Extend PolymorphicJsonAdapterFactory to allow custom handling of unknown labels (#1094)

* PolymorphicJsonAdapter uses JsonAdapter as fallback value instead of defaultValue

* PolymorphicJsonAdapterFactory uses fallbackJsonAdapter instead of (defaultValue, defaultValueSet)

* Add testing coverage

adapters/src/main/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactory.java

@@ -95,38 +95,52 @@ import javax.annotation.Nullable;
  *   <li>Each type identifier must be unique.
  * </ul>
  *
- * <p>For best performance type information should be the first field in the object. Otherwise Moshi
- * must reprocess the JSON stream once it knows the object's type.
+ * <p>For best performance type information should be the first field in the object. Otherwise
+ * Moshi must reprocess the JSON stream once it knows the object's type.
  *
- * <p>If an unknown subtype is encountered when decoding, this will throw a {@link
- * JsonDataException}. If an unknown type is encountered when encoding, this will throw an {@link
- * IllegalArgumentException}. If the same subtype has multiple labels the first one is used when
- * encoding.
+ * <p>If an unknown subtype is encountered when decoding:
+ *  <ul>
+ *    <li> if {@link #withDefaultValue(Object)} is used, then {@code defaultValue} will be returned
+ *    <li> if {@link #withFallbackJsonAdapter(JsonAdapter)} is used, then the
+ *        {@code fallbackJsonAdapter.fromJson(reader)} result will be returned
+ *    <li> otherwise a {@link JsonDataException} will be thrown
+ *  </ul>
  *
- * <p>If you want to specify a custom unknown fallback for decoding, you can do so via
- * {@link #withDefaultValue(Object)}. This instance should be immutable, as it is shared.
+ * <p>If an unknown type is encountered when encoding:
+ *  <ul>
+ *    <li> if {@link #withFallbackJsonAdapter(JsonAdapter)} is used, then the
+ *        {@code fallbackJsonAdapter.toJson(writer, value)} result will be returned
+ *    <li> otherwise a {@link IllegalArgumentException} will be thrown
+ *  </ul>
+ *
+ * <p>If the same subtype has multiple labels the first one is used when encoding.
  */
 public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Factory {
+  /**
+   * Thin wrapper around {@link JsonAdapter} to allow {@link PolymorphicJsonAdapter} to
+   * distinguish between {@code JsonAdapter} added due to a {@code defaultValue} or added
+   * by users of {@link PolymorphicJsonAdapterFactory}.
+   */
+  private abstract static class DefaultJsonAdapter<T> extends JsonAdapter<T> {
+  }
+
   final Class<T> baseType;
   final String labelKey;
   final List<String> labels;
   final List<Type> subtypes;
-  @Nullable final T defaultValue;
-  final boolean defaultValueSet;
+  @Nullable final JsonAdapter<Object> fallbackJsonAdapter;
 
   PolymorphicJsonAdapterFactory(
       Class<T> baseType,
       String labelKey,
       List<String> labels,
       List<Type> subtypes,
-      @Nullable T defaultValue,
-      boolean defaultValueSet) {
+      @Nullable JsonAdapter<Object> fallbackJsonAdapter) {
     this.baseType = baseType;
     this.labelKey = labelKey;
     this.labels = labels;
     this.subtypes = subtypes;
-    this.defaultValue = defaultValue;
-    this.defaultValueSet = defaultValueSet;
+    this.fallbackJsonAdapter = fallbackJsonAdapter;
   }
 
   /**
@@ -143,14 +157,11 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
         labelKey,
         Collections.<String>emptyList(),
         Collections.<Type>emptyList(),
-        null,
-        false);
+        null);
   }
 
   /**
-   * Returns a new factory that decodes instances of {@code subtype}. When an unknown type is found
-   * during encoding an {@linkplain IllegalArgumentException} will be thrown. When an unknown label
-   * is found during decoding a {@linkplain JsonDataException} will be thrown.
+   * Returns a new factory that decodes instances of {@code subtype}.
    */
   public PolymorphicJsonAdapterFactory<T> withSubtype(Class<? extends T> subtype, String label) {
     if (subtype == null) throw new NullPointerException("subtype == null");
@@ -166,21 +177,49 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
         labelKey,
         newLabels,
         newSubtypes,
-        defaultValue,
-        defaultValueSet);
+        fallbackJsonAdapter);
   }
 
   /**
-   * Returns a new factory that with default to {@code defaultValue} upon decoding of unrecognized
-   * labels. The default value should be immutable.
+   * Returns a new factory that with default to {@code fallbackJsonAdapter.fromJson(reader)}
+   * upon decoding of unrecognized labels.
+   *
+   * <p>The {@link JsonReader} instance will not be automatically consumed, so sure to consume it
+   * within your implementation of {@link JsonAdapter#fromJson(JsonReader)}
    */
-  public PolymorphicJsonAdapterFactory<T> withDefaultValue(@Nullable T defaultValue) {
+  public PolymorphicJsonAdapterFactory<T> withFallbackJsonAdapter(
+      @Nullable JsonAdapter<Object> fallbackJsonAdapter) {
     return new PolymorphicJsonAdapterFactory<>(baseType,
         labelKey,
         labels,
         subtypes,
-        defaultValue,
-        true);
+        fallbackJsonAdapter);
+  }
+
+  /**
+   * Returns a new factory that will default to {@code defaultValue} upon decoding of unrecognized
+   * labels. The default value should be immutable.
+   */
+  public PolymorphicJsonAdapterFactory<T> withDefaultValue(@Nullable T defaultValue) {
+    return withFallbackJsonAdapter(buildFallbackJsonAdapter(defaultValue));
+  }
+
+  private JsonAdapter<Object> buildFallbackJsonAdapter(final T defaultValue) {
+    return new DefaultJsonAdapter<Object>() {
+      @Nullable
+      @Override
+      public Object fromJson(JsonReader reader) throws IOException {
+        reader.skipValue();
+        return defaultValue;
+      }
+
+      @Override
+      public void toJson(JsonWriter writer, @Nullable Object value) throws IOException {
+        throw new IOException("This method should never be called. "
+            + "If you find this on your stacktraces please report it "
+            + "to the https://github.com/square/moshi project");
+      }
+    };
   }
 
   @Override
@@ -198,8 +237,7 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
         labels,
         subtypes,
         jsonAdapters,
-        defaultValue,
-        defaultValueSet
+        fallbackJsonAdapter
     ).nullSafe();
   }
 
@@ -208,8 +246,7 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
     final List<String> labels;
     final List<Type> subtypes;
     final List<JsonAdapter<Object>> jsonAdapters;
-    @Nullable final Object defaultValue;
-    final boolean defaultValueSet;
+    @Nullable final JsonAdapter<Object> fallbackJsonAdapter;
 
     /** Single-element options containing the label's key only. */
     final JsonReader.Options labelKeyOptions;
@@ -220,14 +257,12 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
         List<String> labels,
         List<Type> subtypes,
         List<JsonAdapter<Object>> jsonAdapters,
-        @Nullable Object defaultValue,
-        boolean defaultValueSet) {
+        @Nullable JsonAdapter<Object> fallbackJsonAdapter) {
       this.labelKey = labelKey;
       this.labels = labels;
       this.subtypes = subtypes;
       this.jsonAdapters = jsonAdapters;
-      this.defaultValue = defaultValue;
-      this.defaultValueSet = defaultValueSet;
+      this.fallbackJsonAdapter = fallbackJsonAdapter;
 
       this.labelKeyOptions = JsonReader.Options.of(labelKey);
       this.labelOptions = JsonReader.Options.of(labels.toArray(new String[0]));
@@ -243,11 +278,11 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
         peeked.close();
       }
       if (labelIndex == -1) {
-        reader.skipValue();
-        return defaultValue;
-      }
+        return this.fallbackJsonAdapter.fromJson(reader);
+      } else {
         return jsonAdapters.get(labelIndex).fromJson(reader);
       }
+    }
 
     private int labelIndex(JsonReader reader) throws IOException {
       reader.beginObject();
@@ -259,7 +294,7 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
         }
 
         int labelIndex = reader.selectString(labelOptions);
-        if (labelIndex == -1 && !defaultValueSet) {
+        if (labelIndex == -1 && this.fallbackJsonAdapter == null) {
           throw new JsonDataException("Expected one of "
               + labels
               + " for key '"
@@ -277,7 +312,9 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
     @Override public void toJson(JsonWriter writer, Object value) throws IOException {
       Class<?> type = value.getClass();
       int labelIndex = subtypes.indexOf(type);
+      final JsonAdapter<Object> adapter;
       if (labelIndex == -1) {
+        if (fallbackJsonAdapter == null || fallbackJsonAdapter instanceof DefaultJsonAdapter) {
           throw new IllegalArgumentException("Expected one of "
               + subtypes
               + " but found "
@@ -285,10 +322,17 @@ public final class PolymorphicJsonAdapterFactory<T> implements JsonAdapter.Facto
               + ", a "
               + value.getClass()
               + ". Register this subtype.");
+        } else {
+          adapter = fallbackJsonAdapter;
         }
-      JsonAdapter<Object> adapter = jsonAdapters.get(labelIndex);
+      } else {
+        adapter = jsonAdapters.get(labelIndex);
+      }
+
       writer.beginObject();
+      if (adapter != fallbackJsonAdapter) {
         writer.name(labelKey).value(labels.get(labelIndex));
+      }
       int flattenToken = writer.beginFlatten();
       adapter.toJson(writer, value);
       writer.endFlatten(flattenToken);

adapters/src/test/java/com/squareup/moshi/adapters/PolymorphicJsonAdapterFactoryTest.java

@@ -18,6 +18,7 @@ package com.squareup.moshi.adapters;
 import com.squareup.moshi.JsonAdapter;
 import com.squareup.moshi.JsonDataException;
 import com.squareup.moshi.JsonReader;
+import com.squareup.moshi.JsonWriter;
 import com.squareup.moshi.Moshi;
 import java.io.IOException;
 import java.util.Collections;
@@ -25,6 +26,8 @@ import java.util.Map;
 import okio.Buffer;
 import org.junit.Test;
 
+import javax.annotation.Nullable;
+
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.junit.Assert.fail;
 
@@ -105,6 +108,35 @@ public final class PolymorphicJsonAdapterFactoryTest {
     assertThat(message).isNull();
   }
 
+  @Test public void specifiedFallbackJsonAdapter() throws IOException {
+    Moshi moshi = new Moshi.Builder()
+        .add(PolymorphicJsonAdapterFactory.of(Message.class, "type")
+            .withSubtype(Success.class, "success")
+            .withSubtype(Error.class, "error")
+            .withFallbackJsonAdapter(new JsonAdapter<Object>() {
+              @Override
+              public Object fromJson(JsonReader reader) throws IOException {
+                reader.skipValue();
+                return new EmptyMessage();
+              }
+
+              @Override
+              public void toJson(JsonWriter writer, @Nullable Object value) {
+                throw new RuntimeException("Not implemented as not needed for the test");
+              }
+            })
+        )
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    JsonReader reader =
+        JsonReader.of(new Buffer().writeUtf8("{\"type\":\"data\",\"value\":\"Okay!\"}"));
+
+    Message message = adapter.fromJson(reader);
+    assertThat(message).isInstanceOf(EmptyMessage.class);
+    assertThat(reader.peek()).isEqualTo(JsonReader.Token.END_DOCUMENT);
+  }
+
   @Test public void unregisteredSubtype() {
     Moshi moshi = new Moshi.Builder()
         .add(PolymorphicJsonAdapterFactory.of(Message.class, "type")
@@ -125,6 +157,52 @@ public final class PolymorphicJsonAdapterFactoryTest {
     }
   }
 
+  @Test public void unregisteredSubtypeWithDefaultValue() {
+    Error fallbackError = new Error(Collections.<String, Object>emptyMap());
+    Moshi moshi = new Moshi.Builder()
+        .add(PolymorphicJsonAdapterFactory.of(Message.class, "type")
+            .withSubtype(Success.class, "success")
+            .withSubtype(Error.class, "error")
+            .withDefaultValue(fallbackError))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    try {
+      adapter.toJson(new EmptyMessage());
+    } catch (IllegalArgumentException expected) {
+      assertThat(expected).hasMessage("Expected one of [class"
+          + " com.squareup.moshi.adapters.PolymorphicJsonAdapterFactoryTest$Success, class"
+          + " com.squareup.moshi.adapters.PolymorphicJsonAdapterFactoryTest$Error] but found"
+          + " EmptyMessage, a class"
+          + " com.squareup.moshi.adapters.PolymorphicJsonAdapterFactoryTest$EmptyMessage. Register"
+          + " this subtype.");
+    }
+  }
+
+  @Test public void unregisteredSubtypeWithFallbackJsonAdapter() {
+    Moshi moshi = new Moshi.Builder()
+        .add(PolymorphicJsonAdapterFactory.of(Message.class, "type")
+            .withSubtype(Success.class, "success")
+            .withSubtype(Error.class, "error")
+            .withFallbackJsonAdapter(new JsonAdapter<Object>() {
+              @Nullable
+              @Override
+              public Object fromJson(JsonReader reader) {
+                throw new RuntimeException("Not implemented as not needed for the test");
+              }
+
+              @Override
+              public void toJson(JsonWriter writer, @Nullable Object value) throws IOException {
+                writer.name("type").value("injected by fallbackJsonAdapter");
+              }
+            }))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    String json = adapter.toJson(new EmptyMessage());
+    assertThat(json).isEqualTo("{\"type\":\"injected by fallbackJsonAdapter\"}");
+  }
+
   @Test public void nonStringLabelValue() throws IOException {
     Moshi moshi = new Moshi.Builder()
         .add(PolymorphicJsonAdapterFactory.of(Message.class, "type")