Serialization of nested generic inheritance fails: can not specialize

[Felk]

When trying to serialize an object, which itself has generic objects, under some circumstances the serialization fails. It's difficult to put the exact circumstances under which the error occurs into words, so I made a minimal testcase.

• Testcase fails on Jackson 2.9.3, 2.9.4, 2.9.5 and 2.9.6 (latest as of now)
  • EDIT: 2.9.7 fails too, but 2.9.8 works again
• When removing all mentions of Serializable, it fails on 2.9.3, 2.9.4 and 2.9.5, but works on 2.9.6, making me believe it has something to do with issue Failed to specialize Map type during serialization where type key type incompatibility overidden via "raw" types #1964 and commit
  de6ca25
  • EDIT: This continues to work on 2.9.7 and 2.9.8

---

static final ObjectMapper MAPPER = new ObjectMapper();
static {
    MAPPER.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
}

public interface NestedValue<T extends Serializable> extends Serializable {}
public interface ValueHandler<T extends Serializable> {}
public class NestedValueHandler<T extends Serializable> implements ValueHandler<NestedValue<T>> {
    public final ValueHandler<T> handler;
    public NestedValueHandler(ValueHandler<T> handler) {
        this.handler = handler;
    }
}

@Test
public void testJacksonFail() throws JsonProcessingException {

    final ValueHandler<NestedValue<String>> obj1 = new NestedValueHandler<>(null);
    // works:
    MAPPER.writeValueAsString(obj1);

    final ValueHandler<NestedValue<NestedValue<String>>> obj2
            = new NestedValueHandler<>(new NestedValueHandler<>(null));
    // fails:
    MAPPER.writeValueAsString(obj2);
}

failure output:

com.fasterxml.jackson.databind.JsonMappingException: Failed to specialize base type <package>.JacksonTest$ValueHandler<java.io.Serializable> as <package>.JacksonTest$NestedValueHandler, problem: Type parameter #1/1 differs; can not specialize java.io.Serializable with <package>.JacksonTest$NestedValue<$1> (through reference chain: <package>.JacksonTest$NestedValueHandler["plugin"])

	at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:391)
	at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:351)
	at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:316)
	at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:727)
	at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
	at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
	at com.fasterxml.jackson.databind.ObjectMapper._configAndWriteValue(ObjectMapper.java:3905)
	at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:3219)
	at <package>.JacksonTest.testJacksonFail(JacksonTest.java:47)
	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.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$2.evaluate(ParentRunner.java:268)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
	at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:47)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:242)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
Caused by: java.lang.IllegalArgumentException: Failed to specialize base type <package>.JacksonTest$ValueHandler<java.io.Serializable> as <package>.JacksonTest$NestedValueHandler, problem: Type parameter #1/1 differs; can not specialize java.io.Serializable with <package>.JacksonTest$NestedValue<$1>
	at com.fasterxml.jackson.databind.type.TypeFactory._bindingsForSubtype(TypeFactory.java:431)
	at com.fasterxml.jackson.databind.type.TypeFactory.constructSpecializedType(TypeFactory.java:401)
	at com.fasterxml.jackson.databind.cfg.MapperConfig.constructSpecializedType(MapperConfig.java:297)
	at com.fasterxml.jackson.databind.DatabindContext.constructSpecializedType(DatabindContext.java:161)
	at com.fasterxml.jackson.databind.ser.BeanPropertyWriter._findAndAddDynamic(BeanPropertyWriter.java:893)
	at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:705)
	at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
	... 28 more

[cowtowncoder]

First of all, thank you for reporting the issue.

2.9.6 did relax some of the checks, which probably explains why some cases now work.
From what I remember this are did seem likely to have remaining challenges... type assignment compatibility handling is tricky, and one challenge is that whereas limits make sense on deserialization (where mis-assignments are problematic), they are not really that useful on serialization side. But unfortunately type handlers do not know the context (nor ideally should have to worry).

[Felk]

The above test fails on Jackson 2.9.7, but succeeds on 2.9.8.
Is this intentional and just not mentioned here, or did something unexpectedly change?

[cowtowncoder]

@Felk there are always fixes in patch versions, but it is possible issues are sometimes duplicated. Looking at 2.9.8 release notes, I'd guess fix for #2155 may have fixed this problem. Do you that looks likely?

[Felk]

Yes. Not only does that exactly describe and solve my issue, it literally is the exact same issue affecting the exact same codebase, because it's reported by a coworker 😄

Thank you very much!

I'll close this issue, as it is resolved in 2.9.8