java - 使用 Scala 和 Jackson 以及 java.lang.Integer 或 scala.Int 对泛型类型进行奇怪的反序列化问题

Question

我们都知道泛型类型在 Java 和 Scala 下会被类型删除。但是我们在使用 Jackson 和 Scala Jackson 模块的 Scala 中遇到了一个奇怪的问题。

我创建了一个小测试来展示这个问题。

import com.fasterxml.jackson.databind.ObjectMapper
import com.fasterxml.jackson.module.scala.DefaultScalaModule

object GenericTest {

  case class TestWithInt(id: Option[Int])
  case class TestWithInteger(id: Option[Integer])

  def main(args: Array[String]) {

    val mapper = new ObjectMapper()
    mapper.registerModule(DefaultScalaModule)

    // Test with scala's Int
    val test = mapper.readValue[TestWithInt]("""{ "id" : 5 }""", classOf[TestWithInt])
    print("Test 1: ")
    println(test.id.get +  1)

    val test2 = mapper.readValue[TestWithInt]("""{ "id" : "5" }""", classOf[TestWithInt])
    print("Test 2: ")
    try {
      println(test2.id.get + 1)
    } catch {
      case e: ClassCastException => println(e.getMessage)
    }

    // Test with java.lang.Integer
    val test3 = mapper.readValue[TestWithInteger]("""{ "id" : 5 }""", classOf[TestWithInteger])
    print("Test 3: ")
    println(test3.id.get +  1)

    val test4 = mapper.readValue[TestWithInteger]("""{ "id" : "5" }""", classOf[TestWithInteger])
    print("Test 4: ")
    println(test4.id.get + 1)
  }
}

上面的输出是:

Test 1: 6
Test 2: java.lang.String cannot be cast to java.lang.Integer
Test 3: 6
Test 4: 6

这种不同类型的行为从何而来？通用类型删除、Jackson、Jackson Scala 模块？

Answer 1

这个问题变得如此普遍，以至于我写了一个 FAQ为此:

[A]ll primitive type parameters are represented as Object to the JVM. ... The Scala module has informed Jackson that Option is effectively a container type, but it relies on Java reflection to determine the contained type, and comes up with Object.

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.

常见问题解答中有更多详细信息供好奇者使用。