Configure tasks that need "embulkPluginExtension" only when they are needed

build.gradle

@@ -10,7 +10,7 @@ plugins {
 // They want Gradle plugins to be published under the "gradle.plugin" prefix for some security reasons.
 // https://plugins.gradle.org/docs/publish-plugin
 group = "org.embulk"
-version = "0.2.5-SNAPSHOT"
+version = "0.2.6-SNAPSHOT"
 description = "A Gradle plugin to build and publish Embulk plugins"
 
 repositories {

src/main/java/org/embulk/gradle/embulk_plugins/ConfigureGemForEmbulkPlugin.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2019 The Embulk project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.embulk.gradle.embulk_plugins;
+
+import java.io.File;
+import java.util.Arrays;
+import java.util.List;
+import javax.inject.Inject;
+import org.gradle.api.DefaultTask;
+import org.gradle.api.GradleException;
+import org.gradle.api.Project;
+import org.gradle.api.artifacts.Configuration;
+import org.gradle.api.tasks.TaskAction;
+import org.gradle.api.tasks.TaskProvider;
+import org.gradle.api.tasks.bundling.Jar;
+
+/**
+ * Configure a {@code "gem"} task to be available as an Embulk plugin.
+ */
+class ConfigureGemForEmbulkPlugin extends DefaultTask {
+    @Inject
+    public ConfigureGemForEmbulkPlugin() {
+        super();
+    }
+
+    @TaskAction
+    public void configure() {
+        final Project project = this.getProject();
+
+        final EmbulkPluginExtension extension = project.getExtensions().getByType(EmbulkPluginExtension.class);
+
+        final Configuration runtimeConfiguration = project.getConfigurations().getByName("runtime");
+
+        final TaskProvider<Gem> gemTask = project.getTasks().named("gem", Gem.class, task -> {
+            task.setEmbulkPluginMainClass(extension.getMainClass().get());
+            task.setEmbulkPluginCategory(extension.getCategory().get());
+            task.setEmbulkPluginType(extension.getType().get());
+
+            if ((!task.getArchiveBaseName().isPresent())) {
+                // project.getName() never returns null.
+                // https://docs.gradle.org/5.5.1/javadoc/org/gradle/api/Project.html#getName--
+                task.getArchiveBaseName().set(project.getName());
+            }
+            // summary is kept empty -- mandatory.
+            if ((!task.getArchiveVersion().isPresent()) && (!project.getVersion().toString().equals("unspecified"))) {
+                // project.getVersion() never returns null.
+                // https://docs.gradle.org/5.5.1/javadoc/org/gradle/api/Project.html#getVersion--
+                task.getArchiveVersion().set(buildGemVersionFromMavenVersion(project.getVersion().toString()));
+            }
+
+            task.getDestinationDirectory().set(((File) project.property("buildDir")).toPath().resolve("gems").toFile());
+            task.from(runtimeConfiguration, copySpec -> {
+                copySpec.into("classpath");
+            });
+            task.from(((Jar) project.getTasks().getByName("jar")).getArchiveFile(), copySpec -> {
+                copySpec.into("classpath");
+            });
+        });
+
+        project.getTasks().named("gemPush", GemPush.class, task -> {
+            if (!task.getGem().isPresent()) {
+                task.getGem().set(gemTask.get().getArchiveFile());
+            }
+        });
+    }
+
+    private static String buildGemVersionFromMavenVersion(final String mavenVersion) {
+        if (mavenVersion.contains("-")) {
+            final List<String> versionTokens = Arrays.asList(mavenVersion.split("-"));
+            if (versionTokens.size() != 2) {
+                throw new GradleException("Failed to convert the version \"" + mavenVersion + "\" to Gem-style.");
+            }
+            return versionTokens.get(0) + '.' + versionTokens.get(1).toLowerCase();
+        } else {
+            return mavenVersion;
+        }
+    }
+}

src/main/java/org/embulk/gradle/embulk_plugins/ConfigureJarForEmbulkPlugin.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2019 The Embulk project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.embulk.gradle.embulk_plugins;
+
+import javax.inject.Inject;
+import org.gradle.api.DefaultTask;
+import org.gradle.api.Project;
+import org.gradle.api.model.ObjectFactory;
+import org.gradle.api.provider.Property;
+import org.gradle.api.tasks.TaskAction;
+import org.gradle.api.tasks.bundling.Jar;
+
+/**
+ * Configure a {@code "jar"} task to be available as an Embulk plugin.
+ *
+ * <p>It configures the {@code "jar"} task with required MANIFEST.
+ */
+public class ConfigureJarForEmbulkPlugin extends DefaultTask {
+    @Inject
+    public ConfigureJarForEmbulkPlugin() {
+        super();
+
+        final ObjectFactory objectFactory = this.getProject().getObjects();
+
+        this.jar = objectFactory.property(String.class);
+        this.jar.set("jar");
+    }
+
+    @TaskAction
+    public void configure() {
+        final Project project = this.getProject();
+
+        final EmbulkPluginExtension extension = project.getExtensions().getByType(EmbulkPluginExtension.class);
+
+        project.getTasks().named(this.jar.get(), Jar.class, jarTask -> {
+            jarTask.manifest(UpdateManifestAction.builder()
+                             .add("Embulk-Plugin-Main-Class", extension.getMainClass().get())
+                             .add("Embulk-Plugin-Category", extension.getCategory().get())
+                             .add("Embulk-Plugin-Type", extension.getType().get())
+                             .add("Embulk-Plugin-Spi-Version", "0")
+                             .add("Implementation-Title", project.getName())
+                             .add("Implementation-Version", project.getVersion().toString())
+                             .build());
+        });
+    }
+
+    public Property<String> getJar() {
+        return this.jar;
+    }
+
+    private final Property<String> jar;
+}

src/main/java/org/embulk/gradle/embulk_plugins/EmbulkPluginExtension.java

@@ -59,7 +59,7 @@ public Property<String> getType() {
         return this.type;
     }
 
-    void checkValidity() {
+    boolean isValidEmbulkPluginDefined() {
         final ArrayList<String> errors = new ArrayList<>();
         if ((!this.mainClass.isPresent()) || this.mainClass.get().isEmpty()) {
             errors.add("\"mainClass\"");
@@ -71,17 +71,21 @@ void checkValidity() {
             errors.add("\"type\"");
         }
 
-        if (!errors.isEmpty()) {
-            throw new GradleException(
-                    "Failed to configure \"embulkPlugin\" because of insufficient settings: [ "
-                    + String.join(", ", errors) + " ]");
-        }
-
         if (!CATEGORIES.contains(this.category.get())) {
             throw new GradleException(
                     "Failed to configure \"embulkPlugin\" because \"category\" must be one of: [ "
                     + String.join(", ", CATEGORIES_ARRAY) + " ]");
         }
+
+        if (errors.isEmpty()) {
+            return true;
+        }
+
+        this.project.getLogger().lifecycle(
+                "The project \"{}\" is not configured for an Embulk plugin.", this.project.getPath());
+        this.project.getLogger().lifecycle(
+                "\"embulkPlugin { ... }\" is not fully configured. Insufficient setting(s): [ {} ]", String.join(", ", errors));
+        return false;
     }
 
     private static final String[] CATEGORIES_ARRAY = {