java.lang.ClassCastException: class java.util.HashMap cannot be cast : SpringBoot

Question

We are doing a migration from Spring 5.0 to Spring Boot 2.4 version. My Controller code is as follows

@RequestMapping(value = "/getTreeNodesByFilter.action", method = RequestMethod.GET)
    public @ResponseBody Map<String, ? extends Object> getTreeNodesByFilter(@RequestParam("type") String type,
            @RequestParam("id") Long id, @RequestParam("pageNo") int pageNo, @RequestParam("pageSize") int pageSize,
            @RequestParam("filterBy") String filterBy) {
        Map<String, Object> responseMap = commonService.getTreeNodesByFilter(type, id, pageNo, pageSize, filterBy);
        return ResponseUtil.getMap(responseMap);
    }

The response body is a HashMap is seen above. This code works fine in normal spring webapp. The same code when tired with SpringBoot we are getting the following error

java.lang.ClassCastException: class java.util.HashMap cannot be cast to class com.scriptless.web.security.exceptions.JsonError (java.util.HashMap is in module java.base of loader 'bootstrap'; com.scriptless.web.security.exceptions.JsonError is in unnamed module of loader org.springframework.boot.devtools.restart.classloader.RestartClassLoader @5f00ac7a)

This error is thrown from AbstractMessageConverterMethodProcessor.class of Spring-webmvc.jar while executing the code

((HttpMessageConverter) converter).write(body, selectedMediaType, outputMessage);

Can someone please suggest what has to done to resolve this issue?

Answer 1

Try this objectMapper,

public ObjectMapper objectMapper() {
        ObjectMapper objectMapper = new ObjectMapper()
                .configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false)
                .configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false)
                .configure(MapperFeature.USE_GETTERS_AS_SETTERS, false);
        SimpleModule module = new SimpleModule();

        objectMapper.registerModule(module);
        objectMapper.registerModule(new JavaTimeModule());
        PolymorphicTypeValidator ptv = BasicPolymorphicTypeValidator.builder()
            .allowIfSubType("java.util.") //$NON-NLS-1$
                .build();

        objectMapper.setPolymorphicTypeValidator(ptv);
        objectMapper.activateDefaultTyping(ptv, DefaultTyping.NON_FINAL);
        return objectMapper;
    }

This would allow all subtypes of package java.util.* to deserialize.