java.lang.StackOverflowError: null JsonWriter.writ

问题:

I know the title isn't the best, but the error is not clear at all to me. The error I have is this:

019-04-15 14:28:45.270 ERROR 12644 --- [nio-9090-exec-6] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Handler dispatch failed; nested exception is java.lang.StackOverflowError] with root cause

java.lang.StackOverflowError: null
    at com.google.gson.stream.JsonWriter.writeDeferredName(JsonWriter.java:401) ~[gson-2.8.5.jar:na]
    at com.google.gson.stream.JsonWriter.value(JsonWriter.java:480) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.TypeAdapters$3.write(TypeAdapters.java:148) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.TypeAdapters$3.write(TypeAdapters.java:133) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:127) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:245) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:127) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:245) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:97) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:61) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:127) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:245) ~[gson-2.8.5.jar:na]
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:69) ~[gson-2.8.5.jar:na]
    at 

[...]

The [...] is because these lines repeat for a crazy amount of time and they're all the same. For what I understood by searching the error, it should be some problem with recursivity and infinite loop, but in my code I don't get where this is.
This is the method I'm calling:

@PostMapping(path=PathConstants.START_ACTION)
    public ResponseEntity<BaseResponse<ProcessInstance>> start(@PathVariable String processDefinitionId, @RequestBody(required=false) Map<String,Object> params) throws FileNotFoundException {

        LOGGER.info("starting process definition with id {}", processDefinitionId);

        ResponseEntity<BaseResponse<ProcessInstance>> response = null;

        ProcessInstance pi = null;
        LocalDateTime time = LocalDateTime.now();
        try {
            if(params!=null) {
                pi = runtimeService.startProcessInstanceById(processDefinitionId, params);
                List<org.flowable.task.api.Task> tasks = taskService.createTaskQuery().list();
                System.out.println("Length " + tasks.get((tasks.size()-1)).getProcessVariables());
                org.flowable.task.api.Task task = taskService.createTaskQuery().processInstanceId(pi.getId()).singleResult();
                if(task==null) {
                    System.out.println("Task is null");
                } else {
                    System.out.println("Info task " + runtimeService.getVariables(pi.getId()));
                }

            }  else {
                System.out.println("Empty ");
                pi = runtimeService.startProcessInstanceById(processDefinitionId);
            }


            response = new ResponseEntity<>(new BaseResponse<>(Constants.OK, pi), HttpStatus.OK);

            MongoOperations.writeLogSuccess(pi ,pi.getProcessDefinitionName(), "OK"); //if it went well
        } catch (FlowableObjectNotFoundException e) {
            String pdn = repositoryService.getProcessDefinition(processDefinitionId).getName();
            MongoOperations.writeLogFailure(time, pdn, "KO not found");
            response = new ResponseEntity<>(new BaseResponse<>(e.getMessage(), null), HttpStatus.NOT_FOUND);
        }
        catch (FlowableException e) { //TEST EXCEPTION WITH FAILSTATUSCODE
            String pdn = repositoryService.getProcessDefinition(processDefinitionId).getName();
            MongoOperations.writeLogFailure(time, pdn, "KO flowable");
            response = new ResponseEntity<>(new BaseResponse<>(e.getMessage(), null), HttpStatus.INTERNAL_SERVER_ERROR); //other exception are managed here
        } catch(Exception e) {
            String pdn = repositoryService.getProcessDefinition(processDefinitionId).getName();
            MongoOperations.writeLogFailure(time, pdn, "KO general");
            response = new ResponseEntity<>(new BaseResponse<>(e.getMessage(), null), HttpStatus.INTERNAL_SERVER_ERROR); //other exception are managed here
        }



        return response;
    }

I really don't understand what's causing this and why. The params I'm passing through post request is a Json string written in this way:

{
    "input":2
}