ZCW-SpringOne

<#assign project_id="gs-spring-boot">

What you'll build

This guide provides an introduction to Spring Boot by building a simple web application. This doesn't demonstrate all of its features but will help you get started with the concepts Spring Boot has to offer.

What you'll need

• About 15 minutes
• <@prereq_editor_jdk_buildtools/>

<@how_to_complete_this_guide jump_ahead="Warming up with Spring Boot"/>

Set up the project

<@build_system_intro/>

<@create_directory_structure_hello/>

Create a Maven POM

<@snippet path="pom.xml" prefix="initial"/>

Warming up with Spring Boot

What does Spring Boot provide? At the core, it offers a much faster way to build applications because it make reasonable assumptions such as looking at your classpath and other beans you have configured to see what you're missing.

For example:

• Got Spring MVC? There are a handful of needed beans people almost always use in that situation. Spring Boot adds them automatically. But why stop there? A Spring MVC app needs a servlet container so Spring Boot automatically configures embedded Tomcat.
• Got Jetty? You probably do NOT want Tomcat, but instead embedded Jetty. Don't lift a finger; Spring Boot handles it for you.
• Got Thymeleaf? There are a few beans that must always be added to your application context. Why should you have to deal with that? Let Spring Boot handle it for you.
• Doing multipart file uploads? The MultipartConfigElement is part of the servlet 3.0 spec and let's you define upload parameters in pure Java. Why should you have to worry about plugging that into your servlet? Define one in your application context and Spring Boot will snatch it up and plug it into Spring MVC's battle tested DispatcherServlet.

It doesn't stop there. These are just a few examples of the automatic configuration provided by Spring Boot. But it doesn't get in your way. For example, Spring Boot may make assumptions and add a SpringTemplateEngine for your Thymeleaf-based application, unless you've already defined one. At that point, Spring Boot automatically steps aside and lets you take control.

Creating a simple web application

You already have the base build file at the top. Next step is to create a web controller for a simple web application.

<@snippet path="src/main/java/hello/HelloController.java" prefix="initial"/>

The class is flagged as a @Controller meaning it's ready for use by Spring MVC to handle web requests. @RequestMapping maps / to the index() method. When invoked from a browser or using curl on the command line, it returns pure text thanks to the @ResponseBody annotation.

To make it executable, create an Application class:

<@snippet path="src/main/java/hello/Application.java" prefix="initial"/>

• @Configuration tags the class as a source of bean definitions for the application context.
• @EnableAutoConfiguration tells Spring Boot to get going and start adding beans based on classpath settings, other beans, and various property settings.
• @EnableWebMvc signals Spring MVC that this application is a web application and to activate key behaviors such as setting up a DispatcherServlet.
• @ComponentScanning tells Spring to look for other components, configurations, and services in the the hello package, allowing it to find the HelloController.

The main() method uses Spring Boot's SpringApplication.run() method to launch an application. Did you notice that there wasn't a single line of XML? No web.xml file either. This web application is 100% pure Java and you didn't have to deal with configuring any plumbing or infrastructure.

The run() method returns an ApplicationContext and this application then retrieves all the beans that were created either by your app or were automatically added thanks to Spring Boot. It sorts them and prints them out.

To run it, execute:

$ mvn package spring-boot:run

You should see some output like this:

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.

Check out the service.

$ curl localhost:8080
Greetings from Spring Boot!

Switching to Jetty

What if you preferred Jetty over Tomcat? They're both compliant servlet containers, so it should be darn simple to switch. And it is!

Add this to your build file's list of dependencies:

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

Adding multipart upload support

You should also update your configuration and add a MultipartConfigElement to the application context.

<@snippet path="src/main/java/hello/Application.java" prefix="complete"/>

Note: A production version of MultipartConfigElement would not be empty but instead specify things like target upload path, file size upload limits, etc.

Re-run the app

Run the app again:

$ mvn package spring-boot:run

Now check out the output:

Let's inspect the beans provided by Spring Boot:
application
beanNameHandlerMapping
defaultServletHandlerMapping
dispatcherServlet
embeddedServletContainerCustomizerBeanPostProcessor
handlerExceptionResolver
helloController
httpRequestHandlerAdapter
jettyEmbeddedServletContainerFactory
messageSource
multipartConfigElement
multipartResolver
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$DispatcherServletConfiguration
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration$EmbeddedJetty
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration
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
viewControllerHandlerMapping

There is little change from the previous output, except there is no longer a tomcatEmbeddedServletContainerFactory. Instead, there is a new jettyEmbeddedServletContainer.

There is also the multipartConfigElement you added. But along with it came a multipartResolver courtesy of Spring Boot, a bean recommended to support file uploads with Spring MVC.

Other than that, everything else appears the same, as it should be. Most the beans listed above provide Spring MVC's production-grade features. Just swapping one aspect, the container, and adding upload support shouldn't cause a system wide ripple.

That is not all

That last example showed how Spring Boot makes it easy to wire beans you may not be aware you need. But Spring Boot does more than that. It supports not only traditional WAR file deployments, but also makes it easy to put together executable JARs thanks to Spring Boot's loader module. The various guides demonstrate this dual support through the spring-boot-maven-plugin.

Spring Boot provides more than a quick way to wire beans. Spring Boot's actuator module adds common needed business components like health, metrics, audits, and other features.

Spring Boot also have Groovy support, allowing you to dynamically build web apps like this:

@Controller
class ThisWillActuallyRun {

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

}

Spring Boot automatically laces the code with key annotations and Groovy Grapes to pull down needed libraries to make the app run. With Spring Boot's CLI tool, all you need do is:

$ spring run app.groovy
$ curl localhost:8080
Hello World!

Congratulations!

Spring Boot is powerful, but frankly too big to fit into a single guide. This is just a sampling.

As you read more of this site's getting started guides, you will see it used all over the place. It might not be obvious at first, because Spring Boot is so good at adding the things you need without getting in your way. But after using it for a bit, you may wonder how you lived without it.