title: Deseralizing JSON Objects Using Jackson
body: <p>I'm currently developing a Java web application that consumes the JSON data exposed by Magento REST API. An example of data returned by the api is:</p>
  
  <pre><code>{"1":{"entity_id":"1","name":"Wedding dress","description":"White wedding dress"},"2":{"entity_id":"2","name":"Sunglasses", "description":"Black sunglasses"}}
  </code></pre>
  
  <p>and I have a Java class in my application as below:</p>
  
  <pre><code>public class Item {
  
      @JsonProperty("entity_id")
      private String entityId;
      private String name;
      private String description;
  
      //...
  }
  </code></pre>
  
  <p>I would like to deseralize the data and convert that into an <code>ArrayList&lt;Item&gt;</code> but I keep on getting the error below:</p>
  
  <pre><code>com.fasterxml.jackson.databind.JsonMappingException: Can not deserialize instance of java.util.ArrayList out of START_OBJECT token
   at [Source: java.io.StringReader@1ec0f9f6; line: 1, column: 1]
      at com.fasterxml.jackson.databind.JsonMappingException.from(JsonMappingException.java:164)
      at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:575)
      at com.fasterxml.jackson.databind.DeserializationContext.mappingException(DeserializationContext.java:569)
      at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.handleNonArray(CollectionDeserializer.java:259)
      at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:217)
      at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:207)
      at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:23)
      at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:2888)
      at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2048)
      at pricebeaver.service.MagentoItemServiceImpl.getItemsForPriceCheck(MagentoItemServiceImpl.java:111)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:606)
      at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:318)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:183)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150)
      at org.springframework.aop.framework.adapter.AfterReturningAdviceInterceptor.invoke(AfterReturningAdviceInterceptor.java:50)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.aspectj.AspectJAfterThrowingAdvice.invoke(AspectJAfterThrowingAdvice.java:55)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.framework.adapter.MethodBeforeAdviceInterceptor.invoke(MethodBeforeAdviceInterceptor.java:50)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:90)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:202)
      at com.sun.proxy.$Proxy537.getItemsForPriceCheck(Unknown Source)
      at pricebeaver.controller.ItemController.research(ItemController.java:44)
      at pricebeaver.controller.ItemController$$FastClassByCGLIB$$4a51fec1.invoke(&lt;generated&gt;)
      at net.sf.cglib.proxy.MethodProxy.invoke(MethodProxy.java:191)
      at org.springframework.aop.framework.Cglib2AopProxy$CglibMethodInvocation.invokeJoinpoint(Cglib2AopProxy.java:689)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:150)
      at org.springframework.aop.framework.adapter.AfterReturningAdviceInterceptor.invoke(AfterReturningAdviceInterceptor.java:50)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.aspectj.AspectJAfterThrowingAdvice.invoke(AspectJAfterThrowingAdvice.java:55)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.framework.adapter.MethodBeforeAdviceInterceptor.invoke(MethodBeforeAdviceInterceptor.java:50)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:90)
      at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172)
      at org.springframework.aop.framework.Cglib2AopProxy$DynamicAdvisedInterceptor.intercept(Cglib2AopProxy.java:622)
      at pricebeaver.controller.ItemController$$EnhancerByCGLIB$$6b5ba1e6.research(&lt;generated&gt;)
      at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
      at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
      at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
      at java.lang.reflect.Method.invoke(Method.java:606)
      at org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:213)
      at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:126)
      at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:96)
      at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:617)
      at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:578)
      at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80)
      at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:923)
      at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:852)
      at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:882)
      at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:778)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
      at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
      at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:344)
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214)
      at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:77)
      at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
      at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:256)
      at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:214)
      at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:316)
      at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
      at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
      at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
      at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
      at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
      at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:357)
      at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
      at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
      at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
      at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
      at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
      at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
      at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
      at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
      at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
      at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
      at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
      at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
      at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
      at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
      at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:55)
      at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
      at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
      at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
      at java.lang.Thread.run(Thread.java:724)
  </code></pre>
  
  <p>I did some research and have got some idea this <a href="https://stackoverflow.com/questions/20837856/can-not-deserialize-instance-of-java-util-arraylist-out-of-start-object-token?newreg=af8eae10ba7c46b7aaf57453150188ae">post</a>. I tried to experiment it by changing the returned object into an array and get rid of the entity id preceding each object as below:</p>
  
  <pre><code>[{"entity_id":"1","name":"Wedding dress","description":"White wedding dress"},{"entity_id":"2","name":"Sunglasses", "description":"Black sunglasses"}]
  </code></pre>
  
  <p>It works without a drama after the modification, but unfortunately, I can't really modify the JSON in practice.</p>
  
  <p>This is the line of statement I used to deserialise the JSON response into ArrayList:</p>
  
  <pre><code>List&lt;Item&gt; itemsHolder = new ObjectMapper().readValue(jsonString, new TypeReference&lt;ArrayList&lt;Item&gt;&gt;() { });
  </code></pre>
  
  <p>Can anyone share some insight? I saw some examples where the JSON objects returned does not precede with any ID. Is that because I have done something wrong? Any help is much appreciated.</p>