What is the spring-boot-configuration-processor ? Why do people exclude libraries from it? Why is it invisible in dependency tree?

Introduction

So I noticed the following line in the gradle file of the jhipster project:

annotationProcessor ("org.springframework.boot:spring-boot-configuration-processor") {
        exclude group: 'com.vaadin.external.google', module: 'android-json'
    }

https://github.com/jhipster/jhipster-sample-app-gradle/blob/9e9c3db8f3bedba4b1efd85ecb6ff3f12a5f596a/build.gradle#L230

We also used the same configuration in Maven for another project to solve the following problem: https://stackoverflow.com/q/53260385/1688441

Questions

And now I have the following questions:

• What does the spring-boot-configuration-processor dependency do?
• Why is it necessary to sometimes exclude dependencies from the processor?
• Why doesn't the processor necessarily appear in the mvn-dependency tree?
• Why are exclusions used with processor in situations where it's very difficult to exclude a dependency?

spring-boot-configuration-processor is an annotation processor that generates metadata about classes in your application that are annotated with @ConfigurationProperties. This metadata is used by your IDE (Eclipse, IntelliJ, or NetBeans) to provide auto-completion and documentation for the properties when editing application.properties and application.yaml files. You can learn a bit more about it in the relevant section of Spring Boot's reference documentation.

Since Spring Boot 1.5.10, the exclusion is no longer necessary as com.vaadin.external.google:android-json is no longer a dependency of spring-boot-configuration-processor.

> What does the spring-boot-configuration-processor dependency do?

It scans the libraries in the build and sees what properties they use so as to inform the IDE

> Why is it necessary to sometimes exclude dependencies from the processor?

Maven libraries can clash sometimes - the one you reference was excluded by JHipster because it led to errors when on the classpath together with another library in JHipster's dependencies

> Why doesn't the processor necessarily appear in the mvn dependency:tree?

It does for me on the jhipster-sample-app. Presumably you're referring to the comment on the linked issue stating that the android-json library isn't in the tree. I've asked there about that.

> Why are exclusions used with processor in situations where it's very difficult to exclude a dependency?

This is a dependency clash issue like any other really, it just happens that the processor is bringing in the key dependency (or rather was, as @Andy Wilkinson points out com.vaadin.external.google:android-json is no longer used by the processor)