Add use_snake_case_for_field_names option to JsonPrintOptions

src/google/protobuf/util/internal/json_objectwriter.cc

@@ -176,8 +176,14 @@ void JsonObjectWriter::WritePrefix(StringPiece name) {
   if (!name.empty() || empty_key_ok) {
     WriteChar('"');
     if (!name.empty()) {
-      ArrayByteSource source(name);
-      JsonEscaping::Escape(&source, &sink_);
+      if (use_snake_case_for_field_names_) {
+        string snake_name = ToSnakeCase(name);
+        ArrayByteSource source(snake_name);
+        JsonEscaping::Escape(&source, &sink_);
+      } else {
+        ArrayByteSource source(name);
+        JsonEscaping::Escape(&source, &sink_);
+      }
     }
     stream_->WriteString("\":");
     if (!indent_string_.empty()) WriteChar(' ');

src/google/protobuf/util/internal/json_objectwriter.h

@@ -94,6 +94,7 @@ class LIBPROTOBUF_EXPORT JsonObjectWriter : public StructuredObjectWriter {
         sink_(out),
         indent_string_(indent_string.ToString()),
         use_websafe_base64_for_bytes_(false),
+        use_snake_case_for_field_names_(false),
         empty_name_ok_for_next_key_(false) {}
   virtual ~JsonObjectWriter();
 
@@ -118,6 +119,10 @@ class LIBPROTOBUF_EXPORT JsonObjectWriter : public StructuredObjectWriter {
     use_websafe_base64_for_bytes_ = value;
   }
 
+  void set_use_snake_case_for_field_names(bool value) {
+    use_snake_case_for_field_names_ = value;
+  }
+
   // Whether empty strings should be rendered for the next JSON key. This
   // setting is only valid until the next key is rendered, after which it gets
   // reset to false.
@@ -217,6 +222,9 @@ class LIBPROTOBUF_EXPORT JsonObjectWriter : public StructuredObjectWriter {
   // to regular base64 encoding.
   bool use_websafe_base64_for_bytes_;
 
+  // Whether to use snake_case or lowerCamelCase for field names
+  bool use_snake_case_for_field_names_;
+
   // Whether empty strings should be rendered for the next JSON key. This
   // setting is only valid until the next key is rendered, after which it gets
   // reset to false.

src/google/protobuf/util/json_util.cc

@@ -86,6 +86,7 @@ util::Status BinaryToJsonStream(TypeResolver* resolver,
   io::CodedOutputStream out_stream(json_output);
   converter::JsonObjectWriter json_writer(options.add_whitespace ? " " : "",
                                           &out_stream);
+  json_writer.set_use_snake_case_for_field_names(options.use_snake_case_for_field_names);
   if (options.always_print_primitive_fields) {
     converter::DefaultValueObjectWriter default_value_writer(
         resolver, type, &json_writer);

src/google/protobuf/util/json_util.h

@@ -64,10 +64,13 @@ struct JsonPrintOptions {
   // Whether to always print enums as ints. By default they are rendered as
   // strings.
   bool always_print_enums_as_ints;
+  // Whether to convert field names to snake case
+  bool use_snake_case_for_field_names;
 
   JsonPrintOptions() : add_whitespace(false),
                        always_print_primitive_fields(false),
-                       always_print_enums_as_ints(false) {
+                       always_print_enums_as_ints(false),
+                       use_snake_case_for_field_names(false) {
   }
 };