FAQ

Deserializing Option[Int] and other primitive challenges

TL;DR: Use @JsonDeserialize(contentAs = classOf[java.lang.Integer]) on the offending member.

What is happening, and why

Scala, unlike Java, supports using primitive types as type arguments to generic classes. However, this is not supported by the JVM; there is no way to represent java.lang.List<int>, as an example. Prior to Scala 2.9, the compiler would represent Option[Int] as Option[java.lang.Integer] to the JVM, which would appear to naturally fall out of the requirement to use reference types as type parameters.

However, this led to significant problems, for example, implementing bridge methods for traits. The solution, for the time being, is that all primitive type parameters are represented as Object to the JVM. The Scala compiler uses the ScalaSignature annotation to determine what the Scala type needs to be in the cases where it matters.

Enter Jackson, which at its core is a Java library. The Scala module has informed Jackson that Option is effectively a container type, but it relies on Java reflection to determined the contained type, and comes up with Object. Jackson has rules for dealing with this situation, which dictate that the dynamic type of Object values should be the closest natural Java type for the value: java.lang.String for strings, java.lang.Integer, java.lang.Long, or java.math.BigInteger for whole numbers, depending on what size the number fits into, and similarly java.lang.Float, java.lang.Double, or java.lang.BigDecimal for floating point values.

The Scala compiler doesn't know what evil has been done in Java-land. It "knows" the value is a primitive int even if the JVM says Object, so it attempts to cast the value to the relevant boxed type and unbox it. If the primitive type matches the type selected by Jackson, it works; otherwise the actual type protests such treatment in the form of a ClassCastException.

Possible solutions

The best solution for this problem would be to use Scala reflection to determine the Scala type, and tell Jackson which boxed type it should use. This isn't possible for Scala 2.9, which doesn't have Scala reflection, nor for Scala 2.10, whose reflection API is experimental and has bugs.

The second best solution would be to read the @ScalaSignature annotation to determine the correct Scala type. This is a tricky proposition, as the API choices for this are limited (scalap exists but depends on the compiler, which is a large amount of code bloat for the task). However, for the intermediate term this is the best option available, and work is being done to implement it, but it is a load road and is unlikely to be done in time for the next major version of Jackson (2.3).

Current workarounds

The current workaround for this use case is to add the @JsonDeserialize annotation to the member being targeted. Specifically, this annotation has a set of parameters that can be used for different situations:

• contentAs for collections or map values (supported)
• keyAs for Map keys (currently unsupported)

Examples of how to use this annotation can be found in the tests directory.