Custom UnmodifiableSetMixin Fails in Jackson 2.7+

Question

I'd like to be able to deserialize an UnmodifiableSet with default typing enabled. To do this I have created an UnmodifiableSetMixin as shown below:

NOTE: You can find a minimal project with all the source code to reproduce this issue at https://github.com/rwinch/jackson-unmodifiableset-mixin

import com.fasterxml.jackson.annotation.JsonCreator;
import com.fasterxml.jackson.annotation.JsonTypeInfo;

import java.util.Set;

@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY)
public abstract class UnmodifiableSetMixin {

    @JsonCreator
    public UnmodifiableSetMixin(Set<?> s) {}
}

I then try to use this to deserialize an empty set.

public class UnmodifiableSetMixinTest {
    static final String EXPECTED_JSON = "[\"java.util.Collections$UnmodifiableSet\",[]]";

    ObjectMapper mapper;

    @Before
    public void setup() {
        mapper = new ObjectMapper();
        mapper.enableDefaultTyping(ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.PROPERTY);
        mapper.addMixIn(Collections.unmodifiableSet(Collections.<String>emptySet()).getClass(), UnmodifiableSetMixin.class);
    }

    @Test
    @SuppressWarnings("unchecked")
    public void read() throws Exception {
        Set<String> foo = mapper.readValue(EXPECTED_JSON, Set.class);
        assertThat(foo).isEmpty();
    }
}

The test passes with Jackson 2.6, but fails using Jackson 2.7+ with the following stack trace:

java.lang.IllegalStateException: No default constructor for [collection type; class java.util.Collections$UnmodifiableSet, contains [simple type, class java.lang.Object]]
    at com.fasterxml.jackson.databind.deser.std.StdValueInstantiator.createUsingDefault(StdValueInstantiator.java:240)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:249)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:26)
    at com.fasterxml.jackson.databind.jsontype.impl.AsArrayTypeDeserializer._deserialize(AsArrayTypeDeserializer.java:110)
    at com.fasterxml.jackson.databind.jsontype.impl.AsArrayTypeDeserializer.deserializeTypedFromArray(AsArrayTypeDeserializer.java:50)
    at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserializeWithType(CollectionDeserializer.java:310)
    at com.fasterxml.jackson.databind.deser.impl.TypeWrappedDeserializer.deserialize(TypeWrappedDeserializer.java:42)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3788)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2779)
    at sample.UnmodifiableSetMixinTest.read(UnmodifiableSetMixinTest.java:36)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:678)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)

Can anyone help me fix the test for Jackson 2.7+ (I'd like it to work for Jackson 2.8.3)?

Answer 1

It turns out that this is a regression in Jackson. I created https://github.com/FasterXML/jackson-databind/issues/1392 which acknowledges the bug.

A workaround that uses a custom deserializer was provided to me via #4078. For example:

@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY)
@JsonDeserialize(using = UnmodifiableSetDeserializer.class)
public abstract class UnmodifiableSetMixin {
    @JsonCreator
    public UnmodifiableSetMixin(Set<?> s) {}
}

public class UnmodifiableSetDeserializer extends JsonDeserializer<Set> {

    @Override
    public Set deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        ObjectMapper mapper = (ObjectMapper) jp.getCodec();
        JsonNode node = mapper.readTree(jp);
        Set<Object> resultSet = new HashSet<Object>();
        if (node != null) {
            if (node instanceof ArrayNode) {
                ArrayNode arrayNode = (ArrayNode) node;
                Iterator<JsonNode> nodeIterator = arrayNode.iterator();
                while (nodeIterator.hasNext()) {
                    JsonNode elementNode = nodeIterator.next();
                    resultSet.add(mapper.readValue(elementNode.toString(), Object.class));
                }
            } else {
                resultSet.add(mapper.readValue(node.toString(), Object.class));
            }
        }
        return Collections.unmodifiableSet(resultSet);
    }
}