ClassCastException when doing serialization using Jackson ObjectMapper - java.lang.object as value in underlying HashMap used in LinkedHashset

Question

Yes, the title is quite confusing, I know!

I'm using Jackson ObjectMapper to serialize a LnkedHashSet of type MyClass. This is how stuff is added to that HashSet

private LinkedHashSet<MyClass> causeEntities;
this.causeEntities.add(new MyClass("foo"));

Now, when Jackson tries to serialize this, it throws a ClassCastException saying java.lang.Object cannot be cast to MyClass. I debugged into what "add" in LinkedHashSet does and it adds it into a Map, with the key as MyClass and value as a dummy Object

private transient HashMap<E,Object> map;
private static final Object PRESENT = new Object();
public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

So, the final LinkedHashSet that Jackson gets contains a HashMap, which in turn is a key-value pair of MyClass-Object, and this Object is giving me a nightmare.

Any ideas on how to tackle this would highly be appreciated. I'm a novice and I would be glad if someone could help, thanks!

2013/11/12 17:32:30,058 SEVERE [com.servicebridge.service] : org.codehaus.jackson.map.JsonMappingException: [Ljava.lang.Object; cannot be cast to [Lcom.MyClass; (through reference chain: com.blah.ABC["details"]->com.blah.DEF["validationAppErrors"]->com.blah.GHI["causeEntity"])
    at org.codehaus.jackson.map.JsonMappingException.wrapWithPath(JsonMappingException.java:218) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.JsonMappingException.wrapWithPath(JsonMappingException.java:183) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.SerializerBase.wrapAndThrow(SerializerBase.java:140) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:158) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:112) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.ObjectArraySerializer.serializeContents(ObjectArraySerializer.java:123) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.ObjectArraySerializer.serializeContents(ObjectArraySerializer.java:29) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.StdArraySerializers$ArraySerializerBase.serialize(StdArraySerializers.java:56) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:446) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:150) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:112) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:446) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:150) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.BeanSerializer.serialize(BeanSerializer.java:112) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.StdSerializerProvider._serializeValue(StdSerializerProvider.java:610) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.StdSerializerProvider.serializeValue(StdSerializerProvider.java:256) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ObjectMapper._configAndWriteValue(ObjectMapper.java:2568) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ObjectMapper.writeValueAsString(ObjectMapper.java:2090) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at com.blah.JKL.invokeService(JKL.java:106) [classes:]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.7.0_11]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) [rt.jar:1.7.0_11]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.7.0_11]
    at java.lang.reflect.Method.invoke(Method.java:601) [rt.jar:1.7.0_11]
    at org.apache.cxf.service.invoker.AbstractInvoker.performInvocation(AbstractInvoker.java:180) [cxf-api-2.7.5.jar:2.7.5]
    at org.apache.cxf.service.invoker.AbstractInvoker.invoke(AbstractInvoker.java:96) [cxf-api-2.7.5.jar:2.7.5]
    at org.apache.cxf.jaxrs.JAXRSInvoker.invoke(JAXRSInvoker.java:198) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.jaxrs.JAXRSInvoker.invoke(JAXRSInvoker.java:100) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.interceptor.ServiceInvokerInterceptor$1.run(ServiceInvokerInterceptor.java:58) [cxf-api-2.7.5.jar:2.7.5]
    at org.apache.cxf.interceptor.ServiceInvokerInterceptor.handleMessage(ServiceInvokerInterceptor.java:94) [cxf-api-2.7.5.jar:2.7.5]
    at org.apache.cxf.phase.PhaseInterceptorChain.doIntercept(PhaseInterceptorChain.java:271) [cxf-api-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.ChainInitiationObserver.onMessage(ChainInitiationObserver.java:121) [cxf-api-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.http.AbstractHTTPDestination.invoke(AbstractHTTPDestination.java:239) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.servlet.ServletController.invokeDestination(ServletController.java:223) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.servlet.ServletController.invoke(ServletController.java:203) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.servlet.ServletController.invoke(ServletController.java:137) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.servlet.CXFNonSpringServlet.invoke(CXFNonSpringServlet.java:158) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.servlet.AbstractHTTPServlet.handleRequest(AbstractHTTPServlet.java:243) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.cxf.transport.servlet.AbstractHTTPServlet.doPost(AbstractHTTPServlet.java:163) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:754) [jboss-servlet-api_3.0_spec-1.0.1.Final.jar:1.0.1.Final]
    at org.apache.cxf.transport.servlet.AbstractHTTPServlet.service(AbstractHTTPServlet.java:219) [cxf-bundle-jaxrs-2.7.5.jar:2.7.5]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:329) [jbossweb-7.0.17.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.17.Final.jar:]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:366) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:99) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:83) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:109) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:83) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:187) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at com.blah.MNO.doFilter(MNO.java:93) [foo-1.0.jar:]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at com.blah.PQR.doFilter(PQR.java:113) [foo-1.0.jar:]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:79) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:378) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:167) [spring-security-web-3.0.4.RELEASE.jar:3.0.4.RELEASE]
    at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:237) [spring-web-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:167) [spring-web-3.0.5.RELEASE.jar:3.0.5.RELEASE]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:280) [jbossweb-7.0.17.Final.jar:]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:248) [jbossweb-7.0.17.Final.jar:]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:275) [jbossweb-7.0.17.Final.jar:]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:161) [jbossweb-7.0.17.Final.jar:]
    at org.jboss.as.web.security.SecurityContextAssociationValve.invoke(SecurityContextAssociationValve.java:165) [jboss-as-web-7.1.3.Final.jar:7.1.3.Final]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:155) [jbossweb-7.0.17.Final.jar:]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102) [jbossweb-7.0.17.Final.jar:]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109) [jbossweb-7.0.17.Final.jar:]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:372) [jbossweb-7.0.17.Final.jar:]
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:877) [jbossweb-7.0.17.Final.jar:]
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:679) [jbossweb-7.0.17.Final.jar:]
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:931) [jbossweb-7.0.17.Final.jar:]
    at java.lang.Thread.run(Thread.java:722) [rt.jar:1.7.0_11]
Caused by: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Lcom.MyClass;
    at com.blah.GHI.getCauseEntity(GHI.java:80) [web-sim-1.0.jar:]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) [rt.jar:1.7.0_11]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) [rt.jar:1.7.0_11]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) [rt.jar:1.7.0_11]
    at java.lang.reflect.Method.invoke(Method.java:601) [rt.jar:1.7.0_11]
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.get(BeanPropertyWriter.java:483) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:418) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    at org.codehaus.jackson.map.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:150) [jackson-mapper-asl-1.9.3.jar:1.9.3]
    ... 75 more

Answer 1

Code that fixed my problem:

Before:

return (MyClass[]) ((this.causeEntities == null ) ? null : this.causeEntities.toArray());

After:

return (MyClass[]) ((this.causeEntities == null ) ? null : this.causeEntities.toArray(new MyClass[this.causeEntities.size()]));

Answer 2

Caused by: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Lcom.MyClass;
    at com.blah.GHI.getCauseEntity(GHI.java:80) [web-sim-1.0.jar:]

I'll hazard a bit of crystal-ball debugging here: based on the stack trace, the error has nothing to do with serialization itself, but rather with trying to cast an Object[] to a MyClass[] in GHI.java.

Are you by some chance using .toArray() on a generic collection with no argument? This will return an Object[] which cannot be subjected to a narrowing cast. In that case, use the one-argument overload, .toArray(new MyClass[0]), to ensure that the created array is of the correct type.

If you are unable to post actual code and have to resort to obfuscation like GHI, JKL, MyClass and so forth, that will make troubleshooting much more difficult.