How to add OpenAPI client as a subproject?

I can successfully add a generated openapi client to my project via source sets. But then I have to copy dependencies into the main build-gradle, resolve conflicts -> I think it would be a better design to have the client as a subproject with its own build.gradle.

So I add include = 'build:openapi-java-client' to my settings.gradle and compile project(':build:openapi-java-client') to my dependencies. So that I have the following files:
build.gradle:

plugins {
    id 'java'
    id 'application'
    id "org.openapi.generator" version "4.3.1"
}

repositories {
    jcenter()
}

openApiGenerate {
    generatorName = "java"
    inputSpec = "$rootDir/specs/petstore.yaml".toString()
    outputDir = "$buildDir/openapi-java-client".toString()
    apiPackage = "org.openapi.example.api"
    invokerPackage = "org.openapi.example.invoker"
    modelPackage = "org.openapi.example.model"
    configOptions = [
        dateLibrary: "java8"
    ]
}

dependencies {
    implementation 'com.google.guava:guava:29.0-jre'
    testImplementation 'junit:junit:4.13'
    
    compile project(':build:openapi-java-client')
}

application {
    mainClassName = 'a.aa.App'
}

and settings.gradle:

rootProject.name = 'simple-java-app'
include = 'build:openapi-java-client'

I execute openApiGenerate in advance, after adding it as a subproject, I do Gradle -> Refresh Gradle Project and Refresh.

Eclipse then shows me a problem:

Could not run phased build action using Gradle distribution 'https://services.gradle.org/distributions/gradle-6.5.1-bin.zip'.
Settings file 'C:\...\simple-java-app\settings.gradle' line: 11
A problem occurred evaluating settings 'simple-java-app'.
Could not set unknown property 'include' for settings 'simple-java-app' of type org.gradle.initialization.DefaultSettings.

I don't know where to go from here, addressing subprojects in subfolders worked just fine when I worked through https://guides.gradle.org/creating-multi-project-builds/ and put greeting-library in a subfolder.

Solution

You are trying to make build/ a project when that directory specifically is not meant to be a project directory. It's Gradle default build directory and likely 99% of other plugins and other Gradle plugins.

Simply change output directory to something else other than build/:

openApiGenerate {
    generatorName.set("java")
    inputSpec.set("$rootDir/specs/petstore.json")
    outputDir.set("$rootDir/openapi-java-client")
    apiPackage.set("org.openapi.example.api")
    invokerPackage.set("org.openapi.example.invoker")
    modelPackage.set("org.openapi.example.model")
}

Then include the project in your build with the correct syntax:

// settings.gradle
include("openapi-java-client")

However, using the org.openapi.generator seems to generate an invalid build.gradle since I get the following error:

FAILURE: Build failed with an exception.

* Where:
Build file 'C:\Users\fmate\code\example\openapi-java-client\build.gradle' line: 23

* What went wrong:
Could not compile build file 'C:\Users\fmate\code\example\openapi-java-client\build.gradle'.
> startup failed:
  build file 'C:\Users\fmate\code\example\openapi-java-client\build.gradle': 23: unexpected char: '\' @ line 23, column 35.
         main.java.srcDirs = ['src/main\java']

This obviously won't work how you wanted it to since it appears to be an issue with the Gradle plugin itself. If you just need to include the generate code in your project, then just include the generated Java code as part of your main Java source:

openApiGenerate {
    generatorName.set("java")
    inputSpec.set("$rootDir/specs/petstore.json")
    outputDir.set("$buildDir/openapi-java-client")
    apiPackage.set("org.openapi.example.api")
    invokerPackage.set("org.openapi.example.invoker")
    modelPackage.set("org.openapi.example.model")
}

tasks {
    compileJava {
        dependsOn(openApiGenerate)
    }
}

sourceSets {
    main {
        java {
            srcDir(files("${openApiGenerate.outputDir.get()}/src/main"))
        }
    }
}

But with this approach, you'll run into missing imports/dependencies. It doesn't appear this plugin offers the ability to just generate the models/POJOs only, so updating the library property to native and including some missing dependencies manually, it all works:

plugins {
    java
    id("org.openapi.generator") version "5.0.0-beta"
}

repositories {
    mavenCentral()
}

group = "io.mateo.test"

dependencies {
    implementation(platform("com.fasterxml.jackson:jackson-bom:2.11.1"))
    implementation("com.fasterxml.jackson.core:jackson-databind")
    implementation("com.fasterxml.jackson.datatype:jackson-datatype-jsr310")
    implementation("org.openapitools:jackson-databind-nullable:0.2.1")
    implementation("com.google.code.findbugs:jsr305:3.0.2")
    implementation("io.swagger:swagger-core:1.6.2")
}

openApiGenerate {
    generatorName.set("java")
    inputSpec.set("$rootDir/specs/petstore.json")
    outputDir.set("$buildDir/openapi-java-client")
    apiPackage.set("org.openapi.example.api")
    invokerPackage.set("org.openapi.example.invoker")
    modelPackage.set("org.openapi.example.model")
    library.set("native")
    configOptions.put("dateLibrary", "java8")
}

tasks {
    compileJava {
        dependsOn(openApiGenerate)
    }
}

sourceSets {
    main {
        java {
            srcDir(files("${openApiGenerate.outputDir.get()}/src/main"))
        }
    }
}