Issues deserializing scala collections

[allComputableThings]

I'm providing a scala-aware mapper to JSON RPC but finding that its impossible to deserialize return values (via JsonRpcHttpClient) if method returns scala collection types. I've checked and found that the same mapper object can deserialize JSON into the same scala list type.

So... I did some digging and found that using

 method.getReturnType()

as the type parameter to mapper instead of

method.getGenericReturnType()

Seems to solve this issue. i.e. use

ProxyUtil::createClientProxy() ... 
  ... Class[_] clazz = method.getReturnType()
  ... invoke(... clazz ...)

JsonRpcClient::readResponse() ...
  ...
  val tokens = mapper.treeAsTokens(jsonObject.get("result"))
  return mapper.readValue(tokens, clazz).asInstanceOf[Object]

instead of

ProxyUtil::createClientProxy() ... 
  ... Type returnJavaType = method.getGenericReturnType() 
  ... invoke(... returnJavaType ...)
JsonRpcClient::readResponse() ...
        JsonParser returnJsonParser = mapper.treeAsTokens(jsonObject.get("result"));
        JavaType returnJavaType = TypeFactory.defaultInstance().constructType(returnType);          
        return mapper.readValue(returnJsonParser, returnJavaType);

Is there any reason to prefer using getGenericReturnType? Or can a pull request be submitted to replace Type with Class<?> wherever return types are used?

NB. I thought I could fix this by simply providing my own client proxy to use method.getReturnType() (which returns a class). However, although there's a JsonRpcClient::invoke method that takes a Class<?>, this is downcast to a Type, and, I think, by then its not possible to get mapper to instantiate the scala collection class using either:

mapper.readValue(returnJsonParser, );

mapper.readValue(returnJsonParser, <Class[] >);

:( So... better to pass around Class than Type. Any reason not to?

[briandilley]

getGenericReturnType type returns:

List

vs.:

List

so getReturnType() can’t be used.

-- 
Brian C. Dilley

On August 20, 2014 at 7:26:57 PM, stuz5000 ([email protected]) wrote:

I'm providing a scala-aware mapper to JSON RPC but finding that its impossible to deserialize return values (via JsonRpcHttpClient) if method returns scala collection types. I've checked and found that the same mapper object can deserialize JSON into the same scala list type.

So... I did some digging and found that using

method.getReturnType()

as the type parameter to mapper instead of

method.getGenericReturnType()

Seems to solve this issue. i.e. use

ProxyUtil::createClientProxy() ...
... Class[_] clazz = method.getReturnType()
... invoke(... clazz ...)

JsonRpcClient::readResponse() ...
...
val tokens = mapper.treeAsTokens(jsonObject.get("result"))
return mapper.readValue(tokens, clazz).asInstanceOf[Object]

instead of

ProxyUtil::createClientProxy() ...
... Type returnJavaType = method.getGenericReturnType()
... invoke(... returnJavaType ...)
JsonRpcClient::readResponse() ...
JsonParser returnJsonParser = mapper.treeAsTokens(jsonObject.get("result"));
JavaType returnJavaType = TypeFactory.defaultInstance().constructType(returnType);
return mapper.readValue(returnJsonParser, returnJavaType);

Is there any reason to prefer using getGenericReturnType? Or can a pull request be submitted to replace Type with Class<?> wherever return types are used?

NB. I thought I could fix this by simply providing my own client proxy to use method.getReturnType() (which returns a class). However, although there's a JsonRpcClient::invoke method that takes a Class<?>, this is downcast to a Type, and, I think, by then its not possible to get mapper to instantiate the scala collection class using either:

mapper.readValue(returnJsonParser, );

mapper.readValue(returnJsonParser, );

:( So... better to pass around Class than Type. Any reason not to?

—
Reply to this email directly or view it on GitHub.

[allComputableThings]

Hmmm... if I do:

  val userListText = mapper.writeValueAsString(List[ExampleService.User](user, user))

  // Method returns scala.collection.immutable.List[ExampleService.User]
  val method = classOf[ExampleService.UserService].getMethod("getListUser")

  val result = mapper.readValue(userListText, method.getReturnType())

the above successfully deserializes a List[User] from a method.getReturnType().

Though if print...

  println(method.getReturnType())
  >> class scala.collection.immutable.List

  println(method.getGenericReturnType())
  >> scala.collection.immutable.List<net.ailive.scratchbox.ExampleService.User>

it seems somehow that

 method.getReturnType()

carries all the required information.

[allComputableThings]

Ahh... looks like you were right. In this case, the mapper successfully returns a List[User], but the User object data are actual Map objects -- thanks to type erasure.... :(

[allComputableThings]

NB. I'm still blocked on deserializing scala Lists though, even though my ObjectMapper supports this.

Here is the real culprit. In creatClientProxy, I believe

TypeFactory.defaultInstance().constructType(javaType); 

ignores the supplied ObjectMapper. See the discussion here: FasterXML/jackson-module-scala#107

Should prefer to call:

 mapper.getTypeFactory.constructType(javaType)