Allow @AutoService to accept multiple service providers

Fixes https://github.com/google/auto/issues/488
Closes https://github.com/google/auto/issues/548

RELNOTES=Allow @AutoService to accept multiple service providers

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=178621520

8 files changed, 112 insertions, 48 deletions

diff --git a/service/pom.xml b/service/pom.xml
index 7104cb8d..b8b4f5e9 100644
--- a/service/pom.xml
+++ b/service/pom.xml
@@ -79,6 +79,8 @@
         <configuration>
           <!-- disable processing because the definition in META-INF/services breaks javac -->
           <compilerArgument>-proc:none</compilerArgument>
+          <source>1.8</source>
+          <target>1.8</target>
         </configuration>
       </plugin>
     </plugins>
diff --git a/service/src/main/java/com/google/auto/service/AutoService.java b/service/src/main/java/com/google/auto/service/AutoService.java
index f7911169..5fd44a65 100644
--- a/service/src/main/java/com/google/auto/service/AutoService.java
+++ b/service/src/main/java/com/google/auto/service/AutoService.java
@@ -40,6 +40,6 @@ import java.lang.annotation.Target;
 @Retention(SOURCE)
 @Target(TYPE)
 public @interface AutoService {
-  /** Returns the interface implemented by this service provider. */
-  Class<?> value();
+  /** Returns the interfaces implemented by this service provider. */
+  Class<?>[] value();
 }
diff --git a/service/src/main/java/com/google/auto/service/processor/AutoServiceProcessor.java b/service/src/main/java/com/google/auto/service/processor/AutoServiceProcessor.java
index 4a0ab055..71c14174 100644
--- a/service/src/main/java/com/google/auto/service/processor/AutoServiceProcessor.java
+++ b/service/src/main/java/com/google/auto/service/processor/AutoServiceProcessor.java
@@ -15,9 +15,13 @@
  */
 package com.google.auto.service.processor;
 
+import static com.google.auto.common.AnnotationMirrors.getAnnotationValue;
 import static com.google.auto.common.MoreElements.getAnnotationMirror;
+import static com.google.common.collect.ImmutableSet.toImmutableSet;
 
+import com.google.auto.common.MoreTypes;
 import com.google.auto.service.AutoService;
+import com.google.common.annotations.VisibleForTesting;
 import com.google.common.collect.HashMultimap;
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Multimap;
@@ -27,7 +31,7 @@ import java.io.OutputStream;
 import java.io.PrintWriter;
 import java.io.StringWriter;
 import java.util.HashSet;
-import java.util.Map;
+import java.util.List;
 import java.util.Set;
 import java.util.SortedSet;
 import javax.annotation.processing.AbstractProcessor;
@@ -38,7 +42,6 @@ import javax.lang.model.SourceVersion;
 import javax.lang.model.element.AnnotationMirror;
 import javax.lang.model.element.AnnotationValue;
 import javax.lang.model.element.Element;
-import javax.lang.model.element.ExecutableElement;
 import javax.lang.model.element.PackageElement;
 import javax.lang.model.element.TypeElement;
 import javax.lang.model.type.DeclaredType;
@@ -58,6 +61,9 @@ import javax.tools.StandardLocation;
 @SupportedOptions({ "debug", "verify" })
 public class AutoServiceProcessor extends AbstractProcessor {
 
+  @VisibleForTesting
+  static final String MISSING_SERVICES_ERROR = "No service interfaces provided for element!";
+
   /**
    * Maps the class names of service provider interfaces to the
    * class names of the concrete classes which implement them.
@@ -127,26 +133,27 @@ public class AutoServiceProcessor extends AbstractProcessor {
     for (Element e : elements) {
       // TODO(gak): check for error trees?
       TypeElement providerImplementer = (TypeElement) e;
-      AnnotationMirror providerAnnotation = getAnnotationMirror(e, AutoService.class).get();
-      DeclaredType providerInterface = getProviderInterface(providerAnnotation);
-      TypeElement providerType = (TypeElement) providerInterface.asElement();
-
-      log("provider interface: " + providerType.getQualifiedName());
-      log("provider implementer: " + providerImplementer.getQualifiedName());
-
-      if (!checkImplementer(providerImplementer, providerType)) {
-        String message = "ServiceProviders must implement their service provider interface. "
-            + providerImplementer.getQualifiedName() + " does not implement "
-            + providerType.getQualifiedName();
-        error(message, e, providerAnnotation);
+      AnnotationMirror annotationMirror = getAnnotationMirror(e, AutoService.class).get();
+      Set<DeclaredType> providerInterfaces = getValueFieldOfClasses(annotationMirror);
+      if (providerInterfaces.isEmpty()) {
+        error(MISSING_SERVICES_ERROR, e, annotationMirror);
+        continue;
+      }
+      for (DeclaredType providerInterface : providerInterfaces) {
+        TypeElement providerType = MoreTypes.asTypeElement(providerInterface);
+
+        log("provider interface: " + providerType.getQualifiedName());
+        log("provider implementer: " + providerImplementer.getQualifiedName());
+
+        if (checkImplementer(providerImplementer, providerType)) {
+          providers.put(getBinaryName(providerType), getBinaryName(providerImplementer));
+        } else {
+          String message = "ServiceProviders must implement their service provider interface. "
+              + providerImplementer.getQualifiedName() + " does not implement "
+              + providerType.getQualifiedName();
+          error(message, e, annotationMirror);
+        }
       }
-
-      String providerTypeName = getBinaryName(providerType);
-      String providerImplementerName = getBinaryName(providerImplementer);
-      log("provider interface binary name: " + providerTypeName);
-      log("provider implementer binary name: " + providerImplementerName);
-
-      providers.put(providerTypeName, providerImplementerName);
     }
   }
 
@@ -243,23 +250,16 @@ public class AutoServiceProcessor extends AbstractProcessor {
     return getBinaryNameImpl(typeElement, typeElement.getSimpleName() + "$" + className);
   }
 
-  private DeclaredType getProviderInterface(AnnotationMirror providerAnnotation) {
-
-    // The very simplest of way of doing this, is also unfortunately unworkable.
-    // We'd like to do:
-    //    ServiceProvider provider = e.getAnnotation(ServiceProvider.class);
-    //    Class<?> providerInterface = provider.value();
-    //
-    // but unfortunately we can't load the arbitrary class at annotation
-    // processing time. So, instead, we have to use the mirror to get at the
-    // value (much more painful).
-
-    Map<? extends ExecutableElement, ? extends AnnotationValue> valueIndex =
-        providerAnnotation.getElementValues();
-    log("annotation values: " + valueIndex);
-
-    AnnotationValue value = valueIndex.values().iterator().next();
-    return (DeclaredType) value.getValue();
+  /**
+   * Returns the contents of a {@code Class[]}-typed "value" field in a given {@code annotationMirror}.
+   */
+  private ImmutableSet<DeclaredType> getValueFieldOfClasses(AnnotationMirror annotationMirror) {
+    @SuppressWarnings("unchecked")
+    List<AnnotationValue> values =
+        ((List<AnnotationValue>) getAnnotationValue(annotationMirror, "value").getValue());
+    // TODO(ronshapiro): class literals may not always be declared types, i.e. int.class,
+    // int[].class
+    return values.stream().map(value -> (DeclaredType) value.getValue()).collect(toImmutableSet());
   }
 
   private void log(String msg) {
diff --git a/service/src/test/java/com/google/auto/service/processor/AutoServiceProcessorTest.java b/service/src/test/java/com/google/auto/service/processor/AutoServiceProcessorTest.java
index 533373db..6b5e1cfa 100644
--- a/service/src/test/java/com/google/auto/service/processor/AutoServiceProcessorTest.java
+++ b/service/src/test/java/com/google/auto/service/processor/AutoServiceProcessorTest.java
@@ -15,12 +15,10 @@
  */
 package com.google.auto.service.processor;
 
-import static com.google.common.truth.Truth.assert_;
-import static com.google.testing.compile.JavaSourcesSubjectFactory.javaSources;
+import static com.google.auto.service.processor.AutoServiceProcessor.MISSING_SERVICES_ERROR;
+import static com.google.testing.compile.JavaSourcesSubject.assertThat;
 
-import com.google.auto.service.processor.AutoServiceProcessor;
 import com.google.testing.compile.JavaFileObjects;
-import java.util.Arrays;
 import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
@@ -32,18 +30,38 @@ import org.junit.runners.JUnit4;
 public class AutoServiceProcessorTest {
   @Test
   public void autoService() {
-    assert_().about(javaSources())
-        .that(Arrays.asList(
+      assertThat(
             JavaFileObjects.forResource("test/SomeService.java"),
             JavaFileObjects.forResource("test/SomeServiceProvider1.java"),
             JavaFileObjects.forResource("test/SomeServiceProvider2.java"),
             JavaFileObjects.forResource("test/Enclosing.java"),
             JavaFileObjects.forResource("test/AnotherService.java"),
-            JavaFileObjects.forResource("test/AnotherServiceProvider.java")))
+            JavaFileObjects.forResource("test/AnotherServiceProvider.java"))
         .processedWith(new AutoServiceProcessor())
         .compilesWithoutError()
         .and().generatesFiles(
             JavaFileObjects.forResource("META-INF/services/test.SomeService"),
             JavaFileObjects.forResource("META-INF/services/test.AnotherService"));
   }
-}
\ No newline at end of file
+
+  @Test
+  public void multiService() {
+    assertThat(
+            JavaFileObjects.forResource("test/SomeService.java"),
+            JavaFileObjects.forResource("test/AnotherService.java"),
+            JavaFileObjects.forResource("test/MultiServiceProvider.java"))
+        .processedWith(new AutoServiceProcessor())
+        .compilesWithoutError()
+        .and().generatesFiles(
+            JavaFileObjects.forResource("META-INF/services/test.SomeServiceMulti"),
+            JavaFileObjects.forResource("META-INF/services/test.AnotherServiceMulti"));
+  }
+
+  @Test
+  public void badMultiService() {
+    assertThat(JavaFileObjects.forResource("test/NoServices.java"))
+        .processedWith(new AutoServiceProcessor())
+        .failsToCompile()
+        .withErrorContaining(MISSING_SERVICES_ERROR);
+  }
+}
diff --git a/service/src/test/resources/META-INF/services/test.AnotherServiceMulti b/service/src/test/resources/META-INF/services/test.AnotherServiceMulti
new file mode 100644
index 00000000..f6ef36ac
--- /dev/null
+++ b/service/src/test/resources/META-INF/services/test.AnotherServiceMulti
@@ -0,0 +1 @@
+test.MultiServiceProvider
diff --git a/service/src/test/resources/META-INF/services/test.SomeServiceMulti b/service/src/test/resources/META-INF/services/test.SomeServiceMulti
new file mode 100644
index 00000000..f6ef36ac
--- /dev/null
+++ b/service/src/test/resources/META-INF/services/test.SomeServiceMulti
@@ -0,0 +1 @@
+test.MultiServiceProvider
diff --git a/service/src/test/resources/test/MultiServiceProvider.java b/service/src/test/resources/test/MultiServiceProvider.java
new file mode 100644
index 00000000..804904bf
--- /dev/null
+++ b/service/src/test/resources/test/MultiServiceProvider.java
@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2017 Google, Inc.
+ *
+ * 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 test;
+
+import com.google.auto.service.AutoService;
+
+@AutoService({SomeService.class, AnotherService.class})
+public class MultiServiceProvider implements SomeService, AnotherService {}
diff --git a/service/src/test/resources/test/NoServices.java b/service/src/test/resources/test/NoServices.java
new file mode 100644
index 00000000..3687eb55
--- /dev/null
+++ b/service/src/test/resources/test/NoServices.java
@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2008 Google, Inc.
+ *
+ * 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 test;
+
+import com.google.auto.service.AutoService;
+
+@AutoService({})
+public class NoServices {}