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springcloud中feign调用处理mybatis-plus Ipage反序列化问题。

2020年07月23日  | 移动技术网IT编程  | 我要评论

通过feign接口返回的分页列表IPage，出现报错，具体报错内容如下：

feign.codec.DecodeException: Type definition error: [simple type, class com.baomidou.mybatisplus.core.metadata.IPage]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.baomidou.mybatisplus.core.metadata.IPage` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information

通过对该问题的分析，因为com.baomidou.mybatisplus.core.metadata.IPage是一个接口，所以不能构建实例。抽象类型需要映射到实现类型，可以通过自定义反序列化或者包含额外的类型信息。

接下来，我们可以通过这个提示，自定义一个反序列化类来处理该问题，具体代码如下：

public class IPageDeserializer extends StdDeserializer<IPage> {
    protected IPageDeserializer(Class<?> vc) {
        super(vc);
    }

    /**
     * Method that can be called to ask implementation to deserialize
     * JSON content into the value type this serializer handles.
     * Returned instance is to be constructed by method itself.
     * <p>
     * Pre-condition for this method is that the parser points to the
     * first event that is part of value to deserializer (and which
     * is never JSON 'null' literal, more on this below): for simple
     * types it may be the only value; and for structured types the
     * Object start marker or a FIELD_NAME.
     * </p>
     * <p>
     * The two possible input conditions for structured types result
     * from polymorphism via fields. In the ordinary case, Jackson
     * calls this method when it has encountered an OBJECT_START,
     * and the method implementation must advance to the next token to
     * see the first field name. If the application configures
     * polymorphism via a field, then the object looks like the following.
     * <pre>
     *      {
     *          "@class": "class name",
     *          ...
     *      }
     *  </pre>
     * Jackson consumes the two tokens (the <tt>@class</tt> field name
     * and its value) in order to learn the class and select the deserializer.
     * Thus, the stream is pointing to the FIELD_NAME for the first field
     * after the @class. Thus, if you want your method to work correctly
     * both with and without polymorphism, you must begin your method with:
     * <pre>
     *       if (p.getCurrentToken() == JsonToken.START_OBJECT) {
     *         p.nextToken();
     *       }
     *  </pre>
     * This results in the stream pointing to the field name, so that
     * the two conditions align.
     * <p>
     * Post-condition is that the parser will point to the last
     * event that is part of deserialized value (or in case deserialization
     * fails, event that was not recognized or usable, which may be
     * the same event as the one it pointed to upon call).
     * <p>
     * Note that this method is never called for JSON null literal,
     * and thus deserializers need (and should) not check for it.
     *
     * @param p    Parsed used for reading JSON content
     * @param ctxt Context that can be used to access information about
     *             this deserialization activity.
     * @return Deserialized value
     */
    @Override
    public IPage deserialize(JsonParser p, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        JsonNode node = p.getCodec().readTree(p);
        String s  = node.toString();
        ObjectMapper om = new ObjectMapper();
        Page page = om.readValue(s,Page.class);
        return page;
    }
}

在Converter中配置进去即可，代码如下：

@Configuration
public class WebDataConvertConfig extends WebMvcConfigurationSupport {
   

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        converters.add(mappingJackson2HttpMessageConverter());
        super.configureMessageConverters(converters);
    }

    @Bean
    public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter() {
        MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
        ObjectMapper mapper = new ObjectMapper();
        mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

        SimpleModule module = new SimpleModule();
        module.addDeserializer(IPage.class, new IPageDeserializer(IPage.class));
        mapper.registerModule(module);
        
        converter.setSupportedMediaTypes(Arrays.asList(MediaType.APPLICATION_JSON,MediaType.APPLICATION_JSON_UTF8,MediaType.APPLICATION_OCTET_STREAM));
        converter.setObjectMapper(mapper);

        return converter;
    }


}

 

至此，问题解决。

本文地址：https://blog.csdn.net/terminator_J/article/details/107493404

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