Let’s develop a simple “Hello World!” web application in Java that highlights some of Spring Boot’s key features. We’ll use Maven to build this project since most IDEs support it.
![[Tip]](images/tip.png) | Tip |
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The spring.io web site contains many “Getting Started” guides that use Spring Boot. If you’re looking to solve a specific problem; check there first. |
Before we begin, open a terminal to check that you have valid versions of Java and Maven installed.
$ java -version java version "1.7.0_51" Java(TM) SE Runtime Environment (build 1.7.0_51-b13) Java HotSpot(TM) 64-Bit Server VM (build 24.51-b03, mixed mode)
$ mvn -v Apache Maven 3.1.1 (0728685237757ffbf44136acec0402957f723d9a; 2013-09-17 08:22:22-0700) Maven home: /Users/user/tools/apache-maven-3.1.1 Java version: 1.7.0_51, vendor: Oracle Corporation
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This sample needs to be created in its own folder. Subsequent instructions assume that you have created a suitable folder and that it is your “current directory”. |
We need to start by creating a Maven pom.xml file. The pom.xml is the recipe that
will be used to build your project. Open you favorite text editor and add the following:
<?xml version="1.0" encoding="UTF-8"?> <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.example</groupId> <artifactId>myproject</artifactId> <version>0.0.1-SNAPSHOT</version> <parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>1.2.0.M1</version> </parent> <!-- Additional lines to be added here... --> <!-- (you don't need this if you are using a .RELEASE version) --> <repositories> <repository> <id>spring-snapshots</id> <url>http://repo.spring.io/snapshot</url> <snapshots><enabled>true</enabled></snapshots> </repository> <repository> <id>spring-milestones</id> <url>http://repo.spring.io/milestone</url> </repository> </repositories> <pluginRepositories> <pluginRepository> <id>spring-snapshots</id> <url>http://repo.spring.io/snapshot</url> </pluginRepository> <pluginRepository> <id>spring-milestones</id> <url>http://repo.spring.io/milestone</url> </pluginRepository> </pluginRepositories> </project>
This should give you a working build, you can test it out by running mvn package (you
can ignore the “jar will be empty - no content was marked for inclusion!” warning for
now).
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At this point you could import the project into an IDE (most modern Java IDE’s include built-in support for Maven). For simplicity, we will continue to use a plain text editor for this example. |
Spring Boot provides a number of “Starter POMs” that make easy to add jars to your
classpath. Our sample application has already used 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.
Other “Starter POMs” simply provide dependencies that you are likely to need when
developing a specific type of application. Since we are developing a web application, we
will add a spring-boot-starter-web dependency — but before that, let’s look at what we
currently have.
$ mvn dependency:tree [INFO] com.example:myproject:jar:0.0.1-SNAPSHOT
The mvn dependency:tree command prints tree representation of your project dependencies.
You can see that spring-boot-starter-parent provides no
dependencies by itself. Let’s edit our pom.xml and add the spring-boot-starter-web dependency
just below the parent section:
<dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> </dependencies>
If you run mvn dependency:tree again, you will see that there are now a number of
additional dependencies, including the Tomcat web server and Spring Boot itself.
To finish our application we need to create a single Java file. Maven will compile sources
from src/main/java by default so you need to create that folder structure, then add a
file named src/main/java/Example.java:
import org.springframework.boot.*; import org.springframework.boot.autoconfigure.*; import org.springframework.stereotype.*; import org.springframework.web.bind.annotation.*; @RestController @EnableAutoConfiguration public class Example { @RequestMapping("/") String home() { return "Hello World!"; } public static void main(String[] args) throws Exception { SpringApplication.run(Example.class, args); } }
Although there isn’t much code here, quite a lot is going on. Let’s step though the important parts.
The first annotation on our Example 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 will consider it when handling incoming web requests.
The @RequestMapping annotation provides “routing” information. It is telling 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.
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The second class-level annotation is @EnableAutoConfiguration. This annotation tells
Spring Boot to “guess” how you will 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 will assume that you are developing a web application
and setup Spring accordingly.
The final part of our application is the main method. This is just 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 will
bootstrap our application, starting Spring which will in turn start the auto-configured
Tomcat web server. We need to pass Example.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 our application should work. Since we have used the
spring-boot-starter-parent POM we have a useful run goal that we can use to start
the application. Type mvn spring-boot:run from the root project directory to start the
application:
$ mvn spring-boot:run . ____ _ __ _ _ /\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/ ___)| |_)| | | | | || (_| | ) ) ) ) ' |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot :: (v1.2.0.M1) ....... . . . ....... . . . (log output here) ....... . . . ........ Started Example in 2.222 seconds (JVM running for 6.514)
If you open a web browser to http://localhost:8080 you should see the following output:
Hello World!
To gracefully exit the application hit ctrl-c.
Let’s finish our example by creating a completely self-contained executable jar file that we could run in production. Executable jars (sometimes called “fat jars”) are archives containing your compiled classes along with all of the jar dependencies that your code needs to run.
To create an executable jar we need to add the spring-boot-maven-plugin to our
pom.xml. Insert the following lines just below the dependencies section:
<build> <plugins> <plugin> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-maven-plugin</artifactId> </plugin> </plugins> </build>
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Save your pom.xml and run mvn package from the command line:
$ mvn package [INFO] Scanning for projects... [INFO] [INFO] ------------------------------------------------------------------------ [INFO] Building myproject 0.0.1-SNAPSHOT [INFO] ------------------------------------------------------------------------ [INFO] .... .. [INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ myproject --- [INFO] Building jar: /Users/developer/example/spring-boot-example/target/myproject-0.0.1-SNAPSHOT.jar [INFO] [INFO] --- spring-boot-maven-plugin:1.2.0.M1:repackage (default) @ myproject --- [INFO] ------------------------------------------------------------------------ [INFO] BUILD SUCCESS [INFO] ------------------------------------------------------------------------
If you look in the target directory you should see myproject-0.0.1-SNAPSHOT.jar. The
file should be around 10 Mb in size. If you want to peek inside, you can use jar tvf:
$ jar tvf target/myproject-0.0.1-SNAPSHOT.jar
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:
$ java -jar target/myproject-0.0.1-SNAPSHOT.jar . ____ _ __ _ _ /\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/ ___)| |_)| | | | | || (_| | ) ) ) ) ' |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot :: (v1.2.0.M1) ....... . . . ....... . . . (log output here) ....... . . . ........ Started Example in 3.236 seconds (JVM running for 3.764)
As before, to gracefully exit the application hit ctrl-c.