How can I deserialize json to interface

[Daniel Young]

Here is my code:

```java
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

public class TestJackson {

    static interface Entity {
        String getName();
    }

    @Test
    public void test() throws JsonProcessingException {

        String json = "{\"name\": \"good\"}";

        ObjectMapper objectMapper = new ObjectMapper();
        Entity entity = objectMapper.readValue(json, Entity.class);
//        Entity entity = objectMapper.readValue(json, new TypeReference<Entity>(){});

        System.out.println("name: " + entity.getName());
    }
}
```
Get exception:
```java
Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `io.gitlab.donespeak.turtal.TestJackson$Entity` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (String)"{"name": "good"}"; line: 1, column: 1]
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
    at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1589)
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1055)
    at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:265)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4202)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3205)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3173)
    at io.gitlab.donespeak.turtal.TestJackson.test(TestJackson.java:23)
    at io.gitlab.donespeak.turtal.TestJackson.main(TestJackson.java:10)

Process finished with exit code 1
```

I think dynamic proxy could be a solution for deserializing json to a interface.

[Mantas Gridinas]

Whats the issue with generating a concrete type that would have fields for your interface, and then you would have that model class implement your interface? You would still need to provide a concrete type to deserialize to, but later down the chain you would reference to your object only by interface.

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// Mantas

[Tatu Saloranta]

On Sat, Aug 20, 2022 at 11:46 PM Mantas Gridinas <mgri...@gmail.com> wrote:
>
> Whats the issue with generating a concrete type that would have fields for your interface, and then you would have that model class implement your interface? You would still need to provide a concrete type to deserialize to, but later down the chain you would reference to your object only by interface.

There are probably other solutions to this, but one of FasterXML
Jackson modules, "Mr Bean" module:

https://github.com/FasterXML/jackson-modules-base/tree/2.14/mrbean

does this as well. It will be invoked when an attempt is made to
deserialize a non-polymorphic (no `@JsonTypeInfo` or default typing)
for which on explicit concrete type mapping exists.
It only handles relatively basic getter/setter cases, plain Beans.
Contributions for extending functionality would be appreciated of
course, as well (there are a few RFE issues for things like adding
`equals()` and `hashCode()`; default values).
A blog article showing usage:

http://www.cowtowncoder.com/blog/archives/2011/08/entry_459.html

may be helpful as well.

-+ Tatu +-

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