Jackson ignores data class fields that aren't boolean but are of names starting with "is"

I'm using Jackson with Kotlin binding in my project. We have a data class that has a field of type Map<A, B> and is named "isRecommended". When Jackson serializes the data class, this field gets omitted in the resultant JSON string.

A simple test to reproduce the same behavior:

class FooKotlin {
    @Test
    fun testFoo() {
        println(jacksonObjectMapper().writeValueAsString(Foo1(true)))
        println(jacksonObjectMapper().writeValueAsString(Foo2(1)))
        println(jacksonObjectMapper().writeValueAsString(Foo3("true")))
    }
}

data class Foo1(val isFoo: Boolean)
data class Foo2(val isFoo: Int)
data class Foo3(val isFoo: String)

The console prints:

{"foo":true}
{}
{}

When I decompile the Kotlin bytecode, the three classes seem to have almost identical content except for the type of the field. So what is the cause of this behavior of Jackson?

As mentioned by @chrsblck it is related to the jackson-module-kotlin issue #80

On the version 2.10.1 it's not reproducible, although serialized properties names are different (the "is" prefix is not removed):

{"isFoo":true}
{"isFoo":1}
{"isFoo":"true"}

On the earlier versions, the issue can be fixed with a JsonProperty annotation:

data class Foo1(val isFoo: Boolean)
data class Foo2(@get:JsonProperty("foo") val isFoo: Int)
data class Foo3(@get:JsonProperty("foo") val isFoo: String)

{"foo":true}
{"foo":1}
{"foo":"true"}

Technically, naming a non-boolean property "isSomthing" is incorrect and violates JavaBeans specification. Jackson relies on the JavaBeans conventions, thus it gets confused.

If you can avoid such naming, I would advise doing so. Otherwise, you may face the same problems when calling the Foo* classes from Java code.