New @JsonQualifier annotation.

Works like JSR-330's @Qualifier annotation. You may have
multiple qualifiers, or none.

moshi/src/main/java/com/squareup/moshi/AdapterMethodsFactory.java

@@ -16,29 +16,30 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
-import java.lang.reflect.AnnotatedElement;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
 import java.lang.reflect.Type;
-import java.util.LinkedHashMap;
-import java.util.Map;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Set;
 
-// TODO: support qualifier annotations.
 // TODO: support @Nullable
 // TODO: path in JsonWriter.
 
 final class AdapterMethodsFactory implements JsonAdapter.Factory {
-  private final Map<Type, ToAdapter> toAdapters;
-  private final Map<Type, FromAdapter> fromAdapters;
+  private final List<AdapterMethod> toAdapters;
+  private final List<AdapterMethod> fromAdapters;
 
-  AdapterMethodsFactory(Map<Type, ToAdapter> toAdapters, Map<Type, FromAdapter> fromAdapters) {
+  public AdapterMethodsFactory(List<AdapterMethod> toAdapters, List<AdapterMethod> fromAdapters) {
     this.toAdapters = toAdapters;
     this.fromAdapters = fromAdapters;
   }
 
-  @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, final Moshi moshi) {
-    final ToAdapter toAdapter = toAdapters.get(type);
-    final FromAdapter fromAdapter = fromAdapters.get(type);
+  @Override public JsonAdapter<?> create(
+      Type type, Set<? extends Annotation> annotations, final Moshi moshi) {
+    final AdapterMethod toAdapter = get(toAdapters, type, annotations);
+    final AdapterMethod fromAdapter = get(fromAdapters, type, annotations);
     if (toAdapter == null && fromAdapter == null) return null;
 
     final JsonAdapter<Object> delegate = toAdapter == null || fromAdapter == null
@@ -56,7 +57,7 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
             throw new AssertionError();
           } catch (InvocationTargetException e) {
             if (e.getCause() instanceof IOException) throw (IOException) e.getCause();
-            throw new JsonDataException(e.getCause().getMessage()); // TODO: more context?
+            throw new JsonDataException(e.getCause()); // TODO: more context?
           }
         }
       }
@@ -71,7 +72,7 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
             throw new AssertionError();
           } catch (InvocationTargetException e) {
             if (e.getCause() instanceof IOException) throw (IOException) e.getCause();
-            throw new JsonDataException(e.getCause().getMessage()); // TODO: more context?
+            throw new JsonDataException(e.getCause()); // TODO: more context?
           }
         }
       }
@@ -79,29 +80,31 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
   }
 
   public static AdapterMethodsFactory get(Object adapter) {
-    Map<Type, ToAdapter> toAdapters = new LinkedHashMap<>();
-    Map<Type, FromAdapter> fromAdapters = new LinkedHashMap<>();
+    List<AdapterMethod> toAdapters = new ArrayList<>();
+    List<AdapterMethod> fromAdapters = new ArrayList<>();
 
     for (Class<?> c = adapter.getClass(); c != Object.class; c = c.getSuperclass()) {
       for (Method m : c.getDeclaredMethods()) {
         if (m.isAnnotationPresent(ToJson.class)) {
-          ToAdapter toAdapter = toAdapter(adapter, m);
-          ToAdapter replaced = toAdapters.put(toAdapter.type, toAdapter);
-          if (replaced != null) {
+          AdapterMethod toAdapter = toAdapter(adapter, m);
+          AdapterMethod conflicting = get(toAdapters, toAdapter.type, toAdapter.annotations);
+          if (conflicting != null) {
             throw new IllegalArgumentException("Conflicting @ToJson methods:\n"
-                + "    " + replaced.method + "\n"
+                + "    " + conflicting.method + "\n"
                 + "    " + toAdapter.method);
           }
+          toAdapters.add(toAdapter);
         }
 
         if (m.isAnnotationPresent(FromJson.class)) {
-          FromAdapter fromAdapter = fromAdapter(adapter, m);
-          FromAdapter replaced = fromAdapters.put(fromAdapter.type, fromAdapter);
-          if (replaced != null) {
+          AdapterMethod fromAdapter = fromAdapter(adapter, m);
+          AdapterMethod conflicting = get(fromAdapters, fromAdapter.type, fromAdapter.annotations);
+          if (conflicting != null) {
             throw new IllegalArgumentException("Conflicting @FromJson methods:\n"
-                + "    " + replaced.method + "\n"
+                + "    " + conflicting.method + "\n"
                 + "    " + fromAdapter.method);
           }
+          fromAdapters.add(fromAdapter);
         }
       }
     }
@@ -118,7 +121,7 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
    * Returns an object that calls a {@code method} method on {@code adapter} in service of
    * converting an object to JSON.
    */
-  static ToAdapter toAdapter(Object adapter, Method method) {
+  static AdapterMethod toAdapter(Object adapter, Method method) {
     method.setAccessible(true);
     Type[] parameterTypes = method.getGenericParameterTypes();
     final Type returnType = method.getGenericReturnType();
@@ -126,7 +129,10 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
     if (parameterTypes.length == 2
         && parameterTypes[0] == JsonWriter.class
         && returnType == void.class) {
-      return new ToAdapter(parameterTypes[1], adapter, method) {
+      // public void pointToJson(JsonWriter jsonWriter, Point point) throws Exception {
+      Set<? extends Annotation> parameterAnnotations
+          = Util.jsonAnnotations(method.getParameterAnnotations()[1]);
+      return new AdapterMethod(parameterTypes[1], parameterAnnotations, adapter, method) {
         @Override public void toJson(Moshi moshi, JsonWriter writer, Object value)
             throws IOException, InvocationTargetException, IllegalAccessException {
           method.invoke(adapter, writer, value);
@@ -134,10 +140,14 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
       };
 
     } else if (parameterTypes.length == 1 && returnType != void.class) {
-      return new ToAdapter(parameterTypes[0], adapter, method) {
+      // public List<Integer> pointToJson(Point point) throws Exception {
+      final Set<? extends Annotation> returnTypeAnnotations = Util.jsonAnnotations(method);
+      Set<? extends Annotation> parameterAnnotations =
+          Util.jsonAnnotations(method.getParameterAnnotations()[0]);
+      return new AdapterMethod(parameterTypes[0], parameterAnnotations, adapter, method) {
         @Override public void toJson(Moshi moshi, JsonWriter writer, Object value)
             throws IOException, InvocationTargetException, IllegalAccessException {
-          JsonAdapter<Object> delegate = moshi.adapter(returnType, method);
+          JsonAdapter<Object> delegate = moshi.adapter(returnType, returnTypeAnnotations);
           Object intermediate = method.invoke(adapter, value);
           delegate.toJson(writer, intermediate);
         }
@@ -151,26 +161,11 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
     }
   }
 
-  static abstract class ToAdapter {
-    final Type type;
-    final Object adapter;
-    final Method method;
-
-    public ToAdapter(Type type, Object adapter, Method method) {
-      this.type = type;
-      this.adapter = adapter;
-      this.method = method;
-    }
-
-    public abstract void toJson(Moshi moshi, JsonWriter writer, Object value)
-        throws IOException, IllegalAccessException, InvocationTargetException;
-  }
-
   /**
    * Returns an object that calls a {@code method} method on {@code adapter} in service of
    * converting an object from JSON.
    */
-  static FromAdapter fromAdapter(Object adapter, Method method) {
+  static AdapterMethod fromAdapter(Object adapter, Method method) {
     method.setAccessible(true);
     final Type[] parameterTypes = method.getGenericParameterTypes();
     final Type returnType = method.getGenericReturnType();
@@ -179,7 +174,8 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
         && parameterTypes[0] == JsonReader.class
         && returnType != void.class) {
       // public Point pointFromJson(JsonReader jsonReader) throws Exception {
-      return new FromAdapter(returnType, adapter, method) {
+      Set<? extends Annotation> returnTypeAnnotations = Util.jsonAnnotations(method);
+      return new AdapterMethod(returnType, returnTypeAnnotations, adapter, method) {
         @Override public Object fromJson(Moshi moshi, JsonReader reader)
             throws IOException, IllegalAccessException, InvocationTargetException {
           return method.invoke(adapter, reader);
@@ -188,10 +184,13 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
 
     } else if (parameterTypes.length == 1 && returnType != void.class) {
       // public Point pointFromJson(List<Integer> o) throws Exception {
-      return new FromAdapter(returnType, adapter, method) {
+      Set<? extends Annotation> returnTypeAnnotations = Util.jsonAnnotations(method);
+      final Set<? extends Annotation> parameterAnnotations
+          = Util.jsonAnnotations(method.getParameterAnnotations()[0]);
+      return new AdapterMethod(returnType, returnTypeAnnotations, adapter, method) {
         @Override public Object fromJson(Moshi moshi, JsonReader reader)
             throws IOException, IllegalAccessException, InvocationTargetException {
-          JsonAdapter<Object> delegate = moshi.adapter(parameterTypes[0]);
+          JsonAdapter<Object> delegate = moshi.adapter(parameterTypes[0], parameterAnnotations);
           Object intermediate = delegate.fromJson(reader);
           return method.invoke(adapter, intermediate);
         }
@@ -205,18 +204,40 @@ final class AdapterMethodsFactory implements JsonAdapter.Factory {
     }
   }
 
-  static abstract class FromAdapter {
+  /** Returns the matching adapter method from the list. */
+  private static AdapterMethod get(
+      List<AdapterMethod> adapterMethods, Type type, Set<? extends Annotation> annotations) {
+    for (int i = 0, size = adapterMethods.size(); i < size; i++) {
+      AdapterMethod adapterMethod = adapterMethods.get(i);
+      if (adapterMethod.type.equals(type) && adapterMethod.annotations.equals(annotations)) {
+        return adapterMethod;
+      }
+    }
+    return null;
+  }
+
+  static abstract class AdapterMethod {
     final Type type;
+    final Set<? extends Annotation> annotations;
     final Object adapter;
     final Method method;
 
-    public FromAdapter(Type type, Object adapter, Method method) {
+    public AdapterMethod(Type type,
+        Set<? extends Annotation> annotations, Object adapter, Method method) {
       this.type = type;
+      this.annotations = annotations;
       this.adapter = adapter;
       this.method = method;
     }
 
-    public abstract Object fromJson(Moshi moshi, JsonReader reader)
-        throws IOException, IllegalAccessException, InvocationTargetException;
+    public void toJson(Moshi moshi, JsonWriter writer, Object value)
+        throws IOException, IllegalAccessException, InvocationTargetException {
+      throw new AssertionError();
+    }
+
+    public Object fromJson(Moshi moshi, JsonReader reader)
+        throws IOException, IllegalAccessException, InvocationTargetException {
+      throw new AssertionError();
+    }
   }
 }

moshi/src/main/java/com/squareup/moshi/ArrayJsonAdapter.java

@@ -16,11 +16,12 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
-import java.lang.reflect.AnnotatedElement;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Array;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Set;
 
 /**
  * Converts arrays to JSON arrays containing their converted contents. This
@@ -28,9 +29,11 @@ import java.util.List;
  */
 final class ArrayJsonAdapter extends JsonAdapter<Object> {
   public static final Factory FACTORY = new Factory() {
-    @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi) {
+    @Override public JsonAdapter<?> create(
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
       Type elementType = Types.arrayComponentType(type);
       if (elementType == null) return null;
+      if (!annotations.isEmpty()) return null;
       Class<?> elementClass = Types.getRawType(elementType);
       JsonAdapter<Object> elementAdapter = moshi.adapter(elementType);
       return new ArrayJsonAdapter(elementClass, elementAdapter).nullSafe();

moshi/src/main/java/com/squareup/moshi/ClassJsonAdapter.java

@@ -16,12 +16,13 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
-import java.lang.reflect.AnnotatedElement;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Field;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Modifier;
 import java.lang.reflect.Type;
 import java.util.Map;
+import java.util.Set;
 import java.util.TreeMap;
 
 /**
@@ -31,9 +32,11 @@ import java.util.TreeMap;
  */
 final class ClassJsonAdapter<T> extends JsonAdapter<T> {
   public static final JsonAdapter.Factory FACTORY = new JsonAdapter.Factory() {
-    @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi) {
+    @Override public JsonAdapter<?> create(
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
       Class<?> rawType = Types.getRawType(type);
       if (rawType.isInterface() || rawType.isEnum() || isPlatformType(rawType)) return null;
+      if (!annotations.isEmpty()) return null;
 
       if (rawType.getEnclosingClass() != null && !Modifier.isStatic(rawType.getModifiers())) {
         if (rawType.getSimpleName().isEmpty()) {
@@ -66,7 +69,8 @@ final class ClassJsonAdapter<T> extends JsonAdapter<T> {
 
         // Look up a type adapter for this type.
         Type fieldType = Types.resolve(type, rawType, field.getGenericType());
-        JsonAdapter<Object> adapter = moshi.adapter(fieldType, field);
+        Set<? extends Annotation> annotations = Util.jsonAnnotations(field);
+        JsonAdapter<Object> adapter = moshi.adapter(fieldType, annotations);
 
         // Create the binding between field and JSON.
         field.setAccessible(true);

moshi/src/main/java/com/squareup/moshi/CollectionJsonAdapter.java

@@ -16,7 +16,7 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
-import java.lang.reflect.AnnotatedElement;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -27,8 +27,10 @@ import java.util.Set;
 /** Converts collection types to JSON arrays containing their converted contents. */
 abstract class CollectionJsonAdapter<C extends Collection<T>, T> extends JsonAdapter<C> {
   public static final JsonAdapter.Factory FACTORY = new JsonAdapter.Factory() {
-    @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi) {
+    @Override public JsonAdapter<?> create(
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
       Class<?> rawType = Types.getRawType(type);
+      if (!annotations.isEmpty()) return null;
       if (rawType == List.class || rawType == Collection.class) {
         return newArrayListAdapter(type, moshi).nullSafe();
       } else if (rawType == Set.class) {

moshi/src/main/java/com/squareup/moshi/JsonAdapter.java

@@ -16,8 +16,9 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
-import java.lang.reflect.AnnotatedElement;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Type;
+import java.util.Set;
 import okio.Buffer;
 import okio.BufferedSink;
 import okio.BufferedSource;
@@ -107,6 +108,6 @@ public abstract class JsonAdapter<T> {
      * <p>Implementations may use to {@link Moshi#adapter} to compose adapters of other types, or
      * {@link Moshi#nextAdapter} to delegate to the underlying adapter of the same type.
      */
-    JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi);
+    JsonAdapter<?> create(Type type, Set<? extends Annotation> annotations, Moshi moshi);
   }
 }

moshi/src/main/java/com/squareup/moshi/JsonDataException.java

@@ -17,6 +17,9 @@ package com.squareup.moshi;
 
 /** Thrown when a JSON document doesn't match the expected format. */
 public final class JsonDataException extends RuntimeException {
+  public JsonDataException() {
+  }
+
   public JsonDataException(String message) {
     super(message);
   }

moshi/src/main/java/com/squareup/moshi/JsonQualifier.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2015 Square, 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 com.squareup.moshi;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.Retention;
+import java.lang.annotation.Target;
+
+import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+/** Annotates another annotation, causing it to specialize how values are encoded and decoded. */
+@Target(ANNOTATION_TYPE)
+@Retention(RUNTIME)
+@Documented
+public @interface JsonQualifier {
+}

moshi/src/main/java/com/squareup/moshi/MapJsonAdapter.java

@@ -16,9 +16,10 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
-import java.lang.reflect.AnnotatedElement;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Type;
 import java.util.Map;
+import java.util.Set;
 
 /**
  * Converts maps with string keys to JSON objects.
@@ -27,7 +28,9 @@ import java.util.Map;
  */
 final class MapJsonAdapter<K, V> extends JsonAdapter<Map<K, V>> {
   public static final Factory FACTORY = new Factory() {
-    @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi) {
+    @Override public JsonAdapter<?> create(
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
+      if (!annotations.isEmpty()) return null;
       Class<?> rawType = Types.getRawType(type);
       if (rawType != Map.class) return null;
       Type[] keyAndValue = Types.mapKeyAndValueTypes(type, rawType);

moshi/src/main/java/com/squareup/moshi/Moshi.java

@@ -17,11 +17,11 @@ package com.squareup.moshi;
 
 import java.io.IOException;
 import java.lang.annotation.Annotation;
-import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Set;
 
 /**
  * Coordinates binding between JSON values and Java objects.
@@ -51,19 +51,22 @@ public final class Moshi {
     return adapter(type, Util.NO_ANNOTATIONS);
   }
 
-  public <T> JsonAdapter<T> adapter(Type type, AnnotatedElement annotations) {
-    // TODO: support re-entrant calls.
+  public <T> JsonAdapter<T> adapter(Type type, Set<? extends Annotation> annotations) {
     return createAdapter(0, type, annotations);
   }
 
   public <T> JsonAdapter<T> nextAdapter(JsonAdapter.Factory skipPast, Type type,
-      AnnotatedElement annotations) {
+      Set<? extends Annotation> annotations) {
     return createAdapter(factories.indexOf(skipPast) + 1, type, annotations);
   }
 
+  public <T> JsonAdapter<T> nextAdapter(JsonAdapter.Factory skipPast, Type type) {
+    return nextAdapter(skipPast, type, Util.NO_ANNOTATIONS);
+  }
+
   @SuppressWarnings("unchecked") // Factories are required to return only matching JsonAdapters.
   private <T> JsonAdapter<T> createAdapter(
-      int firstIndex, Type type, AnnotatedElement annotations) {
+      int firstIndex, Type type, Set<? extends Annotation> annotations) {
     List<DeferredAdapter<?>> deferredAdapters = reentrantCalls.get();
     if (deferredAdapters == null) {
       deferredAdapters = new ArrayList<>();
@@ -91,7 +94,7 @@ public final class Moshi {
       deferredAdapters.remove(deferredAdapters.size() - 1);
     }
 
-    throw new IllegalArgumentException("no JsonAdapter for " + type);
+    throw new IllegalArgumentException("no JsonAdapter for " + type + " annotated " + annotations);
   }
 
   public static final class Builder {
@@ -103,8 +106,8 @@ public final class Moshi {
 
       return add(new JsonAdapter.Factory() {
         @Override public JsonAdapter<?> create(
-            Type targetType, AnnotatedElement annotations, Moshi moshi) {
-          return Util.typesMatch(type, targetType) ? jsonAdapter : null;
+            Type targetType, Set<? extends Annotation> annotations, Moshi moshi) {
+          return annotations.isEmpty() && Util.typesMatch(type, targetType) ? jsonAdapter : null;
         }
       });
     }
@@ -114,13 +117,16 @@ public final class Moshi {
       if (type == null) throw new IllegalArgumentException("type == null");
       if (annotation == null) throw new IllegalArgumentException("annotation == null");
       if (jsonAdapter == null) throw new IllegalArgumentException("jsonAdapter == null");
+      if (!annotation.isAnnotationPresent(JsonQualifier.class)) {
+        throw new IllegalArgumentException(annotation + " does not have @JsonQualifier");
+      }
 
       return add(new JsonAdapter.Factory() {
         @Override public JsonAdapter<?> create(
-            Type targetType, AnnotatedElement annotations, Moshi moshi) {
-          return Util.typesMatch(type, targetType) && annotations.isAnnotationPresent(annotation)
-              ? jsonAdapter
-              : null;
+            Type targetType, Set<? extends Annotation> annotations, Moshi moshi) {
+          if (!Util.typesMatch(type, targetType)) return null;
+          if (!Util.isAnnotationPresent(annotations, annotation)) return null;
+          return jsonAdapter;
         }
       });
     }
@@ -150,10 +156,10 @@ public final class Moshi {
    */
   private static class DeferredAdapter<T> extends JsonAdapter<T> {
     private Type type;
-    private AnnotatedElement annotations;
+    private Set<? extends Annotation> annotations;
     private JsonAdapter<T> delegate;
 
-    public DeferredAdapter(Type type, AnnotatedElement annotations) {
+    public DeferredAdapter(Type type, Set<? extends Annotation> annotations) {
       this.type = type;
       this.annotations = annotations;
     }

moshi/src/main/java/com/squareup/moshi/StandardJsonAdapters.java

@@ -16,13 +16,16 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
-import java.lang.reflect.AnnotatedElement;
+import java.lang.annotation.Annotation;
 import java.lang.reflect.Type;
 import java.util.Arrays;
+import java.util.Set;
 
 final class StandardJsonAdapters {
   public static final JsonAdapter.Factory FACTORY = new JsonAdapter.Factory() {
-    @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi) {
+    @Override public JsonAdapter<?> create(
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
+      if (!annotations.isEmpty()) return null;
       if (type == boolean.class) return BOOLEAN_JSON_ADAPTER;
       if (type == byte.class) return BYTE_JSON_ADAPTER;
       if (type == char.class) return CHARACTER_JSON_ADAPTER;

moshi/src/main/java/com/squareup/moshi/Util.java

@@ -18,27 +18,39 @@ package com.squareup.moshi;
 import java.lang.annotation.Annotation;
 import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Type;
+import java.util.Collections;
+import java.util.LinkedHashSet;
+import java.util.Set;
 
 final class Util {
-  public static final Annotation[] EMPTY_ANNOTATIONS_ARRAY = new Annotation[0];
-
-  public static final AnnotatedElement NO_ANNOTATIONS = new AnnotatedElement() {
-    @Override public boolean isAnnotationPresent(Class<? extends Annotation> aClass) {
-      return false;
-    }
-    @Override public <T extends Annotation> T getAnnotation(Class<T> tClass) {
-      return null;
-    }
-    @Override public Annotation[] getAnnotations() {
-      return EMPTY_ANNOTATIONS_ARRAY;
-    }
-    @Override public Annotation[] getDeclaredAnnotations() {
-      return EMPTY_ANNOTATIONS_ARRAY;
-    }
-  };
+  public static final Set<Annotation> NO_ANNOTATIONS = Collections.emptySet();
 
   public static boolean typesMatch(Type pattern, Type candidate) {
     // TODO: permit raw types (like Set.class) to match non-raw candidates (like Set<Long>).
     return pattern.equals(candidate);
   }
+
+  public static Set<? extends Annotation> jsonAnnotations(AnnotatedElement annotatedElement) {
+    return jsonAnnotations(annotatedElement.getAnnotations());
+  }
+
+  public static Set<? extends Annotation> jsonAnnotations(Annotation[] annotations) {
+    Set<Annotation> result = null;
+    for (Annotation annotation : annotations) {
+      if (annotation.annotationType().isAnnotationPresent(JsonQualifier.class)) {
+        if (result == null) result = new LinkedHashSet<>();
+        result.add(annotation);
+      }
+    }
+    return result != null ? Collections.unmodifiableSet(result) : Util.NO_ANNOTATIONS;
+  }
+
+  public static boolean isAnnotationPresent(
+      Set<? extends Annotation> annotations, Class<? extends Annotation> annotationClass) {
+    if (annotations.isEmpty()) return false; // Save an iterator in the common case.
+    for (Annotation annotation : annotations) {
+      if (annotation.annotationType() == annotationClass) return true;
+    }
+    return false;
+  }
 }

moshi/src/test/java/com/squareup/moshi/CircularAdaptersTest.java

@@ -16,9 +16,10 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
+import java.lang.annotation.Annotation;
 import java.lang.annotation.Retention;
-import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Type;
+import java.util.Set;
 import org.junit.Test;
 
 import static java.lang.annotation.RetentionPolicy.RUNTIME;
@@ -63,10 +64,12 @@ public final class CircularAdaptersTest {
   }
 
   @Retention(RUNTIME)
+  @JsonQualifier
   public @interface Left {
   }
 
   @Retention(RUNTIME)
+  @JsonQualifier
   public @interface Right {
   }
 
@@ -97,19 +100,20 @@ public final class CircularAdaptersTest {
    * work.
    */
   static class PrefixingNodeFactory implements JsonAdapter.Factory {
-    @Override public JsonAdapter<?> create(Type type, AnnotatedElement annotations, Moshi moshi) {
+    @Override public JsonAdapter<?> create(
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
       if (type != Node.class) return null;
 
       final String prefix;
-      if (annotations.isAnnotationPresent(Left.class)) {
+      if (Util.isAnnotationPresent(annotations, Left.class)) {
         prefix = "L ";
-      } else if (annotations.isAnnotationPresent(Right.class)) {
+      } else if (Util.isAnnotationPresent(annotations, Right.class)) {
         prefix = "R ";
       } else {
         return null;
       }
 
-      final JsonAdapter<Node> delegate = moshi.nextAdapter(this, Node.class, annotations);
+      final JsonAdapter<Node> delegate = moshi.nextAdapter(this, Node.class);
 
       return new JsonAdapter<Node>() {
         @Override public void toJson(JsonWriter writer, Node value) throws IOException {

moshi/src/test/java/com/squareup/moshi/ClassJsonAdapterTest.java

@@ -391,7 +391,7 @@ public final class ClassJsonAdapterTest {
 
   private <T> String toJson(Class<T> type, T value) throws IOException {
     @SuppressWarnings("unchecked") // Factory.create returns an adapter that matches its argument.
-    JsonAdapter<T> jsonAdapter = (JsonAdapter<T>) ClassJsonAdapter.FACTORY.create(
+        JsonAdapter<T> jsonAdapter = (JsonAdapter<T>) ClassJsonAdapter.FACTORY.create(
         type, NO_ANNOTATIONS, moshi);
 
     // Wrap in an array to avoid top-level object warnings without going completely lenient.
@@ -409,7 +409,7 @@ public final class ClassJsonAdapterTest {
 
   private <T> T fromJson(Class<T> type, String json) throws IOException {
     @SuppressWarnings("unchecked") // Factory.create returns an adapter that matches its argument.
-    JsonAdapter<T> jsonAdapter = (JsonAdapter<T>) ClassJsonAdapter.FACTORY.create(
+        JsonAdapter<T> jsonAdapter = (JsonAdapter<T>) ClassJsonAdapter.FACTORY.create(
         type, NO_ANNOTATIONS, moshi);
     // Wrap in an array to avoid top-level object warnings without going completely lenient.
     JsonReader jsonReader = newReader("[" + json + "]");

moshi/src/test/java/com/squareup/moshi/JsonQualifiersTest.java

@@ -0,0 +1,367 @@
+/*
+ * Copyright (C) 2015 Square, 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 com.squareup.moshi;
+
+import java.io.IOException;
+import java.lang.annotation.Retention;
+import java.util.Date;
+import org.junit.Test;
+
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.fail;
+
+public final class JsonQualifiersTest {
+  @Test public void builtInTypes() throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(new BuiltInTypesJsonAdapter())
+        .build();
+    JsonAdapter<StringAndFooString> adapter = moshi.adapter(StringAndFooString.class);
+
+    StringAndFooString v1 = new StringAndFooString();
+    v1.a = "aa";
+    v1.b = "bar";
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":\"aa\",\"b\":\"foobar\"}");
+
+    StringAndFooString v2 = adapter.fromJson("{\"a\":\"aa\",\"b\":\"foobar\"}");
+    assertThat(v2.a).isEqualTo("aa");
+    assertThat(v2.b).isEqualTo("bar");
+  }
+
+  static class BuiltInTypesJsonAdapter {
+    @ToJson String fooPrefixStringToString(@FooPrefix String s) {
+      return "foo" + s;
+    }
+
+    @FromJson @FooPrefix String fooPrefixStringFromString(String s) throws Exception {
+      if (!s.startsWith("foo")) throw new JsonDataException();
+      return s.substring(3);
+    }
+  }
+
+  @Test public void readerWriterJsonAdapter() throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(new ReaderWriterJsonAdapter())
+        .build();
+    JsonAdapter<StringAndFooString> adapter = moshi.adapter(StringAndFooString.class);
+
+    StringAndFooString v1 = new StringAndFooString();
+    v1.a = "aa";
+    v1.b = "bar";
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":\"aa\",\"b\":\"foobar\"}");
+
+    StringAndFooString v2 = adapter.fromJson("{\"a\":\"aa\",\"b\":\"foobar\"}");
+    assertThat(v2.a).isEqualTo("aa");
+    assertThat(v2.b).isEqualTo("bar");
+  }
+
+  static class ReaderWriterJsonAdapter {
+    @ToJson void fooPrefixStringToString(JsonWriter jsonWriter, @FooPrefix String s)
+        throws IOException {
+      jsonWriter.value("foo" + s);
+    }
+
+    @FromJson @FooPrefix String fooPrefixStringFromString(JsonReader reader) throws Exception {
+      String s = reader.nextString();
+      if (!s.startsWith("foo")) throw new JsonDataException();
+      return s.substring(3);
+    }
+  }
+
+  /** Fields with this annotation get "foo" as a prefix in the JSON. */
+  @Retention(RUNTIME)
+  @JsonQualifier
+  public @interface FooPrefix {
+  }
+
+  /** Fields with this annotation get "baz" as a suffix in the JSON. */
+  @Retention(RUNTIME)
+  @JsonQualifier
+  public @interface BazSuffix {
+  }
+
+  static class StringAndFooString {
+    String a;
+    @FooPrefix String b;
+  }
+
+  static class StringAndFooBazString {
+    String a;
+    @FooPrefix @BazSuffix String b;
+  }
+
+  @Test public void builtInTypesWithMultipleAnnotations() throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(new BuiltInTypesWithMultipleAnnotationsJsonAdapter())
+        .build();
+    JsonAdapter<StringAndFooBazString> adapter = moshi.adapter(StringAndFooBazString.class);
+
+    StringAndFooBazString v1 = new StringAndFooBazString();
+    v1.a = "aa";
+    v1.b = "bar";
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+
+    StringAndFooBazString v2 = adapter.fromJson("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+    assertThat(v2.a).isEqualTo("aa");
+    assertThat(v2.b).isEqualTo("bar");
+  }
+
+  static class BuiltInTypesWithMultipleAnnotationsJsonAdapter {
+    @ToJson String fooPrefixAndBazSuffixStringToString(@FooPrefix @BazSuffix String s) {
+      return "foo" + s + "baz";
+    }
+
+    @FromJson @FooPrefix @BazSuffix String fooPrefixAndBazSuffixStringFromString(
+        String s) throws Exception {
+      if (!s.startsWith("foo")) throw new JsonDataException();
+      if (!s.endsWith("baz")) throw new JsonDataException();
+      return s.substring(3, s.length() - 3);
+    }
+  }
+
+  @Test public void readerWriterWithMultipleAnnotations() throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(new ReaderWriterWithMultipleAnnotationsJsonAdapter())
+        .build();
+    JsonAdapter<StringAndFooBazString> adapter = moshi.adapter(StringAndFooBazString.class);
+
+    StringAndFooBazString v1 = new StringAndFooBazString();
+    v1.a = "aa";
+    v1.b = "bar";
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+
+    StringAndFooBazString v2 = adapter.fromJson("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+    assertThat(v2.a).isEqualTo("aa");
+    assertThat(v2.b).isEqualTo("bar");
+  }
+
+  static class ReaderWriterWithMultipleAnnotationsJsonAdapter {
+    @ToJson void fooPrefixAndBazSuffixStringToString(
+        JsonWriter jsonWriter, @FooPrefix @BazSuffix String s) throws IOException {
+      jsonWriter.value("foo" + s + "baz");
+    }
+
+    @FromJson @FooPrefix @BazSuffix String fooPrefixAndBazSuffixStringFromString(
+        JsonReader reader) throws Exception {
+      String s = reader.nextString();
+      if (!s.startsWith("foo")) throw new JsonDataException();
+      if (!s.endsWith("baz")) throw new JsonDataException();
+      return s.substring(3, s.length() - 3);
+    }
+  }
+
+  @Test public void basicTypesAnnotationDelegating() throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(new BuiltInTypesDelegatingJsonAdapter())
+        .add(new BuiltInTypesJsonAdapter())
+        .build();
+    JsonAdapter<StringAndFooBazString> adapter = moshi.adapter(StringAndFooBazString.class);
+
+    StringAndFooBazString v1 = new StringAndFooBazString();
+    v1.a = "aa";
+    v1.b = "bar";
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+
+    StringAndFooBazString v2 = adapter.fromJson("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+    assertThat(v2.a).isEqualTo("aa");
+    assertThat(v2.b).isEqualTo("bar");
+  }
+
+  static class BuiltInTypesDelegatingJsonAdapter {
+    @ToJson @FooPrefix String fooPrefixAndBazSuffixStringToString(@FooPrefix @BazSuffix String s) {
+      return s + "baz";
+    }
+
+    @FromJson @FooPrefix @BazSuffix String fooPrefixAndBazSuffixStringFromString(
+        @FooPrefix String s) throws Exception {
+      if (!s.endsWith("baz")) throw new JsonDataException();
+      return s.substring(0, s.length() - 3);
+    }
+  }
+
+  @Test public void readerWriterAnnotationDelegating() throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(new BuiltInTypesDelegatingJsonAdapter())
+        .add(new ReaderWriterJsonAdapter())
+        .build();
+    JsonAdapter<StringAndFooBazString> adapter = moshi.adapter(StringAndFooBazString.class);
+
+    StringAndFooBazString v1 = new StringAndFooBazString();
+    v1.a = "aa";
+    v1.b = "bar";
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+
+    StringAndFooBazString v2 = adapter.fromJson("{\"a\":\"aa\",\"b\":\"foobarbaz\"}");
+    assertThat(v2.a).isEqualTo("aa");
+    assertThat(v2.b).isEqualTo("bar");
+  }
+
+  @Test public void manualJsonAdapter() throws Exception {
+    JsonAdapter<String> fooPrefixAdapter = new JsonAdapter<String>() {
+      @Override public String fromJson(JsonReader reader) throws IOException {
+        String s = reader.nextString();
+        if (!s.startsWith("foo")) throw new JsonDataException();
+        return s.substring(3);
+      }
+
+      @Override public void toJson(JsonWriter writer, String value) throws IOException {
+        writer.value("foo" + value);
+      }
+    };
+
+    Moshi moshi = new Moshi.Builder()
+        .add(String.class, FooPrefix.class, fooPrefixAdapter)
+        .build();
+    JsonAdapter<StringAndFooString> adapter = moshi.adapter(StringAndFooString.class);
+
+    StringAndFooString v1 = new StringAndFooString();
+    v1.a = "aa";
+    v1.b = "bar";
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":\"aa\",\"b\":\"foobar\"}");
+
+    StringAndFooString v2 = adapter.fromJson("{\"a\":\"aa\",\"b\":\"foobar\"}");
+    assertThat(v2.a).isEqualTo("aa");
+    assertThat(v2.b).isEqualTo("bar");
+  }
+
+  @Test public void noJsonAdapterForAnnotatedType() throws Exception {
+    Moshi moshi = new Moshi.Builder().build();
+    try {
+      moshi.adapter(StringAndFooString.class);
+      fail();
+    } catch (IllegalArgumentException expected) {
+    }
+  }
+
+  @Test public void annotationWithoutJsonQualifierIsIgnoredByAdapterMethods() throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(new MissingJsonQualifierJsonAdapter())
+        .build();
+    JsonAdapter<DateAndMillisDate> adapter = moshi.adapter(DateAndMillisDate.class);
+
+    DateAndMillisDate v1 = new DateAndMillisDate();
+    v1.a = new Date(5);
+    v1.b = new Date(7);
+    assertThat(adapter.toJson(v1)).isEqualTo("{\"a\":5,\"b\":7}");
+
+    DateAndMillisDate v2 = adapter.fromJson("{\"a\":5,\"b\":7}");
+    assertThat(v2.a).isEqualTo(new Date(5));
+    assertThat(v2.b).isEqualTo(new Date(7));
+  }
+
+  /** Despite the fact that these methods are annotated, they match all dates. */
+  static class MissingJsonQualifierJsonAdapter {
+    @ToJson long dateToJson(@Millis Date d) {
+      return d.getTime();
+    }
+
+    @FromJson @Millis Date jsonToDate(long value) throws Exception {
+      return new Date(value);
+    }
+  }
+
+  /** This annotation does nothing. */
+  @Retention(RUNTIME)
+  public @interface Millis {
+  }
+
+  static class DateAndMillisDate {
+    Date a;
+    @Millis Date b;
+  }
+
+  @Test public void annotationWithoutJsonQualifierIsRejectedOnRegistration() throws Exception {
+    JsonAdapter<Date> jsonAdapter = new JsonAdapter<Date>() {
+      @Override public Date fromJson(JsonReader reader) throws IOException {
+        throw new AssertionError();
+      }
+
+      @Override public void toJson(JsonWriter writer, Date value) throws IOException {
+        throw new AssertionError();
+      }
+    };
+
+    try {
+      new Moshi.Builder().add(Date.class, Millis.class, jsonAdapter);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("interface com.squareup.moshi.JsonQualifiersTest$Millis "
+          + "does not have @JsonQualifier");
+    }
+  }
+
+  @Test public void annotationsConflict() throws Exception {
+    try {
+      new Moshi.Builder().add(new AnnotationsConflictJsonAdapter());
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessageContaining("Conflicting @ToJson methods");
+    }
+  }
+
+  static class AnnotationsConflictJsonAdapter {
+    @ToJson String fooPrefixStringToString(@FooPrefix String s) {
+      return "foo" + s;
+    }
+
+    @ToJson String fooPrefixStringToString2(@FooPrefix String s) {
+      return "foo" + s;
+    }
+  }
+
+  @Test public void toButNoFromJson() throws Exception {
+    // Building it is okay.
+    Moshi moshi = new Moshi.Builder()
+        .add(new ToButNoFromJsonAdapter())
+        .build();
+
+    try {
+      moshi.adapter(StringAndFooString.class);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("no JsonAdapter for class java.lang.String "
+          + "annotated [@com.squareup.moshi.JsonQualifiersTest$FooPrefix()]");
+    }
+  }
+
+  static class ToButNoFromJsonAdapter {
+    @ToJson String fooPrefixStringToString(@FooPrefix String s) {
+      return "foo" + s;
+    }
+  }
+
+  @Test public void fromButNoToJson() throws Exception {
+    // Building it is okay.
+    Moshi moshi = new Moshi.Builder()
+        .add(new FromButNoToJsonAdapter())
+        .build();
+
+    try {
+      moshi.adapter(StringAndFooString.class);
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("no JsonAdapter for class java.lang.String "
+          + "annotated [@com.squareup.moshi.JsonQualifiersTest$FooPrefix()]");
+    }
+  }
+
+  static class FromButNoToJsonAdapter {
+    @FromJson @FooPrefix String fooPrefixStringFromString(String s) throws Exception {
+      if (!s.startsWith("foo")) throw new JsonDataException();
+      return s.substring(3);
+    }
+  }
+}

moshi/src/test/java/com/squareup/moshi/MoshiTest.java

@@ -16,8 +16,8 @@
 package com.squareup.moshi;
 
 import java.io.IOException;
+import java.lang.annotation.Annotation;
 import java.lang.annotation.Retention;
-import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Field;
 import java.lang.reflect.Type;
 import java.util.ArrayDeque;
@@ -32,6 +32,7 @@ import org.junit.Test;
 import static com.squareup.moshi.TestUtil.newReader;
 import static java.lang.annotation.RetentionPolicy.RUNTIME;
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.fail;
 
 public final class MoshiTest {
@@ -591,7 +592,8 @@ public final class MoshiTest {
         .add(new UppercaseAdapterFactory())
         .build();
 
-    AnnotatedElement annotations = MoshiTest.class.getDeclaredField("uppercaseString");
+    Field uppercaseString = MoshiTest.class.getDeclaredField("uppercaseString");
+    Set<? extends Annotation> annotations = Util.jsonAnnotations(uppercaseString);
     JsonAdapter<String> adapter = moshi.<String>adapter(String.class, annotations).lenient();
     assertThat(adapter.toJson("a")).isEqualTo("\"A\"");
     assertThat(adapter.fromJson("\"b\"")).isEqualTo("B");
@@ -638,10 +640,14 @@ public final class MoshiTest {
         .build();
 
     Field uppercaseStringsField = MoshiTest.class.getDeclaredField("uppercaseStrings");
-    JsonAdapter<List<String>> adapter = moshi.adapter(uppercaseStringsField.getGenericType(),
-        uppercaseStringsField);
-    assertThat(adapter.toJson(Arrays.asList("a"))).isEqualTo("[\"a\"]");
-    assertThat(adapter.fromJson("[\"b\"]")).isEqualTo(Arrays.asList("b"));
+    try {
+      moshi.adapter(uppercaseStringsField.getGenericType(),
+          Util.jsonAnnotations(uppercaseStringsField));
+      fail();
+    } catch (IllegalArgumentException expected) {
+      assertEquals("no JsonAdapter for java.util.List<java.lang.String> annotated "
+          + "[@com.squareup.moshi.MoshiTest$Uppercase()]", expected.getMessage());
+    }
   }
 
   @Test public void objectArray() throws Exception {
@@ -753,9 +759,8 @@ public final class MoshiTest {
 
   static class MealDealAdapterFactory implements JsonAdapter.Factory {
     @Override public JsonAdapter<?> create(
-        Type type, AnnotatedElement annotations, Moshi moshi) {
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
       if (!type.equals(MealDeal.class)) return null;
-
       final JsonAdapter<Pizza> pizzaAdapter = moshi.adapter(Pizza.class);
       final JsonAdapter<String> drinkAdapter = moshi.adapter(String.class);
       return new JsonAdapter<MealDeal>() {
@@ -778,16 +783,17 @@ public final class MoshiTest {
   }
 
   @Retention(RUNTIME)
+  @JsonQualifier
   public @interface Uppercase {
   }
 
   static class UppercaseAdapterFactory implements JsonAdapter.Factory {
     @Override public JsonAdapter<?> create(
-        Type type, AnnotatedElement annotations, Moshi moshi) {
+        Type type, Set<? extends Annotation> annotations, Moshi moshi) {
       if (!type.equals(String.class)) return null;
-      if (!annotations.isAnnotationPresent(Uppercase.class)) return null;
+      if (!Util.isAnnotationPresent(annotations, Uppercase.class)) return null;
 
-      final JsonAdapter<String> stringAdapter = moshi.nextAdapter(this, String.class, annotations);
+      final JsonAdapter<String> stringAdapter = moshi.nextAdapter(this, String.class);
       return new JsonAdapter<String>() {
         @Override public String fromJson(JsonReader reader) throws IOException {
           String s = stringAdapter.fromJson(reader);