11.5 Creating an executable jar

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.

Executable jars and Java

Java does not provide any standard way to load nested jar files (i.e. jar files that are themselves contained within a jar). This can be problematic if you are looking to distribute a self-contained application.

To solve this problem, many developers use “uber” jars. An uber jar simply packages all classes, from all jars, into a single archive. The problem with this approach is that it becomes hard to see which libraries you are actually using in your application. It can also be problematic if the same filename is used (but with different content) in multiple jars.

Spring Boot takes a different approach and allows you to actually nest jars directly.

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>

	Note
The spring-boot-starter-parent POM includes <executions> configuration to bind the repackage goal. If you are not using the parent POM you will need to declare this configuration yourself. See the plugin documentation for details.

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:1.4.1.BUILD-SNAPSHOT: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 ::  (v1.4.1.BUILD-SNAPSHOT)
....... . . .
....... . . . (log output here)
....... . . .
........ Started Example in 2.536 seconds (JVM running for 2.864)

As before, to gracefully exit the application hit ctrl-c.