The reason for (4) is the good old Java Type Erasure.
What you writing out is essentially `Map<?,?>`, as far as available
type information tells -- there is nothing that could tell otherwise.
So as general rule:
- Do not serialize generic values as root value, if possible.
o but if you do, you MUST provide type information separately
o or, sub-class generic type to make it non-generic
Like so:
// instead of using reflection, construct as `TypeReference`
final TypeReference<?> fooType = new TypeReference<Map<String, Foo>>() { };
// (or using `TypeFactory.constructMapType(...)`)
// important: since root value is generic, MUST provide extra info to avoid
// it being seen as `Map<?,?>`
String json = m.writerFor(fooType)
.writeValueAsString(o);
// after which read succeeds:
System.out.println ("5) " + m.readValue(json, fooType));
Or, with a work-around involving sub-classing to get non-generic type:
static class FooMap extends HashMap<String,Foo> { }
and you do NOT need to specify type for serialization because type
information is now available via super-type declaration (so it's in
class definition; runtime type is still type-erased but that does not
matter).
I hope this helps,
-+ Tatu +-
On Tue, Dec 13, 2016 at 11:53 AM, Lev Kuznetsov
<lev.v.kuznet...@gmail.com> wrote:
> Try the code I pasted without the getter, it won't serialize type
> information if the object is a value in a map I'm trying to serialize (and
> same as part of a collection). The output for the code I pasted above is
>
> 1) {"@class":"foo.Test$Bar","lol":"lol"}
>
> 2) .Bar
>
> 3) {f=.Bar}
>
> 4) {"f":{"lol":"lol"}}
>
> Exception in thread "main"
> com.fasterxml.jackson.databind.JsonMappingException: Unexpected token
> (END_OBJECT), expected FIELD_NAME: missing property '@class' that is to
> contain type id (for class foo.Test$Foo)
>
>
> Where the first line is serialization of the object itself, the fourth line
> is serialization of a map mapping the string "f" to the same object, notice
> how "@class" property is missing, followed by exception where it cannot
> deserialize this json back into a map.
>
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