So why is spring-boot-dependencies to be included as dependencyManagement?

Let's say you have a project named projectA and you add the spring-boot-dependencies to the dependencyManagement section in your pom.xml.

<project>
  <groupId>com.iovation.service</groupId>
  <artifactId>projectA</artifactId>
  <version>1.0.0-SNAPSHOT</version>

  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-dependencies</artifactId>
        <type>pom</type>
        <version>1.5.8.RELEASE</version>
        <scope>import</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>

  <dependencies>
    <!-- Spring Boot Dependencies -->
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
      </dependency>
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
      </dependency>
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
      </dependency>
  ...
</project>

If you notice closely, you will find that all the Spring Boot dependencies declared under the dependencies section don't need to specify the version. It derives the version from the version of spring-boot-dependencies specified in the dependencyManagement section.

Advantages of Dependency Management

• It centralizes dependency information by specifying the Spring Boot version at one place. It really helps during upgrade from one version to another.

• Subsequent declaration of Spring Boot dependencies just mentions the library name without any version. Especially helpful in multi-module projects

• It avoids mismatch of different versions of spring boot libraries in a project.

• No Conflicts.

This is covered in Spring Boot's documentation. In a nutshell, you need to configure your build to use a classifier for the repackaged fat jar so that you can use the original jar as a dependency:

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

<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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.3.0.RELEASE</version>
    <relativePath /> <!-- lookup parent from repository -->
</parent>
<groupId>com.springboot.testapplication</groupId>
<artifactId>testapplication</artifactId>
<version>0.0.1-SNAPSHOT</version>
<name>testapplication</name>
<description>Spring Boot Application</description>
<dependencies>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

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

By default, Spring Boot repackages your JAR into an executable JAR, and it does that by putting all of your classes inside BOOT-INF/classes, and all of the dependent libraries inside BOOT-INF/lib. The consequence of creating this fat JAR is that you can no longer use it as a dependency for other projects.

From Custom repackage classifier:

By default, the repackage goal will replace the original artifact with the repackaged one. That's a sane behaviour for modules that represent an app but if your module is used as a dependency of another module, you need to provide a classifier for the repackaged one.

The reason for that is that application classes are packaged in BOOT-INF/classes so that the dependent module cannot load a repackaged jar's classes.

If you want to keep the original main artifact in order to use it as a dependency, you can add a classifier in the repackage goal configuration:

<plugin>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-maven-plugin</artifactId>
  <version>1.4.1.RELEASE</version>
  <executions>
    <execution>
      <goals>
        <goal>repackage</goal>
      </goals>
      <configuration>
        <classifier>exec</classifier>
      </configuration>
    </execution>
  </executions>
</plugin>

With this configuration, the Spring Boot Maven Plugin will create 2 JARs: the main one will be the same as a usual Maven project, while the second one will have the classifier appended and be the executable JAR.