I´m trying to deserialize a json into scala class, that contains a Collection of types Aclass,Bclass or Cclass
class Results[M](results:util.ArrayList[M]) {
def getResults:util.ArrayList[M]=results
def this() {
this(new util.ArrayList())
}
}
The Json looks like:
{ "results":[{"a":1},{"a":1}]}
or
{ "results":[{"b":1},{"b":1}]}
or
{ "results":[{"b":1},{"b":1}]}
Here my object mapper
val mapper = new ObjectMapper().configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
And here how I deserialize a json of class type Aclass
val results = mapper.readValue(json, classOf[Results[Aclass]])
The problem is that results, return a ArrayList where every element instead to be an instance of Aclass, is a LinkedHashMap.
If I change the code instead to use generic I make it explicit in the type
class Results[Aclass](results:util.ArrayList[Aclass]) {
def getResults:util.ArrayList[Aclass]=results
def this() {
this(new util.ArrayList())
}
}
It works and return an class with result of array of Aclass elements.
What I´m doing wrong here?. I think it´s a bug in the library
Regards.
The reason is that java.lang.Class does not hold information about generic parameters. The value of classOf[Result[Aclass]] is java.lang.Class. It does not have information about the type parameter Aclass. It represents only the Results class.
The mapper does what you ask for: gives you Results of whatever and the best whatever for it is a map.
To solve it, use readValue method that takes JavaType instead. JavaType can be created using TypeFactory:
val results:Results[Aclass] = mapper.readValue(json, mapper.getTypeFactory()
.constructParametricType(classOf[Results[_]], classOf[Aclass]))
Related
I am trying to deserialize a scala enumeration from integer value.
object TestEnum extends Enumeration {
type TestEnum = Value
val None = Value(0)
val One = Value(1)
val Two = Value(2)
val Four = Value(4) // scalastyle:ignore
#JsonCreator
def forValue(value: Int): TestEnum = {
TestEnum.Value(value)
}
}
class TestEnum extends TypeReference[TestEnum.type]
When I try to deserialize this field, it throws an error as - Cannot deserialize value of type com.example.TestEnum$from Integer value (tokenJsonToken.VALUE_NUMBER_INT)
I see the jackson doc suggests to use the JsonCreator for the same in java, however nothing mentioned for scala enum.
I am using the defaultScalaMapper without any customization here.
I was able to solve it with jsonCreator in the class using the enum. as follows -
class Example(testEnum: TestEnum) {
#JsonCreator
def this(testEnumNum: Int) = {
this(TestEnum.forValue(testEnumNum))
}
}
However I need to do this in every class, I was hoping if there can be a better solution for this.
I'm was reading about cats and I encountered the following code snippet which is about serializing objects to JSON!
It starts with a trait like this:
trait JsonWriter[A] {
def write(value: A): Json
}
After this, there are some instances of our domain object:
final case class Person(name: String, email: String)
object JsonWriterInstances {
implicit val stringWriter: JsonWriter[String] =
new JsonWriter[String] {
def write(value: String): Json =
JsString(value)
}
implicit val personWriter: JsonWriter[Person] =
new JsonWriter[Person] {
def write(value: Person): Json =
JsObject(Map(
"name" -> JsString(value.name),
"email" -> JsString(value.email)
))
}
// etc...
}
So far so good! I can then use this like this:
import JsonWriterInstances._
Json.toJson(Person("Dave", "dave#example.com"))
Later on I come across something called the interface syntax, which uses extension methods to extend existing types with interface methods like below:
object JsonSyntax {
implicit class JsonWriterOps[A](value: A) {
def toJson(implicit w: JsonWriter[A]): Json =
w.write(value)
}
}
This then simplifies the call to serializing a Person as:
import JsonWriterInstances._
import JsonSyntax._
Person("Dave", "dave#example.com").toJson
What I don't understand is that how is the Person boxed into JsonWriterOps such that I can directly call the toJson as though toJson was defined in the Person case class itself. I like this magic, but I fail to understand this one last step about the JsonWriterOps. So what is the idea behind this interface syntax and how does this work? Any help?
This is actually a standard Scala feature, since JsonWriterOps is marked implicit and is in scope, the compiler can apply it at compilation-time when needed.
Hence scalac will do the following transformations:
Person("Dave", "dave#example.com").toJson
new JsonWriterOps(Person("Dave", "dave#example.com")).toJson
new JsonWriterOps[Person](Person("Dave", "dave#example.com")).toJson
Side note:
It's much more efficient to implicit classes as value classes like this:
implicit class JsonWriterOps[A](value: A) extends AnyVal
This makes the compiler also optimize away the new object construction, if possible, compiling the whole implicit conversion + method call to a simple function call.
I have a class
class MyClass {
def apply(myRDD: RDD[String]) {
val rdd2 = myRDD.map(myString => {
// do String manipulation
}
}
}
object MyClass {
}
Since I have a block of code performing one task (the area that says "do String manipulation"), I thought I should break it out into its own method. Since the method is not changing the state of the class, I thought I should make it a static method.
How do I do that?
I thought that you can just pop a method inside the companion object and it would be available as a static class, like this:
object MyClass {
def doStringManipulation(myString: String) = {
// do String manipulation
}
}
but when I try val rdd2 = myRDD.map(myString => { doStringManipulation(myString)}), scala doesn't recognize the method and it forces me to do MyClass.doStringManipulation(myString) in order to call it.
What am I doing wrong?
In Scala there are no static methods: all methods are defined over an object, be it an instance of a class or a singleton, as the one you defined in your question.
As you correctly pointed out, by having a class and an object named in the same way in the same compilation unit you make the object a companion of the class, which means that the two have access to each others' private fields and methods, but this does not mean they are available without specifying which object you are accessing.
What you want to do is either using the long form as mentioned (MyClass.doStringManipulation(myString)) or, if you think it makes sense, you can just import the method in the class' scope, as follows:
import MyClass.doStringManipulation
class MyClass {
def apply(myRDD: RDD[String]): Unit = {
val rdd2 = myRDD.map(doStringManipulation)
}
}
object MyClass {
private def doStringManipulation(myString: String): String = {
???
}
}
As a side note, for the MyClass.apply method, you used the a notation which is going to disappear in the future:
// this is a shorthand for a method that returns `Unit` but is going to disappear
def method(parameter: Type) {
// does things
}
// this means the same, but it's going to stay
// the `=` is enough, even without the explicit return type
// unless, that is, you want to force the method to discard the last value and return `Unit`
def method(parameter: Type): Unit = {
// does things
}
You should follow scala's advice.
val rdd2 = myRDD.map(MyClass.doStringManipulation)
Write this inside the class then it will work as expected.
import MyClass._
I'm kinda new to scala. I got into trouble while trying to return object type.
Here is the code.
It shows "error: not found: type A"
object A{}
object B {
def getInstance() : A = {
return A
}
}
If I do similar kind of thing with class instance, it wont show any problem.
class A{}
object B {
def getInstance() : A = {
return new A
}
}
As far as I know object type is a singleton instance of class. What am I missing here?
Compiler complains that can not find type A because in your case A is a name of an object not a type, use A.type to refer to type, like this:
object A
object B {
def getInstance: A.type = A
}
Because object is a singleton is does not define a type, instead in defines a value.
If you would look at the Java equivalent, using scala object produces the:
A$ class //
A$.MODULE$ // singleton instance definition
The type is however masked and can be accessed via A.type.
Using return is also not necessary in Scala. The last statement position in a block is automatically interpreted as return value.
class A{}
object B {
def getInstance() : A = new A
}
(Essentially I need some kind of a synthesis of these two questions (1, 2), but I'm not smart enough to combine them myself.)
I have a set of JAXB representations in Scala like this:
abstract class Representation {
def marshalToXml(): String = {
val context = JAXBContext.newInstance(this.getClass())
val writer = new StringWriter
context.createMarshaller.marshal(this, writer)
writer.toString()
}
}
class Order extends Representation {
#BeanProperty
var name: String = _
...
}
class Invoice extends Representation { ... }
The problem I have is with my unmarshalling "constructor" methods:
def unmarshalFromJson(marshalledData: String): {{My Representation Subclass}} = {
val mapper = new ObjectMapper()
mapper.getDeserializationConfig().withAnnotationIntrospector(new JaxbAnnotationIntrospector())
mapper.readValue(marshalledData, this.getClass())
}
def unmarshalFromXml(marshalledData: String): {{My Representation Subclass}} = {
val context = JAXBContext.newInstance(this.getClass())
val representation = context.createUnmarshaller().unmarshal(
new StringReader(marshalledData)
).asInstanceOf[{{Type of My Representation Subclass}}]
representation // Return the representation
}
Specifically, I can't figure out how to attach these unmarshalling methods in a typesafe and DRY way to each of my classes, and then to call them from Scala (and hopefully sometimes by using only abstract type information). In other words, I would like to do this:
val newOrder = Order.unmarshalFromJson(someJson)
And more ambitiously:
class Resource[R <: Representation] {
getRepresentation(marshalledData: String): R =
{{R's Singleton}}.unmarshalFromXml(marshalledData)
}
In terms of my particular stumbling blocks:
I can't figure out whether I should define my unmarshalFrom*() constructors once in the Representation class, or in a singleton Representation object - if the latter, I don't see how I can automatically inherit that down through the class hierarchy of Order, Invoice etc.
I can't get this.type (as per this answer) to work as a way of self-typing unmarshalFromJson() - I get a compile error type mismatch; found: ?0 where type ?0 required: Representation.this.type on the readValue() call
I can't figure out how to use the implicit Default[A] pattern (as per this answer) to work down my Representation class hierarchy to call the singleton unmarshalling constructors using type information only
I know this is a bit of a mammoth question touching on various different (but related) issues - any help gratefully received!
Alex
The key is to not try and attach the method to the class but rather pass it in as a parameter. To indicate the type you are expecting and let the type system handle passing it in. I tried to make the unmarshal invocation something that reads a little DSL like.
val order = UnMarshalXml( xml ).toRepresentation[Order]
The following is a fully testable code snippet
abstract class Representation {
def marshalToXml(): String = {
val context = JAXBContext.newInstance(this.getClass)
val writer = new StringWriter
context.createMarshaller.marshal(this, writer)
writer.toString
}
}
#XmlRootElement
class Order extends Representation {
#BeanProperty
var name: String = _
}
case class UnMarshalXml( xml: String ) {
def toRepresentation[T <: Representation](implicit m:Manifest[T]): T = {
JAXBContext.newInstance(m.erasure).createUnmarshaller().unmarshal(
new StringReader(xml)
).asInstanceOf[T]
}
}
object test {
def main( args: Array[String] ) {
val order = new Order
order.name = "my order"
val xml = order.marshalToXml()
println("marshalled: " + xml )
val received = UnMarshalXml( xml ).toRepresentation[Order]
println("received order named: " + received.getName )
}
}
You should see the following output if you run test.main
marshalled: <?xml version="1.0" encoding="UTF-8" standalone="yes"?><order><name>my order</name></order>
received name: my order
Here's the updated version of Neil's code which I used to support the second use case as well as the first:
case class UnmarshalXml(xml: String) {
def toRepresentation[T <: Representation](implicit m: Manifest[T]): T =
toRepresentation[T](m.erasure.asInstanceOf[Class[T]])
def toRepresentation[T <: Representation](typeT: Class[T]): T =
JAXBContext.newInstance(typeT).createUnmarshaller().unmarshal(
new StringReader(xml)
).asInstanceOf[T]
}
This supports simple examples like so:
val order = UnmarshalXml(xml).toRepresentation[Order]
But also for abstract type based usage, you can use like this:
val order = UnmarshalXml(xml).toRepresentation[T](typeOfT)
(Where you have grabbed and stored typeOfT using another implicit Manifest at the point of declaring T.)