Skip a UTF-8 BOM if present.

retrofit-converters/moshi/pom.xml

@@ -23,6 +23,12 @@
       <groupId>com.squareup.moshi</groupId>
       <artifactId>moshi</artifactId>
     </dependency>
+    <!-- TODO remove this dependency once Moshi or OkHttp ships with 1.9.0 or newer. -->
+    <dependency>
+      <groupId>com.squareup.okio</groupId>
+      <artifactId>okio</artifactId>
+      <version>1.9.0</version>
+    </dependency>
 
     <dependency>
       <groupId>junit</groupId>

retrofit-converters/moshi/src/main/java/retrofit2/converter/moshi/MoshiResponseBodyConverter.java

@@ -18,9 +18,13 @@ package retrofit2.converter.moshi;
 import com.squareup.moshi.JsonAdapter;
 import java.io.IOException;
 import okhttp3.ResponseBody;
+import okio.BufferedSource;
+import okio.ByteString;
 import retrofit2.Converter;
 
 final class MoshiResponseBodyConverter<T> implements Converter<ResponseBody, T> {
+  private static final ByteString UTF8_BOM = ByteString.decodeHex("EFBBBF");
+
   private final JsonAdapter<T> adapter;
 
   MoshiResponseBodyConverter(JsonAdapter<T> adapter) {
@@ -28,8 +32,14 @@ final class MoshiResponseBodyConverter<T> implements Converter<ResponseBody, T>
   }
 
   @Override public T convert(ResponseBody value) throws IOException {
+    BufferedSource source = value.source();
     try {
-      return adapter.fromJson(value.source());
+      // Moshi has no document-level API so the responsibility of BOM skipping falls to whatever
+      // is delegating to it. Since it's a UTF-8-only library as well we only honor the UTF-8 BOM.
+      if (source.rangeEquals(0, UTF8_BOM)) {
+        source.skip(UTF8_BOM.size());
+      }
+      return adapter.fromJson(source);
     } finally {
       value.close();
     }

retrofit-converters/moshi/src/test/java/retrofit2/converter/moshi/MoshiConverterFactoryTest.java

@@ -26,10 +26,13 @@ import java.io.IOException;
 import java.lang.annotation.Annotation;
 import java.lang.annotation.Retention;
 import java.lang.reflect.Type;
+import java.nio.charset.Charset;
 import java.util.Set;
 import okhttp3.mockwebserver.MockResponse;
 import okhttp3.mockwebserver.MockWebServer;
 import okhttp3.mockwebserver.RecordedRequest;
+import okio.Buffer;
+import okio.ByteString;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -203,4 +206,32 @@ public final class MoshiConverterFactoryTest {
     AnImplementation body = response.body();
     assertThat(body.theName).isEqualTo("value");
   }
+
+  @Test public void utf8BomSkipped() throws IOException {
+    Buffer responseBody = new Buffer()
+        .write(ByteString.decodeHex("EFBBBF"))
+        .writeUtf8("{\"theName\":\"value\"}");
+    MockResponse malformedResponse = new MockResponse().setBody(responseBody);
+    server.enqueue(malformedResponse);
+
+    Call<AnImplementation> call = service.anImplementation(new AnImplementation("value"));
+    Response<AnImplementation> response = call.execute();
+    AnImplementation body = response.body();
+    assertThat(body.theName).isEqualTo("value");
+  }
+
+  @Test public void nonUtf8BomIsNotSkipped() throws IOException {
+    Buffer responseBody = new Buffer()
+        .write(ByteString.decodeHex("FEFF"))
+        .writeString("{\"theName\":\"value\"}", Charset.forName("UTF-16"));
+    MockResponse malformedResponse = new MockResponse().setBody(responseBody);
+    server.enqueue(malformedResponse);
+
+    Call<AnImplementation> call = service.anImplementation(new AnImplementation("value"));
+    try {
+      call.execute();
+      fail();
+    } catch (IOException expected) {
+    }
+  }
 }