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1. Overview

In this tutorial, we'll see how to use a Spring Boot application as a dependency of another project.

2. Spring Boot Packaging

The Spring Boot Maven and Gradle plugins both package our application as executable JARs – such a file can't be used in another project since class files are put into BOOT-INF/classes. This is not a bug, but a feature.

In order to share classes with another project, the best approach to take is to create a separate jar containing shared classes, then make it a dependency of all modules that rely on them.

But if that isn't possible, we can configure the plugin to generate a separate jar that can be used as a dependency.

2.1. Maven Configuration

Let's configure the plugin with a classifier:

...
<build>
    ...
    <plugins>
        ...
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
	    <configuration>
	        <classifier>exec</classifier>
            </configuration>
        </plugin>
    </plugins>
</build>

Though, the configuration for Spring Boot 1.x would be a little different:

...
<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
            <configuration>
                <classifier>exec</classifier>
            </configuration>
        </execution>
    </executions>
</plugin>

This will create two jars, one with the suffix exec as an executable jar, and another as a more typical jar that we can include in other projects.

3. Packaging with Maven Assembly Plugin

We may also use the maven-assembly-plugin to create the dependent jar:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-assembly-plugin</artifactId>
    <configuration>
        <descriptorRefs>
            <descriptorRef>jar-with-dependencies</descriptorRef>
        </descriptorRefs>
    </configuration>
    <executions>
        <execution>
            <id>make-assembly</id>
            <phase>package</phase>
            <goals>
                <goal>single</goal>
            </goals>
        </execution>
    </executions>
</plugin>

If we use this plugin along with the exec classifier in spring-boot-maven-plugin, it will generate three jars. The first two will be the same we saw previously.

The third will have whatever suffix we specified in the <descriptorRef> tag and will contain all the project's transitive dependencies. If we include it in another project, we won't need to separately include Spring dependencies.

4. Conclusion

In this article, we showed a couple of approaches to packaging a Spring Boot application for use as a dependency in other Maven projects.

As always, the code backing the article is available over on GitHub.

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