Expected STRING but was BEGIN_OBJECT

Question

I've read a lot of answers about this error and I don't find the solution for mine maybe the class object isn't the same

My error is :

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected STRING but was BEGIN_OBJECT

So, I just trying to get an Object from JSON like this :

JsonObject mS1SystemStatusJsonObject = parameterEntries.getValue().getAsJsonObject();

MS1SystemStatusResponse mS1SystemStatusResponse = gson.fromJson(mS1SystemStatusJsonObject, MS1SystemStatusResponse.class);

my JsonObject is :

{
 "WARNING_CONDITION_LIST":
  {
   "warningConditionList": {"warningCondition":"BLACK_POSTAL_INK_WF_COND"}
   },
 "DISABLING_CONDITION_LIST": {"disablingConditionList":""},
 "CurrentErrorList":"",
 "LockState":"lockspend",
"SystemReadyState":true
}

My differents POJO (i don't put get and set for short the code) :

public class MS1SystemStatusResponse{

@SerializedName("LockState")
String lockState;

@SerializedName("SystemReadyState")
Boolean systemReadyState;

@SerializedName("WARNING_CONDITION_LIST")
WARNING_CONDITION_LIST warningConditionListMy;

@SerializedName("DISABLING_CONDITION_LIST")
DISABLING_CONDITION_LIST disablingConditionListMy;

@JsonAdapter(EmptyStringAsNullTypeAdapter.class)    
@SerializedName("CurrentErrorList")
CurrentErrorList currentErrorList;


public class WARNING_CONDITION_LIST {

@SerializedName(value = "warningConditionList")
@JsonAdapter(EmptyStringAsNullTypeAdapter.class)
WarningConditionList warningConditionList;


public class DISABLING_CONDITION_LIST {

@SerializedName(value = "disablingConditionList")
@JsonAdapter(EmptyStringAsNullTypeAdapter.class)
DisablingConditionList disablingConditionList;

public class CurrentErrorList {

@SerializedName(value = "Error")
private Error error;


public class WARNING_CONDITION_LIST {

@SerializedName(value = "warningConditionList")
@JsonAdapter(EmptyStringAsNullTypeAdapter.class)
WarningConditionList warningConditionList;


public class WarningConditionList {

@SerializedName(value = "warningCondition")
String warningCondition;

Here is the full stack trace and another class that i've forgot to add before :

Stacktrace :

java.lang.IllegalStateException: Expected STRING but was BEGIN_OBJECT
    com.google.gson.internal.bind.JsonTreeReader.nextString(JsonTreeReader.java:154)
    com.pb.marqueo.connect.plus.api.common.model.response.EmptyStringAsNullTypeAdapter.read(EmptyStringAsNullTypeAdapter.java:24)
    com.pb.marqueo.connect.plus.api.common.model.response.EmptyStringAsNullTypeAdapter.read(EmptyStringAsNullTypeAdapter.java:10)
    com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:117)
    com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:217)
    com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:117)
    com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:217)
    com.google.gson.Gson.fromJson(Gson.java:861)
    com.google.gson.Gson.fromJson(Gson.java:926)
    com.google.gson.Gson.fromJson(Gson.java:899)
    com.pb.marqueo.connect.plus.api.core.service.impl.ConnectPlusServicesImpl.getMS1SystemStatus(ConnectPlusServicesImpl.java:410)
    com.pb.marqueo.connect.plus.api.core.facade.impl.ConnectPlusBusinessServicesImpl.startProduction(ConnectPlusBusinessServicesImpl.java:1236)
    com.pb.marqueo.connect.plus.rest.services.impl.ConnectPlusControllerImpl.startProd(ConnectPlusControllerImpl.java:280)
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    java.lang.reflect.Method.invoke(Method.java:498)
    org.springframework.web.method.support.InvocableHandlerMethod.invoke(InvocableHandlerMethod.java:215)
    org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:132)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:749)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:689)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:938)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:870)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
    org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:644)
    org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:725)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    com.pb.marqueo.common.filter.OriginFilter.doFilter(OriginFilter.java:70)

Class Adapter :

public class EmptyStringAsNullTypeAdapter extends TypeAdapter<String> {

    @Override
    public void write(JsonWriter out, String value) throws IOException {
        out.value(value);
    }

    @Override
    public String read(JsonReader in) throws IOException {
        if (in.peek() == JsonToken.NULL) {
            in.nextNull();
            return null;
        }

        String result = in.nextString();
        if ("".equals(result)) {
            return null;
        }
        return result;
    }
}

I turn around since 1 week :(

Answer 1

Ok, so from the conversation in the comments I understood that you want to be able to deserialize both of these cases:

{
  "WARNING_CONDITION_LIST": {
      "warningConditionList": {"warningCondition":"BLACK_POSTAL_INK_WF_COND"}
  },
  ...
}

and

{
  "WARNING_CONDITION_LIST": ""
  ...
}

In the first scenario, you want the object and in the 2nd scenario, you want null .

The first thing I noticed is that you have the @JsonAdapter annotation in the wrong place. It should actually be in the WARNING_CONDITION_LIST field. Something like this:

public class MS1SystemStatusResponse {
  // ...
  @SerializedName("WARNING_CONDITION_LIST")
  @JsonAdapter(EmptyStringAsNullTypeAdapter.class)
  WARNING_CONDITION_LIST warningConditionListMy;
  // ...
}

However, this alone will not solve your problem. The problem is within the EmptyStringAsNullTypeAdapter . It expects the field to just be a string and doesn't handle the case when it can be an object. For this, we can write the following adapter:

public class EmptyStringAsEmptyWarningConditionlTypeAdapter extends TypeAdapter<WARNING_CONDITION_LIST> {

  @Override
  public void write(JsonWriter out, WARNING_CONDITION_LIST value) throws IOException {
    out.beginObject();
    out.name("warningConditionList");
    out.value(value.warningConditionList.warningCondition);
    out.endObject();
  }

  @Override
  public WARNING_CONDITION_LIST read(JsonReader in) throws IOException {
    WARNING_CONDITION_LIST result = new WARNING_CONDITION_LIST();
    result.warningConditionList =  new WarningConditionList();
    result.warningConditionList.warningCondition = "";

    if (in.peek() == JsonToken.BEGIN_OBJECT) {
        in.beginObject();
        in.nextName();
        in.beginObject();
        in.nextName();
        result.warningConditionList.warningCondition = in.nextString();
        in.endObject();
        in.endObject();
    } else {
        in.nextString();
        return null;
    }
    return result;
  }
}

Essentially it will check if you have a string or an object and return null or the object respectively.

This works only for these 2 cases that I've shown you with the json and should be enough to help you out figuring the rest.

You'll probably face a similar issue with the CurrentErrorList .