This section describes how to develop a simple “Hello World!” web application that highlights some of Spring Boot’s key features. We use Maven to build this project, since most IDEs support it.
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The spring.io web site contains many “Getting Started” guides that use Spring Boot. If you need to solve a specific problem, check there first. You can shortcut the steps below by going to start.spring.io and choosing the "Web" starter from the dependencies searcher. Doing so generates a new project structure so that you can start coding right away. Check the Spring Initializr documentation for more details. |
Before we begin, open a terminal and run the following commands to ensure that you have valid versions of Java and Maven installed:
$ java -version java version "1.8.0_102" Java(TM) SE Runtime Environment (build 1.8.0_102-b14) Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)
$ mvn -v Apache Maven 3.5.4 (1edded0938998edf8bf061f1ceb3cfdeccf443fe; 2018-06-17T14:33:14-04:00) Maven home: /usr/local/Cellar/maven/3.3.9/libexec Java version: 1.8.0_102, 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 is
used to build your project. Open your 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>2.1.0.RELEASE</version> </parent> <!-- Additional lines to be added here... --> </project>
The preceding listing should give you a working build. You can test it by running mvn
package
(for now, you can ignore the “jar will be empty - no content was marked for
inclusion!” warning).
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At this point, you could import the project into an IDE (most modern Java IDEs include built-in support for Maven). For simplicity, we continue to use a plain text editor for this example. |
Spring Boot provides a number of “Starters” that let you 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 “Starters” provide dependencies that you are likely to need when developing a
specific type of application. Since we are developing a web application, we add a
spring-boot-starter-web
dependency. Before that, we can look at what we currently have
by running the following command:
$ mvn dependency:tree [INFO] com.example:myproject:jar:0.0.1-SNAPSHOT
The mvn dependency:tree
command prints a tree representation of your project
dependencies. You can see that spring-boot-starter-parent
provides no dependencies by
itself. To add the necessary dependencies, edit your pom.xml
and add the
spring-boot-starter-web
dependency immediately 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 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. By default, Maven
compiles sources from src/main/java
, so you need to create that folder structure and
then add a file named src/main/java/Example.java
to contain the following code:
import org.springframework.boot.*; import org.springframework.boot.autoconfigure.*; 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 is not much code here, quite a lot is going on. We step through the important parts in the next few sections.
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 considers it when handling incoming web requests.
The @RequestMapping
annotation provides “routing” information. It tells 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 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 assumes that you are developing a web application and sets up 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
bootstraps our application, starting Spring, which, in turn, starts
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, your application should work. Since you used the
spring-boot-starter-parent
POM, you have a useful run
goal that you can use to start
the application. Type mvn spring-boot:run
from the root project directory to start the
application. You should see output similar to the following:
$ mvn spring-boot:run . ____ _ __ _ _ /\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/ ___)| |_)| | | | | || (_| | ) ) ) ) ' |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot :: (v2.1.0.RELEASE) ....... . . . ....... . . . (log output here) ....... . . . ........ Started Example in 2.222 seconds (JVM running for 6.514)
If you open a web browser to localhost:8080
, you should see the following output:
Hello World!
To gracefully exit the application, press ctrl-c
.
We 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
. To do so, 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, as follows:
$ 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:2.1.0.RELEASE: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
,
as follows:
$ 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, as follows:
$ java -jar target/myproject-0.0.1-SNAPSHOT.jar . ____ _ __ _ _ /\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/ ___)| |_)| | | | | || (_| | ) ) ) ) ' |____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot :: (v2.1.0.RELEASE) ....... . . . ....... . . . (log output here) ....... . . . ........ Started Example in 2.536 seconds (JVM running for 2.864)
As before, to exit the application, press ctrl-c
.