Fix service files not merged with shadow plugin

This commit fixes shadow jar building, by making sure that the generated
jar merges service files.

Fixes #406

functional-tests/src/test/groovy/io/micronaut/gradle/shadow/ShadowJarSpec.groovy

@@ -0,0 +1,80 @@
+package io.micronaut.gradle.shadow
+
+import io.micronaut.gradle.fixtures.AbstractFunctionalTest
+import org.gradle.testkit.runner.TaskOutcome
+import spock.lang.Issue
+
+class ShadowJarSpec extends AbstractFunctionalTest {
+
+    @Issue("https://github.com/micronaut-projects/micronaut-gradle-plugin/issues/406")
+    def "merges service files when building shadow jar"() {
+        def shadowJar = file("build/libs/hello-world-1.0-all.jar")
+
+        given:
+        settingsFile << "rootProject.name = 'hello-world'"
+        buildFile << """
+            plugins {
+                id "io.micronaut.minimal.application"
+                id "com.github.johnrengelman.shadow" version "7.1.0"
+            }
+
+            version = "1.0"
+            
+            micronaut {
+                version "3.4.0"
+                runtime "none"
+                processing {
+                    annotations("example.*")
+                }
+            }
+            
+            $repositoriesBlock
+            mainClassName="example.Application"
+            
+            dependencies {
+                annotationProcessor("info.picocli:picocli-codegen")
+                implementation("info.picocli:picocli")
+                implementation("io.micronaut.picocli:micronaut-picocli")
+            }
+
+            tasks.register("shadowRun") {
+                def shadowJar = tasks.named("shadowJar")
+                inputs.file(shadowJar.flatMap { it.archiveFile })
+                doLast {
+                    def exec = services.get(ExecOperations)
+                    exec.javaexec {
+                        it.classpath = files(shadowJar.flatMap { it.archiveFile }.get())
+                        it.main = "example.Application"
+                    }
+                }
+            }
+        """
+        testProjectDir.newFolder("src", "main", "java", "example")
+
+        file("src/main/java/example/Application.java") << """package example;
+
+import io.micronaut.configuration.picocli.PicocliRunner;
+import picocli.CommandLine.Command;
+
+@Command(name = "demo", description = "...", mixinStandardHelpOptions = true)
+public class Application implements Runnable {
+
+    public static void main(String[] args) {
+        PicocliRunner.run(Application.class, args);
+    }
+
+    public void run() {
+        System.out.println("Hello, all!");
+    }
+}"""
+
+        when:
+        def result = build('shadowRun')
+
+        then:
+        result.task(":shadowJar").outcome == TaskOutcome.SUCCESS
+        result.task(":shadowRun").outcome == TaskOutcome.SUCCESS
+        shadowJar.exists()
+        result.output.contains("Hello, all!")
+    }
+}

minimal-plugin/src/main/java/io/micronaut/gradle/MicronautMinimalApplicationPlugin.java

@@ -15,6 +15,7 @@
  */
 package io.micronaut.gradle;
 
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar;
 import io.micronaut.gradle.graalvm.GraalUtil;
 import org.apache.tools.ant.taskdefs.condition.Os;
 import org.gradle.api.Plugin;
@@ -176,6 +177,12 @@ private void configureMicronautRuntime(Project project) {
                         project.setProperty("mainClassName", mainClass.get());
                     }
                 }
+
+                // If shadow JAR is enabled it must be configured to merge
+                // all META-INF/services file into a single file otherwise this
+                // will break the application
+                project.getTasks().withType(ShadowJar.class).configureEach(ShadowJar::mergeServiceFiles);
+
             });
         });
     }