Building an Application with Spring Boot :: Learn how to build an application with minimal configuration.
:spring_boot_version: current
:spring-boot: https://github.com/spring-projects/spring-boot
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:project_id: gs-spring-boot
This guide provides a sampling of how {spring-boot}[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
https://start.spring.io/[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
:java_version: 17
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== 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:
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.
[[scratch]]
== Starting with Spring Initializr
You can use this https://start.spring.io/#!type=maven-project&language=java&packaging=jar&jvmVersion=17&groupId=com.example&artifactId=spring-boot&name=spring-boot&description=Demo project for Spring Boot&packageName=com.example.spring-boot&dependencies=web[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:
. 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.
. Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.
. Click Dependencies and select Spring Web.
. Click Generate.
. Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.
NOTE: If your IDE has the Spring Initializr integration, you can complete this process from your IDE.
NOTE: You can also fork the project from Github and open it in your IDE or other editor.
NOTE: For Spring 3.0 you need Java 17 or later, regardless of whether you use Spring Initializr.
[[initial]]
== 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:
====
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
(from src/main/java/com/example/springboot/Application.java):
====
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:
====
If you use Maven, run the following command in a terminal window (in the complete)
directory:
====
You should see output similar to the following:
====
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):
====
== 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:
If you use Maven, add the following to your pom.xml file:
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:
====
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/HelloControllerITest.java):
====
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 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
http://docs.spring.io/spring-boot/docs/{spring_boot_version}/reference/htmlsingle/#actuator[actuator module].
If you use Gradle, add the following dependency to your build.gradle file:
If you use Maven, add the following dependency to your pom.xml file:
Then restart the application. If you use Gradle, run the following command in a terminal
window (in the complete directory):
====
If you use Maven, run the following command in a terminal window (in the complete
directory):
====
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:
====
The actuator exposes the following:
NOTE: There is also an /actuator/shutdown endpoint, but, by default, it is visible only
through JMX. To http://docs.spring.io/spring-boot/docs/{spring_boot_version}/reference/htmlsingle/#production-ready-endpoints-enabling-endpoints[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:
====
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:
====
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
the http://docs.spring.io/spring-boot/docs/{spring_boot_version}/reference/htmlsingle/#production-ready-endpoints[documentation about the endpoints].
== View Spring Boot’s Starters
You have seen some of
http://docs.spring.io/spring-boot/docs/{spring_boot_version}/reference/htmlsingle/#using-boot-starter[Spring Boot’s “starters”].
You can see them all
https://github.com/spring-projects/spring-boot/tree/main/spring-boot-project/spring-boot-starters[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
http://docs.spring.io/spring-boot/docs/current/reference/htmlsingle[Spring Boot’s online docs]
for much more information.
== See Also
The following guides may also be helpful:
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