Jackson自定义反序列化器及简单使用

适用情况:在使用jackson时,遇到转特殊对象不能成功时,这时需要自定义转换规则.

本文以以json字符串转jsonorg.springframework.validation.FieldError对象为例.

一. 问题:FieldError中字段private修饰字段只有getter方法,所以直接用常规转Bean方式是失败的.以下是FieldError部分源码:

@SuppressWarnings("serial")
public class FieldError extends ObjectError {

    private final String field;

    @Nullable
    private final Object rejectedValue;

    private final boolean bindingFailure;

    /**
     * Create a new FieldError instance.
     * @param objectName the name of the affected object
     * @param field the affected field of the object
     * @param defaultMessage the default message to be used to resolve this message
     */
    public FieldError(String objectName, String field, String defaultMessage) {
        this(objectName, field, null, false, null, null, defaultMessage);
    }

    /**
     * Create a new FieldError instance.
     * @param objectName the name of the affected object
     * @param field the affected field of the object
     * @param rejectedValue the rejected field value
     * @param bindingFailure whether this error represents a binding failure
     * (like a type mismatch); else, it is a validation failure
     * @param codes the codes to be used to resolve this message
     * @param arguments the array of arguments to be used to resolve this message
     * @param defaultMessage the default message to be used to resolve this message
     */
    public FieldError(String objectName, String field, @Nullable Object rejectedValue, boolean bindingFailure,
            @Nullable String[] codes, @Nullable Object[] arguments, @Nullable String defaultMessage) {

        super(objectName, codes, arguments, defaultMessage);
        Assert.notNull(field, "Field must not be null");
        this.field = field;
        this.rejectedValue = rejectedValue;
        this.bindingFailure = bindingFailure;
    }

...
}

二 解决:自定义反序列化器.

1.定义FieldErrorDeserializer,实现JsonDeserializer,覆写deserialize方法,通过FieldError构造方法进行转换.
public class FieldErrorDeserializer extends JsonDeserializer<FieldError> {

    @Override
    public FieldError deserialize(JsonParser jsonParser, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        ObjectCodec oc = jsonParser.getCodec();
        JsonNode node = oc.readTree(jsonParser);
        
        String objectName = JsonUtil.getJsonNodeAsText(node, "objectName");    
        String defaultMessage = JsonUtil.getJsonNodeAsText(node, "defaultMessage");
        String field = JsonUtil.getJsonNodeAsText(node, "field");
        String[] codes = JsonUtil.getJsonNodeAsStringArray(node, "codes");
        
        String[] arguments = JsonUtil.getJsonNodeAsStringArray(node, "arguments");
        if(!ArrayUtils.isEmpty(arguments) && StringUtils.isEmpty(field) ){
            for(JsonNode temp:node.get("arguments")){
                field = JsonUtil.getJsonNodeAsText(temp, "code");
            }    
        }    
        
        return new FieldError(objectName, field, null, false, codes, arguments, defaultMessage);
    }

}

2.将自定义的反序列化器,"注册"到jackson中.定义SelfJackson2Helper实现Jackson2Helper,覆写buildObjectMapper方法.

public class SelfJackson2Helper extends ch.mfrey.jackson.antpathfilter.Jackson2Helper{
    
    public Jackson2Helper(){
        super();
    }
    
    public ObjectMapper buildObjectMapper(final String... filters) {        
        ObjectMapper oObjectMapper = super.buildObjectMapper(filters);
        oObjectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
        oObjectMapper.configure(SerializationFeature.WRITE_NULL_MAP_VALUES, false);
        oObjectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
        oObjectMapper.configure(SerializationFeature.FAIL_ON_SELF_REFERENCES, false);
        oObjectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        oObjectMapper.getSerializerProvider().setNullValueSerializer(new ObjectNullSerializer());
        //If property don't have @json annotation, will ingore the error. 
        oObjectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        //Allow json don't use standard format
        oObjectMapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_FIELD_NAMES, true);
        
        SimpleModule module = new SimpleModule();
        // add FieldErrorDeserializer() to Jackson.
        module.addDeserializer(FieldError.class, new FieldErrorDeserializer()); 
        oObjectMapper.registerModule(module); 
        return oObjectMapper;
    }
    
    public SimpleFilterProvider buildFilterProvider(final String... filters) {
        SimpleFilterProvider oSimpleFilterProvider = super.buildFilterProvider(filters);
        oSimpleFilterProvider.setFailOnUnknownId(false);
        //oSimpleFilterProvider.addFilter("customFilter", new AntPathPropertyFilter(filters));
        return oSimpleFilterProvider;
    }

}

3.使用, 以下是简单验证.

public class JsonUtil {
    // json2Bean
    public static <T> T convertValue(Object value,TypeReference<?> toValueTypeRef){
        T result = null;
        SelfJackson2Helper jackson2Helper = ApplicationContextHolder.getContext().getBean(SelfJackson2Helper.class);
        ObjectMapper mapper = jackson2Helper.buildObjectMapper();
        try {
            result = mapper.convertValue(value, toValueTypeRef);
        } catch (Exception e) {
            LOGGER.error("JsonUtil.convertValue: "+e.getMessage());
        }
        return result;
    }
    //main 方法验证...
    public static void main(String[] args) {

    BindingResult bindingResult = new BeanPropertyBindingResult();
    bindingResult.rejectValue("sampleField", "msg_sampleField_not_null");//error
    
    Map<String,Object> map = new HashMap<String,String>();
    map.put("errors",bindingResult.getAllErrors())
    //构造一个FieldError的jsonStr.
    String jsonStr = objectMapper.writeValueAsString(map);
    //转为FieldError对象.
    List<FieldError> errorList = convertValue(jsonStr,new TypeReference<List<FieldError>>(){});
    
    errorList.stream().forEach(error -> {
             System.out.println(error.getField());
         });
    }
}