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1. Represent messages in .proto files, package them as a separate jar, and ship it to target consumers of the service --which is basically the default approach I guess.

2. Do the same but also include hand crafted wrappers (not sub-classes!) around each message that implement a contract supporting at least these two methods (T is the wrapper class, V is the message class (using generics but simplified syntax for brevity):

   public V toProtobufMessage() { //Build V from T }

   public static T fromProtobufMessage(V message_) { //Build and return T from V }

message ItemList {
    repeated item = 1;
}

message CustomerInvoice {
    required ShippingAddress address = 1;
    required ItemList = 2;
    required double amount = 3;
}

message CustomerInvoice {
    required ShippingAddress address = 1;
    repeated Item item = 2;
    required double amount = 3;
}

1. Represent messages in .proto files, package them as a separate jar, and ship it to target consumers of the service --which is basically the default approach I guess.

2. Do the same but also include hand crafted wrappers (not sub-classes!) around each message that implement a contract supporting at least these two methods (T is the wrapper class, V is the message class (using generics but simplified syntax for brevity):

    public V toProtobufMessage() {
        V.Builder builder = V.newBuilder();
        for (Item item : getItemList()) {
            builder.addItem(item);
        }
        return builder.setAmountPayable(getAmountPayable()).
                       setShippingAddress(getShippingAddress()).
                       build();
    }
   
    public static T fromProtobufMessage(V message_) { 
        return new T(message_.getShippingAddress(), 
                     message_.getItemList(),
                     message_.getAmountPayable());
    }
   

message ItemList {
    repeated item = 1;
}

message CustomerInvoice {
    required ShippingAddress address = 1;
    required ItemList = 2;
    required double amountPayable = 3;
}

message CustomerInvoice {
    required ShippingAddress address = 1;
    repeated Item item = 2;
    required double amountPayable = 3;
}