spring Initializr, fill in your project details, pick your options, and download a bundled-up project as a zip file.

What You Will build

You will build a simple web application with Spring Boot and add some useful services to it.

What You Need

How to complete this guide

Like most Spring Getting Started guides, you can start from scratch and complete each step or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.

To start from scratch, move on to Starting with Spring Initializr.

To skip the basics, do the following:

When you finish, you can check your results against the code in gs-spring-boot/complete.

Learn What You Can Do with Spring Boot

Spring Boot offers a fast way to build applications. It looks at your classpath and at the beans you have configured, makes reasonable assumptions about what you are missing, and adds those items. With Spring Boot, you can focus more on business features and less on infrastructure.

The following examples show what Spring Boot can do for you:

  • Is Spring MVC on the classpath? There are several specific beans you almost always need, and Spring Boot adds them automatically. A Spring MVC application also needs a servlet container, so Spring Boot automatically configures embedded Tomcat.

  • Is Jetty on the classpath? If so, you probably do NOT want Tomcat but instead want embedded Jetty. Spring Boot handles that for you.

  • Is Thymeleaf on the classpath? If so, there are a few beans that must always be added to your application context. Spring Boot adds them for you.

These are just a few examples of the automatic configuration Spring Boot provides. At the same time, Spring Boot does not get in your way. For example, if Thymeleaf is on your path, Spring Boot automatically adds a SpringTemplateEngine to your application context. But if you define your own SpringTemplateEngine with your own settings, Spring Boot does not add one. This leaves you in control with little effort on your part.

Spring Boot does not generate code or make edits to your files. Instead, when you start your application, Spring Boot dynamically wires up beans and settings and applies them to your application context.

Starting with Spring Initializr

You can use this pre-initialized project and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.

To manually initialize the project:

  1. Navigate to https://start.spring.io. This service pulls in all the dependencies you need for an application and does most of the setup for you.

  2. Choose either Gradle or Maven and the language you want to use.

  3. Click Dependencies and select Spring Web.

  4. Click Generate.

  5. Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.

If your IDE has the Spring Initializr integration, you can complete this process from your IDE.
You can also fork the project from GitHub and open it in your IDE or other editor.

Create a Simple Web Application

Now you can create a web controller for a simple web application, as the following listing shows:

Java 
package com.example.springboot; import org.springframework.web.bind.annotation.GetMapping; import org.springframework.web.bind.annotation.RestController; @RestController public class HelloController {	@GetMapping("/")	public String index() {	return "Greetings from Spring Boot!";	} }
Kotlin 
package com.example.springboot import org.springframework.web.bind.annotation.GetMapping import org.springframework.web.bind.annotation.RestController @RestController class HelloController { @GetMapping("/") fun index(): String = "Greetings from Spring Boot!" }

The class is flagged as a @RestController, meaning it is ready for use by Spring MVC to handle web requests. @GetMapping maps / to the index() method. When invoked from a browser or by using curl on the command line, the method returns pure text. That is because @RestController combines @Controller and @ResponseBody, two annotations that results in web requests returning data rather than a view.

Create an Application class

The Spring Initializr creates a simple application class for you. However, in this case, it is too simple. You need to modify the application class to match the following listing:

Java 
package com.example.springboot; import java.util.Arrays; import org.springframework.boot.CommandLineRunner; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; import org.springframework.context.ApplicationContext; import org.springframework.context.annotation.Bean; @SpringBootApplication public class Application {	public static void main(String[] args) {	SpringApplication.run(Application.class, args);	}	@Bean	public CommandLineRunner commandLineRunner(ApplicationContext ctx) {	return args -> {	System.out.println("Let's inspect the beans provided by Spring Boot:");	String[] beanNames = ctx.getBeanDefinitionNames();	Arrays.sort(beanNames);	for (String beanName : beanNames) {	System.out.println(beanName);	}	};	} }
Kotlin 
package com.example.springboot import org.springframework.boot.CommandLineRunner import org.springframework.boot.autoconfigure.SpringBootApplication import org.springframework.boot.runApplication import org.springframework.context.ApplicationContext import org.springframework.context.annotation.Bean @SpringBootApplication class Application { @Bean fun commandLineRunner(ctx: ApplicationContext) = CommandLineRunner { println("Let's inspect the beans provided by Spring Boot:") val beanNames = ctx.beanDefinitionNames beanNames.sorted().forEach { println(it) } } } fun main(args: Array<String>) { runApplication<Application>(*args) }

@SpringBootApplication is a convenience annotation that adds all of the following:

  • @Configuration: Tags the class as a source of bean definitions for the application context.

  • @EnableAutoConfiguration: Tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings. For example, if spring-webmvc is on the classpath, this annotation flags the application as a web application and activates key behaviors, such as setting up a DispatcherServlet.

  • @ComponentScan: Tells Spring to look for other components, configurations, and services in the com/example package, letting it find the controllers.

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there was not a single line of XML? There is no web.xml file, either. This web application is 100% pure Java and you did not have to deal with configuring any plumbing or infrastructure.

There is also a CommandLineRunner method marked as a @Bean, and this runs on startup. It retrieves all the beans that were created by your application or that were automatically added by Spring Boot. It sorts them and prints them out.

Run the Application

To run the application, run the following command in a terminal window directory:

./gradlew bootRun

If you use Maven, run the following command in a terminal window directory:

./mvnw spring-boot:run

You should see output similar to the following:

Let's inspect the beans provided by Spring Boot: application applicationAvailability applicationTaskExecutor applicationTaskExecutorAsyncConfigurer basicErrorController beanNameHandlerMapping beanNameViewResolver ...

You can see org.springframework.boot.autoconfigure beans. There is also a tomcatServletWebServerFactory one.

Now run the service with curl (in a separate terminal window) by running the following command (shown with its output):

$ curl http://localhost:8080 Greetings from Spring Boot!

Add Unit Tests

You will want to add a test for the endpoint you added, and Spring Test provides some machinery for that.

If you use Gradle, add the following dependency to your build.gradle(.kts) file:

Groovy 
testImplementation('org.springframework.boot:spring-boot-starter-test')
Kotlin 
testImplementation("org.springframework.boot:spring-boot-starter-test")

If you use Maven, add the following to your pom.xml file:

<dependency>	<groupId>org.springframework.boot</groupId>	<artifactId>spring-boot-starter-test</artifactId>	<scope>test</scope> </dependency>

Now write a simple unit test that mocks the servlet request and response through your endpoint, as the following listing shows:

Java 
package com.example.springboot; import static org.hamcrest.Matchers.equalTo; import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content; import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status; import org.junit.jupiter.api.Test; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc; import org.springframework.boot.test.context.SpringBootTest; import org.springframework.http.MediaType; import org.springframework.test.web.servlet.MockMvc; import org.springframework.test.web.servlet.request.MockMvcRequestBuilders; @SpringBootTest @AutoConfigureMockMvc public class HelloControllerTest {	@Autowired	private MockMvc mvc;	@Test	public void getHello() throws Exception {	mvc.perform(MockMvcRequestBuilders.get("/").accept(MediaType.APPLICATION_JSON))	.andExpect(status().isOk())	.andExpect(content().string(equalTo("Greetings from Spring Boot!")));	} }
Kotlin 
package com.example.springboot import org.junit.jupiter.api.Test import org.springframework.beans.factory.annotation.Autowired import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc import org.springframework.boot.test.context.SpringBootTest import org.springframework.http.MediaType import org.springframework.test.web.servlet.MockMvc import org.springframework.test.web.servlet.get @SpringBootTest @AutoConfigureMockMvc class HelloControllerTest { @Autowired private lateinit var mvc: MockMvc @Test fun getHello() { mvc.get("/") { accept(MediaType.APPLICATION_JSON) }.andExpect { status { isOk() } content { string("Greetings from Spring Boot!") } } } }

MockMvc comes from Spring Test and lets you, through a set of convenient builder classes, send HTTP requests into the DispatcherServlet and make assertions about the result. Note the use of @AutoConfigureMockMvc and @SpringBootTest to inject a MockMvc instance. Having used @SpringBootTest, we are asking for the whole application context to be created. An alternative would be to ask Spring Boot to create only the web layers of the context by using @WebMvcTest. In either case, Spring Boot automatically tries to locate the main application class of your application, but you can override it or narrow it down if you want to build something different.

As well as mocking the HTTP request cycle, you can also use Spring Boot to write a simple full-stack integration test. For example, instead of (or as well as) the mock test shown earlier, we could create the following test:

Java 
package com.example.springboot; import org.junit.jupiter.api.Test; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.boot.test.context.SpringBootTest; import org.springframework.boot.test.web.client.TestRestTemplate; import org.springframework.http.ResponseEntity; import static org.assertj.core.api.Assertions.assertThat; @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT) public class HelloControllerITest {	@Autowired	private TestRestTemplate template; @Test public void getHello() throws Exception { ResponseEntity<String> response = template.getForEntity("/", String.class); assertThat(response.getBody()).isEqualTo("Greetings from Spring Boot!"); } }
Kotlin 
package com.example.springboot import org.assertj.core.api.Assertions.assertThat import org.junit.jupiter.api.Test import org.springframework.beans.factory.annotation.Autowired import org.springframework.boot.test.context.SpringBootTest import org.springframework.boot.test.web.client.TestRestTemplate import org.springframework.boot.test.web.client.getForEntity @SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT) class HelloControllerITest { @Autowired private lateinit var template: TestRestTemplate @Test fun getHello() { val response = template.getForEntity<String>("/") assertThat(response.body).isEqualTo("Greetings from Spring Boot!") } }

The embedded server starts on a random port because of webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT, and the actual port is configured automatically in the base URL for the TestRestTemplate.

Add Production-grade Services

If you are building a website for your business, you probably need to add some management services. Spring Boot provides several such services (such as health, audits, beans, and more) with its actuator module.

If you use Gradle, add the following dependency to your build.gradle(.kts) file:

Groovy 
implementation 'org.springframework.boot:spring-boot-starter-actuator'
Kotlin 
implementation("org.springframework.boot:spring-boot-starter-actuator")

If you use Maven, add the following dependency to your pom.xml file:

<dependency>	<groupId>org.springframework.boot</groupId>	<artifactId>spring-boot-starter-actuator</artifactId> </dependency>

Then restart the application. If you use Gradle, run the following command in a terminal window:

./gradlew bootRun

If you use Maven, run the following command in a terminal window:

./mvnw spring-boot:run

You should see that a new set of RESTful end points has been added to the application. These are management services provided by Spring Boot. The following listing shows a typical output:

management.endpoint.configprops-org.springframework.boot.actuate.autoconfigure.context.properties.ConfigurationPropertiesReportEndpointProperties management.endpoint.env-org.springframework.boot.actuate.autoconfigure.env.EnvironmentEndpointProperties management.endpoint.health-org.springframework.boot.actuate.autoconfigure.health.HealthEndpointProperties management.endpoint.logfile-org.springframework.boot.actuate.autoconfigure.logging.LogFileWebEndpointProperties management.endpoints.jmx-org.springframework.boot.actuate.autoconfigure.endpoint.jmx.JmxEndpointProperties management.endpoints.web-org.springframework.boot.actuate.autoconfigure.endpoint.web.WebEndpointProperties management.endpoints.web.cors-org.springframework.boot.actuate.autoconfigure.endpoint.web.CorsEndpointProperties management.health.diskspace-org.springframework.boot.actuate.autoconfigure.system.DiskSpaceHealthIndicatorProperties management.info-org.springframework.boot.actuate.autoconfigure.info.InfoContributorProperties management.metrics-org.springframework.boot.actuate.autoconfigure.metrics.MetricsProperties management.metrics.export.simple-org.springframework.boot.actuate.autoconfigure.metrics.export.simple.SimpleProperties management.server-org.springframework.boot.actuate.autoconfigure.web.server.ManagementServerProperties

The actuator exposes the following:

There is also an /actuator/shutdown endpoint, but, by default, it is visible only through JMX. To enable it as an HTTP endpoint, add management.endpoint.shutdown.enabled=true to your application.properties file and expose it with management.endpoints.web.exposure.include=health,info,shutdown. However, you probably should not enable the shutdown endpoint for a publicly available application.

You can check the health of the application by running the following command:

$ curl http://localhost:8080/actuator/health {"status":"UP"}

You can try also to invoke shutdown through curl, to see what happens when you have not added the necessary line (shown in the preceding note) to application.properties:

$ curl -X POST http://localhost:8080/actuator/shutdown {"timestamp":1401820343710,"error":"Not Found","status":404,"message":"","path":"/actuator/shutdown"}

Because we did not enable it, the requested endpoint is not available (because the endpoint does not exist).

For more details about each of these REST endpoints and how you can tune their settings with an application.properties file (in src/main/resources), see the documentation about the endpoints.

View Spring Boot’s Starters

You have seen some of Spring Boot’s “starters”. You can see them all here in source code.

JAR Support

The last example showed how Spring Boot lets you wire beans that you may not be aware you need. It also showed how to turn on convenient management services.

However, Spring Boot does more than that. It supports not only traditional WAR file deployments but also lets you put together executable JARs, thanks to Spring Boot’s loader module. The various guides demonstrate this dual support through the spring-boot-gradle-plugin and spring-boot-maven-plugin.

Summary

Congratulations! You built a simple web application with Spring Boot and learned how it can ramp up your development pace. You also turned on some handy production services. This is only a small sampling of what Spring Boot can do. See Spring Boot’s online docs for much more information.

See Also

The following guides may also be helpful:

Want to write a new guide or contribute to an existing one? Check out our contribution guidelines.

All guides are released with an ASLv2 license for the code, and an Attribution, NoDerivatives creative commons license for the writing.

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