json使用Scala、Jackson和java对泛型类型进行奇怪的反序列化问题。整型或scala。Int

Question

我们都知道，在Java和Scala下，泛型类型会受到类型擦除的影响。但是我们在使用Jackson和Scala-Jackson模块时遇到了一个奇怪的问题

我创建了一个小测试来说明这个问题

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

这种不同的行为从何而来？泛型擦除，Jackson，Jackson Scala模块

Answer 1

这似乎是因为java.lang.Integer有一个接受字符串的constructor，它允许反序列化程序将该字符串用于Integer，而Int的情况并非如此

Answer 2

这是一个非常普遍的问题，我为此写了一篇FAQ：

[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.

对于好奇的人来说，FAQ中有更多的细节