Don't decode into memory in RuntimeJsonAdapterFactory

Instead use our new flatten API to decode directly to the stream.

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

@@ -26,9 +26,7 @@ import java.lang.annotation.Annotation;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.LinkedHashMap;
 import java.util.List;
-import java.util.Map;
 import java.util.Set;
 import javax.annotation.CheckReturnValue;
 
@@ -182,12 +180,12 @@ final class RuntimeJsonAdapterFactory<T> implements JsonAdapter.Factory {
             + ". Register this subtype.");
       }
       JsonAdapter<Object> adapter = jsonAdapters.get(labelIndex);
-      Map<String, Object> jsonValue = (Map<String, Object>) adapter.toJsonValue(value);
-
-      Map<String, Object> valueWithLabel = new LinkedHashMap<>(1 + jsonValue.size());
-      valueWithLabel.put(labelKey, labels.get(labelIndex));
-      valueWithLabel.putAll(jsonValue);
-      objectJsonAdapter.toJson(writer, valueWithLabel);
+      writer.beginObject();
+      writer.name(labelKey).value(labels.get(labelIndex));
+      int flattenToken = writer.beginFlatten();
+      adapter.toJson(writer, value);
+      writer.endFlatten(flattenToken);
+      writer.endObject();
     }
 
     @Override public String toString() {

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

@@ -180,6 +180,24 @@ public final class RuntimeJsonAdapterFactoryTest {
     }
   }
 
+  /**
+   * Longs that do not have an exact double representation are problematic for JSON. It is a bad
+   * idea to use JSON for these values! But Moshi tries to retain long precision where possible.
+   */
+  @Test public void unportableTypes() throws IOException {
+    Moshi moshi = new Moshi.Builder()
+        .add(RuntimeJsonAdapterFactory.of(Message.class, "type")
+            .withSubtype(MessageWithUnportableTypes.class, "unportable"))
+        .build();
+    JsonAdapter<Message> adapter = moshi.adapter(Message.class);
+
+    assertThat(adapter.toJson(new MessageWithUnportableTypes(9007199254740993L)))
+        .isEqualTo("{\"type\":\"unportable\",\"long_value\":9007199254740993}");
+    MessageWithUnportableTypes decoded = (MessageWithUnportableTypes) adapter.fromJson(
+        "{\"type\":\"unportable\",\"long_value\":9007199254740993}");
+    assertThat(decoded.long_value).isEqualTo(9007199254740993L);
+  }
+
   interface Message {
   }
 
@@ -226,4 +244,12 @@ public final class RuntimeJsonAdapterFactoryTest {
       return "EmptyMessage";
     }
   }
+
+  static final class MessageWithUnportableTypes implements Message {
+    final long long_value;
+
+    MessageWithUnportableTypes(long long_value) {
+      this.long_value = long_value;
+    }
+  }
 }