Spring Boot autoconfigure and its dependencies

Question

I checked the source code of module spring-boot-autoconfigure

It has configurations classes for plenty of technologies : data, redis, cassandra, JPA, LDAP etc...

How can this module can compile properly without including all theses technologies dependencies jar in its POM ?

If I take the example of HibernateJpaAutoConfiguration class :

It uses beans/classes from other Spring modules like spring-orm :

import org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean;

However spring-boot-autoconfigure has no dependency to spring-orm in its POM. So how is compilation possible ?

Although I don't find a POM in the actual main branch of the Spring Boot project: In the Gradle-script the Spring ORM dependency is listed (as optional) [here](https://github.com/spring-projects/spring-boot/blob/11cbb2549e2c327c2c7426af035846c537c4353a/spring-boot-project/spring-boot-autoconfigure/build.gradle#L129). — hjoeren, Jun 06 '22 at 16:42

score 4 · Accepted Answer · answered Jun 06 '22 at 18:50

This is possible because they apply Maven's concept of optional dependencies:

Optional dependencies are used when it's not possible (for whatever reason) to split a project into sub-modules. The idea is that some of the dependencies are only used for certain features in the project and will not be needed if that feature isn't used. (...) However, since the project cannot be split up (again, for whatever reason), these dependencies are declared optional. If a user wants to use functionality related to an optional dependency, they have to redeclare that optional dependency in their own project.

In Maven, it would usually look like this:

<dependency>
    <groupId>sample.ProjectA</groupId>
    <artifactId>Project-A</artifactId>
    <version>1.0</version>
    <optional>true</optional>
</dependency>

In this example, the project is compiled with Project-A. However, Project-A is not shared as transitive dependency.

The developers of Spring Boot use Gradle instead of Maven. They wrote their own Gradle plugin to replicate this behavior. The result looks something like this:

dependencies {
    ...
    optional("org.springframework:spring-orm")

(see spring-boot-autoconfigure/build.gradle)

Spring Boot autoconfigure and its dependencies

1 Answers1