Library for easy conversion between GPB and Scala case classes.

Cactus

Build Status Download

Library for mapping between GPB generated Java classes and Scala's case classes. It has support for both GPB v2 and GPB v3 (including gRPC).

The library automatically converts common data types (String, primitive types, collections) and can map optional fields into Option. This process is extensible via Converter.

The library uses Either[NonEmptyList[CactusFailure], A] as a return type. There is collection of errors used which means not only the first but all found errors are returned in the cats.data.NonEmptyList.

The conversion never throws an exception. If there is an exception caught during the conversion, it's transformed into UnknownFailure.

In case of failure (e.g. MissingRequiredField), a whole variable path (e.g. gpb.fieldGpb2RepeatedRecurse.fieldGpb.fieldBlob) is reported inside the failure.

Dependency

Gradle:

compile "com.avast.cactus:ARTIFACT_2.12:VERSION"

SBT:

"com.avast.cactus" %% "ARTIFACT" % "VERSION"

where current version is available here and ARTIFACT is one of:

  1. cactus-gpbv2 (conversion of GPB v2.x.x) Use import com.avast.cactus._ and import com.avast.cactus.v2._.
  2. cactus-gpbv3 (conversion of GPB v3.x.x) Use import com.avast.cactus._ and import com.avast.cactus.v3._.
  3. cactus-bytes-gpbv2 (additional converters for support Avast Bytes) Use import com.avast.cactus.bytes._.
  4. cactus-bytes-gpbv3 (same as bytes-gpbv2 but supports some GPBv3 types too) Use import com.avast.cactus.bytes._.
  5. cactus-grpc-client (support for mapping of gRPC client, see docs) Use import com.avast.cactus.grpc.client._.
  6. cactus-grpc-server (support for mapping of gRPC server, see docs) Use import com.avast.cactus.grpc.client._.

GPB to case class

We often need to map GPB message to business object (which is usually a case class) when GPB is used for communication over the network. It is a good practice to follow Google's recommendation to have all fields optional. However that is not very handy during mapping because some validation is necessary.

Since v3.0, optional keyword is removed. See GPB v3 official docs.

This library solves both problems - it generates mapping (using macros) between Java GPB class and Scala case class based on the fields of the case class and validates that all the required fields (not wrapped in Option) are present. It also automatically converts between common data types.

There are several types of failure which can happen. The most common one is MissingFieldFailure which contains a name of the missing field.

It is common that the field has different name in the GPB and in the case class. You can use the GpbName annotation to override the expected name - see the example below.

Sometimes the case class has more fields than the GPB and you just want to ignore them during the conversion. You can use the GpbIgnored annotation to achieve this. Please note that the ignored field in the case class must have default value!

GPBs 2.x do not have map type and it's usually solved by using repeated field with message, which contains key and value fields. There is an automatic conversion to (and from) Scala Map supported via the GpbMap annotation - see the CaseClassA in example. See GPBv3 section for native maps mapping.

Mapping of complex messages (message contains another message which contains another message) is supported. However recursive mapping (field with Data type in Data class) is NOT supported.

When using your own converters, please note it's necessary to use exactly the same type as present in the GPB instance/case class. For example see JavaIntegerListStringConverter in test.

Don't forget to add import com.avast.cactus.v3._ (or import com.avast.cactus.v2._ respectively).

Example

GPB:

message Data {
    optional string     field = 1;                          // REQUIRED
    optional int32      field_int_name = 2;                 // REQUIRED
    optional int32      field_option = 3;                   // OPTIONAL
    repeated string     field_strings = 4;                  
    repeated string     field_strings_name = 5;             
    repeated int32      field_option_integers = 6;          
    repeated int32      field_option_integers_empty = 7;    
    optional Data2      field_gpb_option = 8; 	            // OPTIONAL
    optional Data2      field_gpb_option_empty = 9;         // OPTIONAL
    optional Data2      field_gpb = 10;   		            // REQUIRED
    optional bytes      field_blob = 11;                    // REQUIRED
    repeated MapMessage field_map = 12;                     // OPTIONAL
    repeated Data2      field_gpb_repeated = 13;            // OPTIONAL
}

message Data2 {
    optional double     field_double = 1;	  	            // REQUIRED
    optional bytes      field_blob = 2;	                    // REQUIRED
}

message MapMessage {
    optional string key = 1;                                // REQUIRED
    optional string value = 2;                              // REQUIRED
    optional string other = 3;                              // OPTIONAL
}

Scala:

// your own converters:
implicit val StringToByteStringConverter: Converter[String, ByteString] = Converter((b: String) => ByteString.copyFromUtf8(b))
implicit val ByteStringToStringConverter: Converter[ByteString, String] = Converter(_.toStringUtf8)

case class CaseClassA(
  field: String,
  @GpbName("fieldIntName") // different name in GPB than in case class 
  fieldInt: Int,
  fieldOption: Option[Int],
  fieldBlob: ByteString,
  @GpbName("fieldStringsName")
  fieldStrings2: List[String],
  fieldGpb: CaseClassB,
  fieldGpbOption: Option[CaseClassB],
  fieldGpbOptionEmpty: Option[CaseClassB],
  fieldGpbRepeated: Seq[CaseClassB],
  fieldStrings: immutable.Seq[String],
  fieldOptionIntegers: Seq[Int],
  fieldOptionIntegersEmpty: List[Int],
  @GpbMap(key = "key", value = "value")
  fieldMap: Map[String, String]
)

case class CaseClassB(
  fieldDouble: Double, 
  @GpbName("fieldBlob")
  fieldString: String // this is possible thanks to the user-specified converter, the field type is `ByteString` in the GPB
)

object Test extends App {
  // Warning: You may be missing some version-specific support without this import, e.g. GPBv3 value wrappers.
  import com.avast.cactus.v2._ // or v3
  
  val gpbInternal = Data2.newBuilder()
   .setFieldDouble(0.9)
   .setFieldBlob(ByteString.copyFromUtf8("text"))
   .build()
  
  val dataRepeated = Seq(gpbInternal, gpbInternal, gpbInternal)
  
  val gpb = TestMessage.Data.newBuilder()
   .setField("ahoj")
   .setFieldIntName(9)
   .setFieldOption(13)
   .setFieldBlob(ByteString.EMPTY)
   .setFieldGpb(gpbInternal)
   .setFieldGpbOption(gpbInternal)
   .addAllFieldStrings(Seq("a", "b").asJava)
   .addAllFieldStringsName(Seq("a").asJava)
   .addAllFieldOptionIntegers(Seq(3, 6).map(int2Integer).asJava)
   .addAllFieldMap(map.map { case (key, value) => TestMessage.MapMessage.newBuilder().setKey(key).setValue(value).build() }.asJava)
   .addAllFieldGpbRepeated(dataRepeated.asJava)
   .build()

  gpb.asCaseClass[CaseClassA] match {
    case Right(inst) =>
      println(inst)

    case Left(e) =>
      println(s"Missing required fields: '${e.toList.mkString(", ")}'")
  }
}

See unit tests for more examples.

Collections (java.util.List<T>) are converted to scala.collection.immutable.Seq[T] - concretely to scala.collection.immutable.Vector[T] by default. Another option is to use scala.collection.immutable.List[T] - you have to use the List type explicitly. By specifying scala.collection.Seq[T], which can contain also mutable collections, you get the Vector[T].

Case class to GPB

When using your own converters, please note it's necessary to use exactly the same type as present in the GPB builder/case class. For example see StringJavaIntegerListConverter in test.

Don't forget to add import com.avast.cactus.v3._ (or import com.avast.cactus.v2._ respectively).

Example

GPB:

message Data {
    optional string     field = 1;                          // REQUIRED
    optional int32      field_int_name = 2;                 // REQUIRED
    optional int32      field_option = 3;                   // OPTIONAL
    repeated string     field_strings = 4;                  
    repeated string     field_strings_name = 5;             
    repeated int32      field_option_integers = 6;          
    repeated int32      field_option_integers_empty = 7;    
    optional Data2      field_gpb_option = 8; 	            // OPTIONAL
    optional Data2      field_gpb_option_empty = 9;         // OPTIONAL
    optional Data2      field_gpb = 10;   		            // REQUIRED
    optional bytes      field_blob = 11;                    // REQUIRED
    repeated Data2      field_gpb_repeated = 12;            // OPTIONAL
}

message Data2 {
    optional double     field_double = 1;	  	            // REQUIRED
    optional bytes      field_blob = 2;	                    // REQUIRED
}

Scala:

case class CaseClassA(field: String,
  @GpbName("fieldIntName")
  fieldInt: Int,
  fieldOption: Option[Int],
  fieldBlob: ByteString,
  @GpbName("fieldStringsName")
  fieldStrings2: Vector[String],
  fieldGpb: CaseClassB,
  fieldGpbOption: Option[CaseClassB],
  fieldGpbOptionEmpty: Option[CaseClassB],
  fieldGpbRepeated: Seq[CaseClassB],
  fieldStringsList: Seq[String],
  fieldOptionIntegersList: Option[Seq[Int]],
  fieldOptionIntegersEmptyList: Option[List[Int]])

case class CaseClassB(fieldDouble: Double, @GpbName("fieldBlob") fieldString: String)


object Test extends App {
  // Warning: You may be missing some version-specific support without this import, e.g. GPBv3 value wrappers.
  import com.avast.cactus.v2._ // or v3
  
  val caseClassB = CaseClassB(0.9, "text")
  
  val caseClassBRepeated = Seq(caseClassB, caseClassB, caseClassB)
  
  val caseClass = CaseClassA(
    field = "ahoj",
    fieldInt = 9,
    fieldOption = Some(13),
    fieldBlob = ByteString.EMPTY,
    fieldStrings2 = Vector("a"),
    fieldGpb = CaseClassB(0.9, "text"),
    fieldGpbOption = Some(caseClassB),
    fieldGpbOptionEmpty = None,
    fieldGpbRepeated = caseClassBRepeated,
    fieldStringsList = Seq("a", "b"),
    fieldOptionIntegersList = Some(Seq(3, 6)),
    fieldOptionIntegersEmptyList = None
  )

  caseClass.asGpb[Data] match {
    case Right(inst) =>
      println(inst)

    case Left(e) =>
      println("Errors: " + e)
  }
}

See unit tests for more examples.

GPB v3 support

Don't forget to always add import com.avast.cactus.v3._.

See more in gpbv3 module.

Defining own converters

There are following ways how to implement own Converter:

  • Plain conversion
    implicit val StringToByteStringConverter: Converter[String, ByteString] = Converter((b: String) => ByteString.copyFromUtf8(b))
    This is to be used when just simple A -> B conversion is needed.

  • Checked conversion

    // unsafe way:
    implicit val StringJavaIntegerListConverter: Converter[String, java.lang.Iterable[_ <: Integer]] = Converter(_.split(", ").map(_.toInt).map(int2Integer).toSeq.asJava)
    
    // safe (checked) way:
    implicit val StringJavaIntegerListConverter2: Converter[String, java.lang.Iterable[_ <: Integer]] = Converter.checked { (fieldPath, str) =>
        val parts = str.split(", ")
        
        if (parts.nonEmpty && parts.forall(_.matches("\\d+"))) {
          Right(parts.map(_.toInt).map(int2Integer).toSeq.asJava)
        } else {
          Left(NonEmptyList.one(CustomFailure(fieldPath, s"Wrong format of '$fieldPath' field")))
        }
    }

    This is to be used when the conversion can go wrong and it's better to cover possible failures right in the converter.
    Please note, that even if the StringJavaIntegerListConverter fails, the exception will not be thrown outside. The UnknownFailure(..., ...) error will be returned instead.

  • Just implement new Converter

    implicit val StringJavaIntegerListConverter2: Converter[String, java.lang.Iterable[_ <: Integer]] = new Converter[String, java.lang.Iterable[_ <: Integer]] {
      override def apply(fieldPath: String)(str: String): ResultOrErrors[java.lang.Iterable[_ <: Integer]] = {
          val parts = str.split(", ")
    
          if (parts.nonEmpty && parts.forall(_.matches("\\d+"))) {
            Right(parts.map(_.toInt).map(int2Integer).toSeq.asJava)
          } else {
            Left(NonEmptyList.one(CustomFailure(fieldPath, s"Wrong format of '$fieldPath' field")))
          }
      }
    }
  • Derive converter from existing one

    There are multiple methods which allow you to manually derive and combine converters on the Converter trait (see ConverterMethods).
    For example:

    • You have Converter[A, B] - you can derive Converter[A, C] by using converterAB.map((b: B) => ???:C).
    • You have Converter[A, B] and Converter[B, C] - you can derive Converter[A, C] by using converterAB.andThen(converterBC)

    See unit tests for more examples.

    Note: You can let Cactus derive Converter[A, B] automatically and use it then, e.g.

      val convAtoB = implicitly[Converter[A, B]]
      val convAtoC: Converter[A, C] = convAtoB.map((b: B) => C(b))

Examples of custom converters may be seen in examples above or in unit tests.

Advanced example 1

Scenario:

Having proto messages

message Event {
    optional int32 number = 1;
}

message EventsResponse {
    repeated Event events = 1;
}

you want to have a following code in Scala:

import com.avast.cactus.ResultOrErrors
import com.avast.cactus.v3._

import my.gpb.{Event => GpbEvent, EventsResponse => GpbEventsResopnse}

case class Event(number: Int)

val response: Seq[Event] = myMethodCall()

val gpbResponse: ResultOrErrors[GpbEventsResponse] = response.asGpb[GpbEventsResponse]

Unfortunately this code won't compile because Cactus is unable to generate Converter[Seq[Event], GpbEventsResponse] automatically.

There are three possible solutions:

  1. Implement the converter manually - but that is what Cactus is trying to help you from ;-)
  2. Wrap Seq[Event] into some case class (e.g. case class EventsResponse(events: Seq[Event])) which would add basically useless code into the application
  3. Implement the converter but let Cactus do the dirty part
    1. Cactus is able to generate Converter[Event, GpbEvent] and derive Converter[Seq[Event], Seq[GpbEvent]] from it
    2. You have to implement just Seq[GpbEvent] => GpbEventsResponse which is an easy task:
         implicit val theConverter: Converter[Seq[Event], GpbEventsResponse] = {
           import scala.jdk.CollectionConverters._
      
           implicitly[Converter[Seq[Event], Seq[GpbEvent]]]
               .map(events => GpbEventsResponse.newBuilder().addAllEvents(events.asJava).build())
         }

Advanced example 2

Scenario:

You have findUser endpoint in your service. As it's usual for all find* methods, it's valid case that the user is not found. You want to express this as Option[User] in Scala API and now you need to model GPB messages and related converter.

message OptionalUserResponseMessage {
    UserMessage user = 1;
}

message UserMessage {
    string name = 1;                                        // REQUIRED
    int32 age = 2;                                          // REQUIRED
}
case class OptionalUserResponse(user: Option[User])

case class User(name: String, age: Int)

Now, Cactus is able to convert OptionalUserResponse to OptionalUserResponseMessage (and back), but you want to have just Option[User] in your API, not the whole OptionalUserResponse.

Creating converter for a client side is very easy:

implicit val convGpbToOptUser: Converter[OptionalUserResponseMessage, Option[User]] = {
  implicitly[Converter[OptionalUserResponseMessage, OptionalUserResponse]]
    .map(_.user) // append `OptionalUserResponse => Option[User]` to the converter
}

But on a server side, you call DAO which gets you class DbUser:

// def findUser: Option[DbUser] = ???

case class DbUser(firstName: String, surName: String, dateOfBirth: LocalDate)

You decide to return Option[DbUser] from your handler and thus to convert DbUser directly to GPB. That is obviously something Cactus cannot do by its own. However, you can derive your custom converter if you provide a way how to convert DbUser to User:

def toAge(d: Duration): Int = ???

implicit val dbUserToUser: Converter[DbUser, User] = Converter{ dbUser =>
  User(s"${dbUser.firstName} ${dbUser.surName}", toAge(Duration.between(dbUser.dateOfBirth, Instant.now())))
}

implicit val convToGpb: Converter[Option[DbUser], OptionalUserResponseMessage] = {
  implicitly[Converter[OptionalUserResponse, OptionalUserResponseMessage]] // 1 -> Converter[OptionalUserResponse, OptionalUserResponseMessage]
    .contraMap(OptionalUserResponse.apply) // 2 -> Converter[Option[User], OptionalUserResponseMessage]
    .compose[Option[DbUser]] // 3 -> Converter[Option[DbUser], OptionalUserResponseMessage]
}

This deserves more detailed explanation:

  1. Cactus derives Converter[OptionalUserResponse, OptionalUserResponseMessage] for you.
  2. You prepend conversion function Option[User] => OptionalUserResponse to the converter from previous step.
  3. You prepend your custom Converter[DbUser, User] to the converter from previous step.
    (In fact, the Converter[Option[DbUser], Option[User]] is needed here, but Cactus will lift your custom converter automatically to fit.)

Optional modules

Bytes

There are modules cactus-bytes-gpbv2 and cactus-bytes-gpbv3 containing support for conversion between GPB types and Avast Bytes.
You have to use this import to make mentioned converters available:

import com.avast.cactus.bytes._

Then the proto

message Message {
    optional double     number = 1;	  	            // REQUIRED
    optional bytes      blob = 2;	                // REQUIRED
}

becomes convertible to

case class TheCaseClass(number: Double, blob: com.avast.bytes.Bytes)

When using cactus-bytes-gpbv3 module, you can use google.protobuf.BytesValue in the GPB message too:

message Message {
    optional double                       number = 1;	            // REQUIRED
    optional google.protobuf.BytesValue   blob = 2;	                // REQUIRED
}

Using Cactus in your own library

Sometimes you need to wrap Cactus parsing under the hood of your own library and be GPB-version-agnostic at the same time.
For example you could have method for parsing some event into a case class but the event is encoded in the GPB like this:

import cats.syntax.either._
import com.google.protobuf.{MessageLite, Parser}
import com.avast.cactus.{CactusParser, Converter}

import CactusParser._ // enables the asCaseClass and asGpb methods!

def parse[GpbMessage <: MessageLite: Parser: ClassTag, CaseClass: Converter[GpbMessage, *]]: Either[Exception, CaseClass] = {
  Try(implicitly[Parser[GpbMessage]].parseFrom(eventBody.newInputStream())) match {
    case Success(gpb) => gpb.asCaseClass[CaseClass].leftMap(fs => new RuntimeException("Errors: " + fs.toList.mkString("[", ", ", "]")))
    case Failure(NonFatal(e)) => Left(new RuntimeException("Could not parse GPB message", e))
  }
}

This is where com.avast.cactus.CactusParser from cactus-common module should be used - depending on this module does not add dependency on neither the GPBv2 nor GPBv3.

Debugging

Hidden errors

It happens the Cactus is not able to derive your converter despite of you're convinced it should. Because the derivation is called implicitly, the compiler will not output the real reason (error) which happened there. As a workaround for this, you can do this to call the derivation directly and to unveil the error output:

import com.avast.cactus.Converter

Converter.deriveConverter[MySource, MyDestination]

where MySource and MyDestination are obviously the two types which you need to be Converter derived for.

Notes

  1. There is Kind Projector library used in some examples above.
  2. In case of any problem (or you just want to see the generated code), you can turn on debug of cactus by passing cactus.debug=true system property to the compiler. E.g. in IDEA go to File | Settings | Build, Execution, Deployment | Compiler | Scala Compiler | Scala Compile Server and add -Dcactus.debug=true to JVM options.
  3. Name you GPB fields with a common sense. Some combinations will cause clashes: e.g. Enum enum and map<string, Enum> enumMap will cause the mapping/compilation to fail with a nonsense message.