/gs-spring-boot

Building an Application with Spring Boot :: Learn how to build an application with minimal configuration.

Primary LanguageJava

Table of Contents

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

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.

Note
 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

For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the setup for you. This example needs only the Spring Web dependency.

The following listing shows the pom.xml file that is created when you choose Maven:

link:initial/pom.xml[role=include]

The following listing shows the build.gradle file that is created when you choose Gradle:

link:initial/build.gradle[role=include]

Create a Simple Web Application

Now you can create a web controller for a simple web application, as the following listing (from src/main/java/com/example/springboot/HelloController.java) shows:

link:initial/src/main/java/com/example/springboot/HelloController.java[role=include]

The class is flagged as a @RestController, meaning it is ready for use by Spring MVC to handle web requests. @RequestMapping 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 (from src/main/java/com/example/springboot/Application.java):

link:complete/src/main/java/com/example/springboot/Application.java[role=include]

There is also a CommandLineRunner method marked as a @Bean, and this runs on start up. 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 (in the complete) directory:

./gradlew bootRun

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

./mvnw spring-boot:run

You should see output similar to the following:

Let's inspect the beans provided by Spring Boot:
application
beanNameHandlerMapping
defaultServletHandlerMapping
dispatcherServlet
embeddedServletContainerCustomizerBeanPostProcessor
handlerExceptionResolver
helloController
httpRequestHandlerAdapter
messageSource
mvcContentNegotiationManager
mvcConversionService
mvcValidator
org.springframework.boot.autoconfigure.MessageSourceAutoConfiguration
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration$DispatcherServletConfiguration
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration$EmbeddedTomcat
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration
org.springframework.boot.context.embedded.properties.ServerProperties
org.springframework.context.annotation.ConfigurationClassPostProcessor.enhancedConfigurationProcessor
org.springframework.context.annotation.ConfigurationClassPostProcessor.importAwareProcessor
org.springframework.context.annotation.internalAutowiredAnnotationProcessor
org.springframework.context.annotation.internalCommonAnnotationProcessor
org.springframework.context.annotation.internalConfigurationAnnotationProcessor
org.springframework.context.annotation.internalRequiredAnnotationProcessor
org.springframework.web.servlet.config.annotation.DelegatingWebMvcConfiguration
propertySourcesBinder
propertySourcesPlaceholderConfigurer
requestMappingHandlerAdapter
requestMappingHandlerMapping
resourceHandlerMapping
simpleControllerHandlerAdapter
tomcatEmbeddedServletContainerFactory
viewControllerHandlerMapping

You can clearly see org.springframework.boot.autoconfigure beans. There is also a tomcatEmbeddedServletContainerFactory.

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

$ curl 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 file:

link:complete/build.gradle[role=include]

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

link:complete/pom.xml[role=include]

Now write a simple unit test that mocks the servlet request and response through your endpoint, as the following listing (from src/test/java/com/example/springboot/HelloControllerTest.java) shows:

link:complete/src/test/java/com/example/springboot/HelloControllerTest.java[role=include]

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 (from src/test/java/com/example/springboot/HelloControllerIT.java):

link:complete/src/test/java/com/example/springboot/HelloControllerIT.java[role=include]

The embedded server starts on a random port because of webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT, and the actual port is discovered at runtime with @LocalServerPort.

Add Production-grade Services

If you are building a web site 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 file:

link:complete/build.gradle[role=include]

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

link:complete/pom.xml[role=include]

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

./gradlew bootRun

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

./mvnw spring-boot:run

You should see that a new set of RESTful end points have been added to the application. These are management services provided by Spring Boot. The following listing shows 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.status-org.springframework.boot.actuate.autoconfigure.health.HealthIndicatorProperties
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
management.trace.http-org.springframework.boot.actuate.autoconfigure.trace.http.HttpTraceProperties

The actuator exposes the following:

Note
 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.endpoints.web.exposure.include=health,info,shutdown to your application.properties file. 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 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 localhost:8080/actuator/shutdown
{"timestamp":1401820343710,"error":"Method Not Allowed","status":405,"message":"Request method 'POST' not supported"}

Because we did not enable it, the request is blocked (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 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 and Groovy 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.

On top of that, Spring Boot also has Groovy support, letting you build Spring MVC web applications with as little as a single file.

Create a new file called app.groovy and put the following code in it:

@RestController
class ThisWillActuallyRun {

    @RequestMapping("/")
    String home() {
        return "Hello, World!"
    }

}
Note
 It does not matter where the file is. You can even fit an application that small inside a single tweet!

Run the Groovy application by running the following command:

$ spring run app.groovy
Note
 Shut down the previous application, to avoid a port collision.

From a different terminal window, run the following curl command (shown with its output):

$ curl localhost:8080
Hello, World!

Spring Boot does this by dynamically adding key annotations to your code and using Groovy Grape to pull down the libraries that are needed to make the app run.

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: