/gs-convert-jar-to-war

Converting a Spring Boot JAR Application to a WAR :: Learn how to convert your Spring Boot JAR-based application to a WAR file.

Primary LanguageJava

tags projects
spring-boot
spring-boot
Table of Contents

This guide walks you through the process of converting a runnable JAR application that was built with Spring Boot into a WAR file that you can run in any standard servlet container.

What you’ll build

You’ll take a simple Spring MVC web app and build a WAR file using Spring Boot.

What you’ll need

Set up the project

build.gradle

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

Create a basic web application

Now that you’ve set up the project, you can create a Spring MVC application. You’ll get it to run using Spring Boot’s embedded servlet container. Then, you’ll modify things slightly to build a WAR file that can run in any servlet 3.0 container.

Spring MVC allows you to quickly build controllers for your web site.

src/main/java/hello/HelloController.java

link:initial/src/main/java/hello/HelloController.java[role=include]

This controller is concise and simple, but there’s plenty going on under the hood. Let’s break it down step by step.

The @Controller annotation signals that this class contains web application paths.

The @RequestMapping annotation ensures that HTTP requests to / are mapped to the index() method.

Note
 In this example, the application only handles GET.

The implementation of the method body returns the string index, signaling the name of the view that needs to be rendered.

The web controller returns the string index when someone does GET / on your web site. Spring Boot has automatically added Thymeleaf beans to the application context to convert this into a request for the Thymeleaf template located at src/main/resources/templates/index.html.

src/main/resources/templates/index.html

link:initial/src/main/resources/templates/index.html[role=include]

This template has some very basic HTML elements and no actual Thymeleaf-specific code. You could augment it as needed.

Make the application executable

In this guide, you’ll first make the application an executable JAR file. You package everything in a single, executable JAR, driven by a main() method. Along the way, you use Spring’s support for embedding the Tomcat servlet container as the HTTP runtime, instead of deploying to an external instance. Later on, you’ll see how to build a [WAR][u-war] file and run it on a standard container.

src/main/java/hello/Application.java

link:initial/src/main/java/hello/Application.java[role=include]

The main() method defers to the SpringApplication helper class, providing Application.class as an argument to its run() method. This tells Spring to read the annotation metadata from Application and to manage it as a component in the Spring application context.

The @ComponentScan annotation tells Spring to search recursively through the hello package and its children for classes marked directly or indirectly with Spring’s @Component annotation. This directive ensures that Spring finds and registers the HelloController, because it is marked with @Controller, which in turn is a kind of @Component annotation.

The @EnableAutoConfiguration annotation switches on reasonable default behaviors based on the content of your classpath. For example, because the application depends on the embeddable version of Tomcat (tomcat-embed-core.jar), a Tomcat server is set up and configured with reasonable defaults on your behalf. And because the application also depends on Spring MVC (spring-webmvc.jar), a Spring MVC DispatcherServlet is configured and registered for you — no web.xml necessary! Auto-configuration is a powerful, flexible mechanism. See the {@EnableAutoConfiguration}[API documentation] for further details.

Now that your Application class is ready, you simply instruct the build system to create a single, executable jar containing everything. This makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

./gradlew clean build

Logging output is displayed. The service should be up and running within a few seconds. With your browser, click on http://localhost:8080. You should see the "Hello, world!" text rendered by the template.

Create a WAR file

The application you built up to this point is configured to generate a JAR artifact. To switch it to a WAR file, you need add another plugin to your build.gradle:

apply plugin: 'war'

You also need to change the jar settings to war settings:

war {
    baseName = 'gs-convert-jar-to-war'
    version =  '0.1.0'
}

and, if you want to deploy the WAR file to an external container, you also need to mark the embedded container dependencies as "provided":

---
configurations {
    providedRuntime
}

dependencies { …​ providedRuntime("org.springframework.boot:spring-boot-starter-tomcat") …​ } ---

This signals Gradle to build a WAR. Here is the new version of the build.gradle:

build.gradle

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

Now when you run ./gradlew clean build a WAR file is created, as you’ll see below.

Note
 If you are using Spring Tool Suite or Eclipse, you need to run ./gradlew eclipseWtp to reconfigure your project so you can drag the app directly to tc Server.

Initialize the servlet

Previously, the application contained a public static void main() method which the spring-boot-gradle-plugin was configured to run when using the java -jar command.

By converting this into a WAR file with no XML files, you need a different signal to the servlet container on how to launch the application.

src/main/java/hello/HelloWebXml.java

link:complete/src/main/java/hello/HelloWebXml.java[role=include]

HelloWebXml is a pure Java class that provides an alternative to creating a web.xml. It extends the SpringServletInitializer class. This extension offers many configurable options by overriding methods. But one required method is configure().

configure() provides the means to register the classes that are needed to launch the application. This is where you supply a handle to your Application configuration. Remember: Application has the @ComponentScan, so it will find the web controller automatically.

Even though public static void main() is no longer needed, you can leave that code in place.

Note
 If you didn’t use @ComponentScan, you would either need to add all other components manually as @Bean`s or include the other components inside `configure().

Run the WAR file

Once Application is loaded, it will trigger Spring Boot to automatically configure other beans. In this example, it adds the the Spring MVC beans and Thymeleaf beans. But Spring Boot adds other beans driven by a combination factors such as what’s on your classpath as well as other settings in the application context.

At this stage, you are ready to build a WAR file.

./gradlew clean build

This command creates build/libs/gs-convert-jar-to-war-0.1.0.war, a deployable artifact.

You can download Tomcat 7.0.52 (the same version Spring Boot currently uses), Jetty, or any other container, as long as it has servlet 3.0 support. Unpack it and drop the WAR file in the proper directory. Then start the server.

If you are using Spring Tool Suite to develop your application, you can use its built-in support for tc Server v2.9.

  • Drag the entire application’s root folder down to the server instance.

  • Click the Start button to start the app.

  • You should see tc Server logging appear in one of the console windows.

  • Right-click on the app, and select Open Home Page. A browser tab inside STS should open up and display the "Hello, world!" text.

That means you can then either navigate to:

From there you will see the "Hello, world!" text.

Summary

Congratulations! You’ve just converted an executable JAR application into a WAR-file based application that can be run in any servlet 3.0+ container.