ACCEPT_SINGLE_VALUE_AS_ARRAY partially broken in 2.7.x, 2.8.x

[Spikhalskiy]

In 2.7.x, 2.8.x versions following test fails with an exception:

public class Test {
    private static final String JSON = "[{\"message\":\"messageHere\"}]";

    static class A {
        List<B> bs = Collections.emptyList();

        @JsonCreator
        A(final List<B> bs) {
            this.bs = bs;
        }
    }

    static class B {
        List<C> cs = Collections.emptyList();

        @JsonCreator
        B(final List<C> cs) {
            this.cs = cs;
        }
    }

    public static class C {
        String message;

        @JsonCreator
        C(@JsonProperty("message") String message) {
            this.message = message;
        }
    }

    @Test
    public void test() throws IOException {
        ObjectMapper om = new ObjectMapper();
        om.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);
        om.readValue(JSON, A.class);
    }
}


com.fasterxml.jackson.databind.exc.InputMismatchException: Can not construct instance of com.fasterxml.jackson.databind.creators.JsonCreatorWithCollectionTest$B, problem: no suitable constructor found, can not deserialize from Object value (missing default constructor or creator, or perhaps need to add/enable type information?)
 at [Source: [{"message":"site is missing from bid request (breq) object"}]; line: 1, column: 3] (through reference chain: java.util.ArrayList[0])

    at com.fasterxml.jackson.databind.exc.InputMismatchException.from(InputMismatchException.java:58)
    at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1354)
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1019)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1207)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:314)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:148)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:289)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:261)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:26)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromArray(BeanDeserializerBase.java:1336)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:174)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:150)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3806)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2849)
    at com.fasterxml.jackson.databind.creators.JsonCreatorWithCollectionTest.test(JsonCreatorWithCollectionTest.java:51)

While on 2.5 and 2.6 it works fine.

[cowtowncoder]

Thank you for the test. I'll have to re-read this some more to know if this is a bug or not.
The challenge is multiple levels of containment, and to what degree should it work in ambiguous cases.
But perhaps the underlying problem itself is something different. :)

[Spikhalskiy]

@cowtowncoder It works fine in 2.6.x and 2.5.x branches and we use and rely on this ability. So, for us, it's definitely a bug and a blocker for updating... Thanks!

[yuesong]

I run into what I think is the same issue. Here's the test

package com.addthis.codec;

import java.io.IOException;

import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

import org.junit.Assert;
import org.junit.Test;

public class MyTest {

    @Test
    public void test() throws IOException {
        A<String> a = new ObjectMapper().readValue("\"test\"", new TypeReference<A<String>>() {});
        Assert.assertEquals("test", a.value);
    }

    @Test
    public void test2() throws IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);
        A<List<String>> a = objectMapper.readValue("\"test2\"", new TypeReference<A<List<String>>>() {});
        List<String> expected = new ArrayList<>();
        expected.add("test2");
        Assert.assertEquals(expected, a.value);
    }

    public static class A<T> {

        private T value;

        @JsonCreator
        public A(T value) {
            this.value = value;
        }
    }
}

test2 succeeds with 2.5.4, but fails with 2.8.4. The exception is:

com.fasterxml.jackson.databind.JsonMappingException: Can not construct instance of com.addthis.codec.MyTest$A: no String-argument constructor/factory method to deserialize from String value ('test2')
 at [Source: "test2"; line: 1, column: 1]

    at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:270)
    at com.fasterxml.jackson.databind.DeserializationContext.instantiationException(DeserializationContext.java:1456)
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1012)
    at com.fasterxml.jackson.databind.deser.ValueInstantiator._createFromStringFallbacks(ValueInstantiator.java:370)
    at com.fasterxml.jackson.databind.deser.std.StdValueInstantiator.createFromString(StdValueInstantiator.java:315)
    at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromString(BeanDeserializerBase.java:1279)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeOther(BeanDeserializer.java:159)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:150)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3798)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2861)
    at com.addthis.codec.MyTest.test2(MyTest.java:28)
...

[cowtowncoder]

@yuesong thank you for providing an additional test. While it is not certain it's the same issue (error message differs), that sounds plausible.

[cowtowncoder]

Ok; I suspect 2nd failure at least is due to #1010, so that there are 2 separate delegators. And perhaps/probably 1st one too. So there's something I can try to resolve; array-delegator is set correctly, but not used.

[cowtowncoder]

Ok, fixed for 2.8.5.

[Spikhalskiy]

Yay! Thanks @cowtowncoder!