Spring-boot: Spring Boot 2.2.8 XML/JSON mix-up with arbitrary maps

[dlvenable]

Serializing a model which has a Map<String, ?> appears to serialize as XML in all cases.

We found this in:
Spring Boot 2.2.8
Spring Cloud Hoxton.SR6

It appears that the issue does not exist in Spring Boot 2.2.5 and Spring Cloud Hoxton.SR2.

[dlvenable]

I put together a simple app demonstrating the issue:

https://github.com/dlvenable/spring-boot-22415

I created it from the Spring Boot starter. I added a couple of Spring Cloud dependencies that we regularly make use of. I'm unsure if those Spring Cloud dependencies are part of the issue or not.

You may have to run ./gradlew build a few times. The test does not always fail. However, it does fail about 50% of the time for me. It appears that Jackson is trying to deserialize XML.

Sample stack trace:

org.springframework.web.client.RestClientException: Error while extracting response for type [class com.example.demo.model.BrokenModel] and content type [application/xml;charset=UTF-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Unexpected character '2' (code 50) in content after '<' (malformed start element?).
 at [row,col {unknown-source}]: [1,22]; nested exception is com.fasterxml.jackson.core.JsonParseException: Unexpected character '2' (code 50) in content after '<' (malformed start element?).
 at [row,col {unknown-source}]: [1,22]
 at [Source: (PushbackInputStream); line: 1, column: 23]
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:120)
    at org.springframework.web.client.RestTemplate$ResponseEntityResponseExtractor.extractData(RestTemplate.java:998)
    at org.springframework.web.client.RestTemplate$ResponseEntityResponseExtractor.extractData(RestTemplate.java:981)
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:741)
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:674)
    at org.springframework.web.client.RestTemplate.getForEntity(RestTemplate.java:342)
    at org.springframework.boot.test.web.client.TestRestTemplate.getForEntity(TestRestTemplate.java:233)
    at com.example.demo.DemoApplicationTests.GET_broken(DemoApplicationTests.java:29)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:675)
    at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:125)
    at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:132)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:124)
    at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:74)
    at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:104)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:62)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:43)
    at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:35)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
    at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:202)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:198)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:135)
    at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:69)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:135)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at java.base/java.util.ArrayList.forEach(ArrayList.java:1540)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:125)
    at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:135)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:123)
    at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:122)
    at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:80)
    at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
    at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:248)
    at org.junit.platform.launcher.core.DefaultLauncher.lambda$execute$5(DefaultLauncher.java:211)
    at org.junit.platform.launcher.core.DefaultLauncher.withInterceptedStreams(DefaultLauncher.java:226)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:199)
    at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:132)
    at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.processAllTestClasses(JUnitPlatformTestClassProcessor.java:99)
    at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.access$000(JUnitPlatformTestClassProcessor.java:79)
    at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor.stop(JUnitPlatformTestClassProcessor.java:75)
    at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.stop(SuiteTestClassProcessor.java:61)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
    at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
    at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:94)
    at com.sun.proxy.$Proxy2.stop(Unknown Source)
    at org.gradle.api.internal.tasks.testing.worker.TestWorker.stop(TestWorker.java:132)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
    at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
    at org.gradle.internal.remote.internal.hub.MessageHubBackedObjectConnection$DispatchWrapper.dispatch(MessageHubBackedObjectConnection.java:182)
    at org.gradle.internal.remote.internal.hub.MessageHubBackedObjectConnection$DispatchWrapper.dispatch(MessageHubBackedObjectConnection.java:164)
    at org.gradle.internal.remote.internal.hub.MessageHub$Handler.run(MessageHub.java:413)
    at org.gradle.internal.concurrent.ExecutorPolicy$CatchAndRecordFailures.onExecute(ExecutorPolicy.java:64)
    at org.gradle.internal.concurrent.ManagedExecutorImpl$1.run(ManagedExecutorImpl.java:48)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at org.gradle.internal.concurrent.ThreadFactoryImpl$ManagedThreadRunnable.run(ThreadFactoryImpl.java:56)
    at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Unexpected character '2' (code 50) in content after '<' (malformed start element?).
 at [row,col {unknown-source}]: [1,22]; nested exception is com.fasterxml.jackson.core.JsonParseException: Unexpected character '2' (code 50) in content after '<' (malformed start element?).
 at [row,col {unknown-source}]: [1,22]
 at [Source: (PushbackInputStream); line: 1, column: 23]
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:275)
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:257)
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:105)
    ... 95 more
Caused by: com.fasterxml.jackson.core.JsonParseException: Unexpected character '2' (code 50) in content after '<' (malformed start element?).
 at [row,col {unknown-source}]: [1,22]
 at [Source: (PushbackInputStream); line: 1, column: 23]
    at com.fasterxml.jackson.dataformat.xml.util.StaxUtil.throwAsParseException(StaxUtil.java:37)
    at com.fasterxml.jackson.dataformat.xml.deser.FromXmlParser.nextToken(FromXmlParser.java:509)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:365)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:159)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4218)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3267)
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:269)
    ... 97 more
Caused by: com.ctc.wstx.exc.WstxUnexpectedCharException: Unexpected character '2' (code 50) in content after '<' (malformed start element?).
 at [row,col {unknown-source}]: [1,22]
    at com.ctc.wstx.sr.StreamScanner.throwUnexpectedChar(StreamScanner.java:666)
    at com.ctc.wstx.sr.BasicStreamReader.nextFromTree(BasicStreamReader.java:2926)
    at com.ctc.wstx.sr.BasicStreamReader.next(BasicStreamReader.java:1122)
    at com.fasterxml.jackson.dataformat.xml.deser.XmlTokenStream._collectUntilTag(XmlTokenStream.java:425)
    at com.fasterxml.jackson.dataformat.xml.deser.XmlTokenStream._next(XmlTokenStream.java:364)
    at com.fasterxml.jackson.dataformat.xml.deser.XmlTokenStream.next(XmlTokenStream.java:202)
    at com.fasterxml.jackson.dataformat.xml.deser.FromXmlParser.nextToken(FromXmlParser.java:507)
    ... 102 more

[dlvenable]

There are two endpoints:
/ok
/broken

The /broken/ endpoint returns a BrokenModel, which has a field: Map<String, List<Node>> myMap. This is the only test which has failed for me. The OkModel succeeds every time. This is why I think the cause is the arbitrary Map.

Also, we found this issue in our internal code with an object with an arbitrary Map.

[wilkinsona]

Thanks for the sample. I've reproduced the intermittent failure in GET_broken().

I don't think that there's an XML/JSON mixup here. You have jackson-dataformat-xml on the classpath and the request indicates that application/xml is acceptable so I think it's expected for the server to produce XML and, therefore, for the client to be attempting to deserialize that XML. What I don't yet understand is why either broken XML is being served some of the time or, alternatively, why the client-side is unable to deserialize valid XML some of the time.

[wilkinsona]

And having just said I don't understand, I now do.

When serialized to XML, your BrokenModel contains a map that uses the toString() of a UUID as its keys. These keys are serialised into XML elements. It is illegal for an element name to start with a number. Interestingly, Jackson doesn't enforce this during serialisation but does enforce it during deserialisation. Your GET_broken() test fails when the randomly generated UUID starts with 0-9.