无法写入 JSON：JsonObject；嵌套异常是 com.fasterxml.jackson.databind.JsonMappingException: JsonObject

Question

春季启动 2.5

    @PostMapping("/cart/product")
    public Response addProduct(@RequestBody Map<String, Object> payloadMap) {
        logger.info("addProduct: payloadMap: " + payloadMap);
        String userName = payloadMap.get("user_name").toString();
        final Product product = new ObjectMapper().convertValue(payloadMap.get("product"), Product.class);
        int quantity = (int) payloadMap.get("quantity");
        Cart findCart = cartRepository.findByUsername(userName);
        if (findCart == null) {
            Cart cart = new Cart();
            cart.setCreated(new Date());
            cart.setUsername(userName);
            cart.addProduct(product, quantity);
            cartRepository.save(cart);
            logger.info("addProduct: success_add_product_to_new_cart: " + cart);
            return ResponseService.getSuccessResponse(GsonUtil.gson.toJsonTree(cart));
        } else {
            findCart.addProduct(product, quantity);
            logger.info("addProduct: before_save_exist_cart: " + findCart);
            cartRepository.save(findCart);
            logger.info("addProduct: success_add_product_to_exist_cart: " + findCart);
            return ResponseService.getSuccessResponse(GsonUtil.gson.toJsonTree(findCart));
        }
    }


public class ResponseService {
    private static final int SUCCESS_CODE = 0;
    private static final String SUCCESS_MESSAGE = "Success";
    private static final int ERROR_CODE = -1;

    private static Logger logger = LogManager.getLogger(ResponseService.class);

    public static Response getSuccessResponse(JsonElement body) {
        Response response = new Response(SUCCESS_CODE, SUCCESS_MESSAGE, body);
        logger.info("getSuccessResponse: response = " + response);
        return response;
    }


import com.google.gson.JsonElement;

public class Response {
    private int code;
    private String message;
    private JsonElement body;

    public Response(int code, String message) {
        this.code = code;
        this.message = message;
    }

    public Response(int code, String message, JsonElement body) {
        this.code = code;
        this.message = message;
        this.body = body;
    }

但是当我尝试返回响应时出现错误：

2020-04-12 12:02:18.825  INFO 9584 --- [nio-8092-exec-1] r.o.s.e.controllers.CartController       : addProduct: success_add_product_to_new_cart: Cart{id=130, username='admin@admin.com', created=Sun Apr 12 12:02:18 EEST 2020, updated=null, productEntities=[
ProductEntity{id=131, created=Sun Apr 12 12:02:18 EEST 2020, updated=null, quantity=1, orders=null, product=
Product{id=132, name='product name', description='product description', created=Tue Mar 10 22:34:15 EET 2020, updated=null, price=11.15, currency='EUR', images=[http://www.gravatar.com/avatar/44444?s=200x200&d=identicon, http://www.gravatar.com/avatar/33333?s=200x200&d=identicon]}}], totalAmount=11.15, currency='EUR'}
2020-04-12 12:02:18.836  INFO 9584 --- [nio-8092-exec-1] r.o.s.e.service.ResponseService          : getSuccessResponse: response = Response{code = 0, message = 'Success', body = '{"id":130,"username":"admin@admin.com","created":"Apr 12, 2020, 12:02:18 PM","productEntities":[{"id":131,"created":"Apr 12, 2020, 12:02:18 PM","quantity":1,"product":{"id":132,"name":"product name","description":"product description","created":"Mar 10, 2020, 10:34:15 PM","price":11.15,"currency":"EUR","images":["http://www.gravatar.com/avatar/44444?s=200x200&d=identicon","http://www.gravatar.com/avatar/33333?s=200x200&d=identicon"]}}],"totalAmount":11.15,"currency":"EUR"}'}
2020-04-12 12:02:18.861  WARN 9584 --- [nio-8092-exec-1] .w.s.m.s.DefaultHandlerExceptionResolver : Resolved [org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: JsonObject; nested exception is com.fasterxml.jackson.databind.JsonMappingException: JsonObject (through reference chain: ru.otus.software_architect.eshop_orders.api.Response["body"]->com.google.gson.JsonObject["asBoolean"])]

Answer 1

Spring Boot 默认使用 jackson 来序列化 json。在响应对象中，您有字段 JsonElement body。它是来自 gson 包的对象，而杰克逊不知道如何序列化它。

解决方案：添加属性（在application.properties 文件中）以使用 gson 而不是 jackson。请注意，Spring Boot 版本很重要。

Spring Boot >= 2.3.0.RELEASE： spring.mvc.converters.preferred-json-mapper=gson

Spring Boot spring.http.converters.preferred-json-mapper=gson

更多信息：

Configuring Spring Boot to use Gson instead of Jackson

Spring Boot 2.3 Release Notes

Answer 2

Application.properties

spring.mvc.converters.preferred-json-mapper=gson

Answer 3

我找到了一种解决方法，即保留 Jackson，但为导致 Jackson 序列化问题的类实现我自己的序列化器。

这是一个骇人听闻的解决方案，但它现在可以工作了。

public class GSONObjectSerializer extends SimpleSerializers {

private static final long serialVersionUID = -8745250727467996655L;

private class EmptySerializer extends StdSerializer<Object> {
    
    /**
     * 
     */
    private static final long serialVersionUID = 5435498165882848947L;

    protected EmptySerializer(Class t) {
        super(t);
    }

    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider provider) throws IOException {
        // --> Will write an empty JSON string
        gen.writeStartObject();
        gen.writeEndObject();
    }
}

@Override
public JsonSerializer<?> findSerializer(SerializationConfig config, JavaType type, BeanDescription beanDesc) {

    // --> Here is to setup which class will be serialised by the custom serializer.
    if (type.isTypeOrSubTypeOf(JsonObject.class) || type.isTypeOrSubTypeOf(StripeResponse.class)) {
        return new EmptySerializer(type.getRawClass());
    }
    return super.findSerializer(config, type, beanDesc);
}

}

你可以像这样注册你的序列化器

@Configuration
public class SerialisationConfig {
    @Bean
    public ObjectMapper createMapper() {
        SimpleModule simpleModule = new SimpleModule();
        simpleModule.setSerializers(new GSONObjectSerializer());
        return  Jackson2ObjectMapperBuilder.json().modules(Arrays.asList(simpleModule)).build();
    }
}