Added support for Unwrapped Serialization.

src/main/java/com/hubspot/jackson/datatype/protobuf/builtin/serializers/MessageSerializer.java

@@ -11,6 +11,7 @@
 import com.fasterxml.jackson.core.JsonGenerator;
 import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.util.NameTransformer;
 import com.google.protobuf.Descriptors.Descriptor;
 import com.google.protobuf.Descriptors.FieldDescriptor;
 import com.google.protobuf.Descriptors.FieldDescriptor.JavaType;
@@ -28,6 +29,7 @@
 public class MessageSerializer extends ProtobufSerializer<MessageOrBuilder> {
   @SuppressFBWarnings(value="SE_BAD_FIELD")
   private final ProtobufJacksonConfig config;
+  private final boolean unwrappingSerializer;
   private final Map<Descriptor, PropertyNamingCache> propertyNamingCache;
 
   /**
@@ -39,9 +41,17 @@ public MessageSerializer(ExtensionRegistryWrapper extensionRegistry) {
   }
 
   public MessageSerializer(ProtobufJacksonConfig config) {
-    super(MessageOrBuilder.class);
+    this(config, false);
+  }
+
+  public MessageSerializer(ProtobufJacksonConfig config, boolean unwrappingSerializer) {
+    this(config, null, unwrappingSerializer);
+  }
 
+  public MessageSerializer(ProtobufJacksonConfig config, NameTransformer nameTransformer, boolean unwrappingSerializer) {
+    super(MessageOrBuilder.class);
     this.config = config;
+    this.unwrappingSerializer = unwrappingSerializer;
     this.propertyNamingCache = new ConcurrentHashMap<>();
   }
 
@@ -51,7 +61,9 @@ public void serialize(
           JsonGenerator generator,
           SerializerProvider serializerProvider
   ) throws IOException {
-    generator.writeStartObject();
+    if (!isUnwrappingSerializer()) {
+      generator.writeStartObject();
+    }
 
     boolean proto3 = message.getDescriptorForType().getFile().getSyntax() == Syntax.PROTO3;
     Include include = serializerProvider.getConfig().getDefaultPropertyInclusion().getValueInclusion();
@@ -99,7 +111,19 @@ public void serialize(
       }
     }
 
-    generator.writeEndObject();
+    if (!isUnwrappingSerializer()) {
+      generator.writeEndObject();
+    }
+  }
+
+  @Override
+  public boolean isUnwrappingSerializer() {
+    return unwrappingSerializer;
+  }
+
+  @Override
+  public MessageSerializer unwrappingSerializer(NameTransformer nameTransformer) {
+    return new MessageSerializer(config, nameTransformer, true);
   }
 
   private Function<FieldDescriptor, String> getPropertyNaming(Descriptor descriptor, SerializerProvider serializerProvider) {

src/test/java/com/hubspot/jackson/datatype/protobuf/builtin/UnwrappedSerializationTest.java

@@ -0,0 +1,53 @@
+package com.hubspot.jackson.datatype.protobuf.builtin;
+
+import static com.hubspot.jackson.datatype.protobuf.util.ObjectMapperHelper.camelCase;
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.io.IOException;
+
+import org.junit.Test;
+
+import com.fasterxml.jackson.annotation.JsonUnwrapped;
+import com.google.protobuf.ListValue;
+import com.google.protobuf.NullValue;
+import com.google.protobuf.Value;
+import com.hubspot.jackson.datatype.protobuf.util.BuiltInProtobufs;
+
+public class UnwrappedSerializationTest {
+    @Test
+    public void itWritesUnwrappedNullValue() throws IOException {
+        BuiltInProtobufs.HasValue message = BuiltInProtobufs.HasValue
+                .newBuilder()
+                .setValue(Value.newBuilder().setNullValue(NullValue.NULL_VALUE).build())
+                .build();
+        ValueBean bean = new ValueBean();
+        bean.setHasValue(message);
+        String json = camelCase().writeValueAsString(bean);
+        assertThat(json).isEqualTo("{\"value\":null}");
+    }
+
+    @Test
+    public void itWritesUnwrappedListValue() throws IOException {
+        ListValue list = ListValue.newBuilder().addValues(Value.newBuilder().setStringValue("test").build()).build();
+        BuiltInProtobufs.HasValue message = BuiltInProtobufs.HasValue
+                .newBuilder()
+                .setValue(Value.newBuilder().setListValue(list).build())
+                .build();
+        ValueBean bean = new ValueBean();
+        bean.setHasValue(message);
+        String json = camelCase().writeValueAsString(bean);
+        assertThat(json).isEqualTo("{\"value\":[\"test\"]}");
+    }
+
+    public static class ValueBean {
+        @JsonUnwrapped
+        private BuiltInProtobufs.HasValue hasValue;
+
+        public BuiltInProtobufs.HasValue getHasValue() {
+            return hasValue;
+        }
+        public void setHasValue(BuiltInProtobufs.HasValue hasValue) {
+            this.hasValue = hasValue;
+        }
+    }
+}