NPE deserializing collection with @JsonCreator and ACCEPT_CASE_INSENSITIVE_PROPERTIES when JSON and Java property names match

[rue-jw]

Attempting to deserialize JSON fails when the following conditions are met:

• Jackson version >2.8.7 (introduced in 2.8.8)
• ObjectMapper has MapperFeature.ACCEPT_CASE_INSENSITIVE_PROPERTIES enabled.
• Deserializing to @JsonCreator on constructor or factory method
• The @JsonProperty name is identical to the Java POJO property name
• The property is a collection

Stack trace:

feign.FeignException: (was java.lang.NullPointerException) (through reference chain: com.test.jackson.deserialize.Obj["Items"]) reading GET /somepath
	at feign.FeignException.errorReading(FeignException.java:49)
	at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:144)
	at feign.SynchronousMethodHandler.invoke(SynchronousMethodHandler.java:76)
	at feign.ReflectiveFeign$FeignInvocationHandler.invoke(ReflectiveFeign.java:103)
	at com.sun.proxy.$Proxy6.fetchOrder(Unknown Source)
	at com.test.jackson.deserialize.ApiClientTest.fetchOrders_deserializeOrders_success(ApiClientTest.java:76)
	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)
Caused by: com.fasterxml.jackson.databind.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: com.test.jackson.deserialize.Obj["Items"])
	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.deser.BeanDeserializerBase.wrapAndThrow(BeanDeserializerBase.java:1689)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeWithErrorWrapping(BeanDeserializer.java:529)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeUsingPropertyBased(BeanDeserializer.java:416)
	at com.fasterxml.jackson.databind.deser.BeanDeserializerBase.deserializeFromObjectUsingNonDefault(BeanDeserializerBase.java:1265)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:325)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
	at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4001)
	at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3051)
	at feign.jackson.JacksonDecoder.decode(JacksonDecoder.java:65)
	at feign.SynchronousMethodHandler.decode(SynchronousMethodHandler.java:165)
	at feign.SynchronousMethodHandler.executeAndDecode(SynchronousMethodHandler.java:133)
	... 28 more
Caused by: java.lang.NullPointerException
	at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:271)
	at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:244)
	at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:27)
	at com.fasterxml.jackson.databind.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:519)
	at com.fasterxml.jackson.databind.deser.BeanDeserializer._deserializeWithErrorWrapping(BeanDeserializer.java:527)
	... 37 more

Sample maven project attached. Runs unit test against a Feign client with JacksonDecoder.
Vary the Jackson version to 2.8.7 or below and it passes.
Disable ACCEPT_CASE_INSENSITIVE_PROPERTIES and it passes.
Use getters instead of @JsonCreator and it passes.

jackson-test.zip

[cowtowncoder]

Ok at this point I will need a simplified version to remove all unrelated aspects (client, server framework).
See if I can make something that can be run without additional deps.
Test itself does fail as explained.

Interestingly enough, 3.0.0-SNAPSHOT does not fail, so something in handling has been improved.

[cowtowncoder]

Added simplified stand-alone test which reproduces the problem.

[cowtowncoder]

Quick note on work-around: I haven't quite figured out why assignment of value deserializer fails, at low level, but it is due to mismatch of property names (before being unified).
So the work-around, until fix, is to remove discrepancy wrt declaration

private final List<ChildObj> items;

by either adding name match (@JsonProperty("Items")) OR by ignoring field explicitly (@JsonIgnore). Either works around specific problem.

[cowtowncoder]

Looks like there was a conflict in BeanPropertyMap, resulting from separate accessor methods for names that map to same case-insensitive input (Items vs items specifically). Fix is to add second argument to a call.

Unfortunately at this point I think I will only backport the fix to 2.9 branch (in addition to master being updated, although it did not exhibit the bug). I am bit hesitant to try to fix 2.8 since it is nearing end of its maintenance cycle, and since this is not a trivial fix (that is, seems that there is non-zero chance for regressions although all tests obviously pass).
So if at all possible, upgrade to 2.9 would be recommended as soon as possible; and in the meantime use of work-around mentioned above (which will resolve naming problem that triggers the bug).

[rue-jw]

Thank you!