Gradle Spring Boot Devtools: developmentOnly and runtimeClasspath

Question

I am puzzled by this block of code to be used in a gradle file, suggested by Spring Boot Documentation on Developer Tools

configurations {
    developmentOnly
    runtimeClasspath {
        extendsFrom developmentOnly
    }
}
dependencies {
    developmentOnly("org.springframework.boot:spring-boot-devtools")
}

I think I must declare the developmentOnly configuration because it is to be used in the dependencies {} block, but why do I need the lines for runtimeClasspath ? I actually tried removing the lines in my project and the project built prefectly fine.

configurations {
    developmentOnly
}
dependencies {
    developmentOnly("org.springframework.boot:spring-boot-devtools")
}

Is runtimeClasspath used by the Java Plugin? (As suggested by this doc ) Will there be any bad side-effect if I do not include those lines for runtimeClasspath ?

Update (2019-12-10)

I can also confirm that the built executable jar built without the runtimeClasspath directive ran prefectly okay. So I really don't know what that directive is doing.

Answer 1

You need spring-boot-devtools only at runtime, that's why we're using runtimeClasspath config.

more details: https://docs.gradle.org/current/userguide/java_library_plugin.html#sec:java_library_configurations_graph

Answer 2

• The developmentOnly is a new configuration that you add.
• The runtimeClasspath configuration is added by the Java Library Plugin .
• You specify that the runtimeClasspath configuration extend from your developmentOnly configuration.
• You set spring-boot-devtools as a dependency for your developmentOnly configuration, which will make the runtimeClasspath depend on spring-boot-devtools too.

I actually tried removing the lines in my project and the project built prefectly fine.

I think this is because the dependency is for run time, not for build time.

I can also confirm that the built executable jar built without the runtimeClasspath directive ran prefectly okay.

I think this is because spring-boot-devtools only works on development mode, eg when you execute the bootRun task with ./gradlew bootRun .