Gradle multi-project build dependencies of subprojects cannot be resolved

[aemaem]

Summary

When building a Gradle multi-project with Spring Boot 2.0.0.M7 the project cannot resolve the dependency of another project in the same root project.

Sample

https://github.com/aemaem/spring-boot-gradle-multi-project
Branches:

• master --> Spring Boot 2.0.0.M7
• boot-1.5.9 --> Spring Boot 1.5.9.RELEASE

Background

When running ./gradlew clean build in the sample project the dependency of the demo-api project cannot be resolved. If the same configuration is configured with Spring Boot 1.5.9.REALEASE then the build works as expected.

Gradle Output

> Task :demo-app:compileJava
/home/mmair/Downloads/demo/demo-app/src/main/java/com/example/demo/DemoApplication.java:3: error: package com.example.demo.api does not exist
import com.example.demo.api.FooApi;
                           ^
/home/mmair/Downloads/demo/demo-app/src/main/java/com/example/demo/DemoApplication.java:13: error: cannot find symbol
        private FooApi foo;
                ^
  symbol:   class FooApi
  location: class DemoApplication
/home/mmair/Downloads/demo/demo-app/src/main/java/com/example/demo/FooService.java:3: error: package com.example.demo.api does not exist
import com.example.demo.api.FooApi;
                           ^
/home/mmair/Downloads/demo/demo-app/src/main/java/com/example/demo/FooService.java:7: error: cannot find symbol
public class FooService implements FooApi {
                                   ^
  symbol: class FooApi
/home/mmair/Downloads/demo/demo-app/src/main/java/com/example/demo/FooService.java:9: error: method does not override or implement a method from a supertype
    @Override
    ^
5 errors

FAILURE: Build failed with an exception.

Project structure is as follows:

demo
├── build.gradle
├── demo-api
│   ├── build.gradle
│   └── src
│       └── main
│           └── java
│               └── com
│                   └── example
│                       └── demo
│                           └── api
│                               └── FooApi.java
├── demo-app
│   ├── build.gradle
│   └── src
│       └── main
│           ├── java
│           │   └── com
│           │       └── example
│           │           └── demo
│           │               ├── DemoApplication.java
│           │               └── FooService.java
├── gradle
│   └── wrapper
│       ├── gradle-wrapper.jar
│       └── gradle-wrapper.properties
├── gradle.properties
├── gradlew
├── gradlew.bat
└── settings.gradle

[wilkinsona]

demo-api isn't a Spring Boot application so it should not really have the Spring Boot plugin applied to it.

Having the Spring Boot plugin applied causes a problem as it is disabling demo-api's jar task, the output of which is what compile project(":demo-api") adds to the classpath. You can resolve the problem by either only applying the Spring Boot plugin to projects that are Spring Boot applications or by enabling the jar task in the demo-api project.

If the same configuration is configured with Spring Boot 1.5.9.REALEASE then the build works as expected.

Your configuration in your boot-1.5.9 branch isn't the equivalent of the configuration in your master branch. Enabling and disabling bootRepackage isn't the same as enabling and disabling the bootJar task. As explained above you also need to re-enable the jar task. That said, narrowing down the projects to which you apply the Spring Boot plugin is the recommended approach, and one that will work in Boot 1.5 and 2.0.

Here's a diff for your master branch that illustrates how to only apply the Spring Boot plugin to the project that needs it. It also shows how to use Boot's dependency management in other projects (#11059).

diff --git a/build.gradle b/build.gradle
index 54e7f15..86ae66e 100644
--- a/build.gradle
+++ b/build.gradle
@@ -14,7 +14,6 @@ buildscript {

 subprojects {
        apply plugin: "java"
-       apply plugin: "org.springframework.boot"
        apply plugin: "io.spring.dependency-management"

        repositories {
@@ -23,6 +22,12 @@ subprojects {
                maven { url "https://repo.spring.io/milestone" }
        }

+       dependencyManagement {
+               imports {
+                       mavenBom "org.springframework.boot:spring-boot-dependencies:$springBootVersion"
+               }
+       }
+
        dependencies {
                compile "org.springframework.boot:spring-boot-starter"
                compile "org.springframework.boot:spring-boot-starter-actuator"
@@ -33,6 +38,4 @@ subprojects {

        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
-
-       bootJar.enabled = false
 }
\ No newline at end of file
diff --git a/demo-app/build.gradle b/demo-app/build.gradle
index 776bcdb..1e89e6f 100644
--- a/demo-app/build.gradle
+++ b/demo-app/build.gradle
@@ -1,5 +1,4 @@
-springBoot.mainClassName = "com.example.demo.app.DemoApplication"
-bootJar.enabled = true
+apply plugin: 'org.springframework.boot'

 dependencies {
     compile project(":demo-api")