On Wed, Feb 27, 2019 at 1:38 PM Samuel <[email protected]> wrote:
>
> Following classes:
>
> public class StructuralRule {
>
> private List<Statement> statements;
>
> .. /getter & setter
>
> }
>
>
> @JsonIdentityInfo(generator = ObjectIdGenerators.IntSequenceGenerator.class,
> property = "id")
> @JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "operator")
> @JsonSubTypes({
> @JsonSubTypes.Type(value = StatementEquals.class, name = "equals"),
> @JsonSubTypes.Type(value = StatementIn.class, name = "in"),
> @JsonSubTypes.Type(value = StatementLike.class, name = "like"),
> })
> public abstract class Statement<T> {
> private String key;
> private T value;
>
> public Statement() {
> }
>
> public Statement(String key, T value) {
> this.key = key;
> this.value = value;
> }
> .. /getter & setter
> }
>
>
>
> When serializing a StructuralRule-Object the property "operator" inside
> Statement is serialized too.
>
> But when the List of Statements is serialized the "operator" disappears.
>
> And when a single Statement is serialized then the "operator" is there.
>
>
> new ObjectMapper().writeValueAsString(structuralRule)
>
> {"statements":[{"operator":"equals","id":1,"key":"keyx","value":"valy"}]}
>
> new ObjectMapper().writeValueAsString(structuralRule.getStatements()) //
> missing "operator"
> [{"id":1,"key":"keyx","value":"valy"}]
>
> new ObjectMapper().writeValueAsString(structuralRule.getStatements().get(0))
> {"operator":"equals","id":1,"key":"keyx","value":"valy"}
>
>
> I think it's bug.
>
> What do you think?
It's not a bug in Jackson, but limitation of Java generics, known as
"Type Erasure".
Whereas value of property `statements` is known as `List<Statement>`
(from method/field declaration), runtime value return by
`getStatements()` can only be known as `List<?>` (that is, `List<?
extends Object>`), so element type is known only as `Object or
subtype`.
Because of `Object` does not have type info inclusion enabled, no type
information is included.
So: the main rule is to avoid using generic types as root values. This
mainly affects Collections and Maps.
There are 3 ways to resolve the problem
1. Use a wrapper POJO (non-generic) to contain generic types --
`StructuralRule` works fine
2. Use concrete subtype of generic types -- public class StatementList
extends List<Statement> { }
3. Force use of specific type on serialization too (you will always
need it on deser):
objectMapper.writerFor(new TypeReference<List<Statement>>() { }).
writeValue(listValue);
I hope this helps,
-+ Tatu +-
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