fastjson 能否提供字段级别的null输出控制，类似Jackson2 的 @JsonInclude(value=Include.NON_NULL) 注解

[ldcsaa]

需求或者改进建议

fastjson2 的 JSONWriter.Feature.WriteMapNullValue 是对象级别的，序列化对象时null字段要么全部输出，要么不输出。
我现在有一个场景：校验错误字段validationErrors在有值的时候才输出，没有值就不输出，无论是否设置了 JSONWriter.Feature.WriteMapNullValue。
Jackson2 的 @JsonInclude(value=Include.NON_NULL) 能很好实现这个需求，但fastjson2 我没看到类似的功能

附加信息

源码示例

public class Response<T> implements Serializable
{
	public static final String MSG_OK = "ok";	

	@Schema(title="状态码", example="0", requiredMode=RequiredMode.REQUIRED, nullable=false)
	private Integer statusCode = OK;
	
	@Schema(title="状态描述", example=MSG_OK, requiredMode=RequiredMode.NOT_REQUIRED, nullable=true)
	private String msg = MSG_OK;

	/* fastjson2 如何实现？？？ */
	@JsonInclude(value=Include.NON_NULL)
	@Schema(title="参数校验错误列表", example="name is empty", requiredMode=RequiredMode.NOT_REQUIRED, nullable=true)
	private Map<String, List<String>> validationErrors;
	
	@Schema(title="业务模型对象", example="Any Object", requiredMode=RequiredMode.NOT_REQUIRED, nullable=true)
	private T result;
}

目前的解决办法

我目前是通过自定义 PropertyFilter 实现，每个属性判断都要经过这个Filter，还涉及到反射操作，不知对运行效率有多大影响

@Documented
@Retention(RUNTIME)
@Target(ElementType.FIELD)
public @interface JSONFieldExclude
{
	Exclude value() default Exclude.ALWAYS;
	
	public static enum Exclude
	{
		NULL,
		ABSENT,
		EMPTY,
		ALWAYS
	}
}

public class FastJsonExcludePropertyFilter implements PropertyFilter
{
	@Override
	public boolean apply(Object object, String name, Object value)
	{
		if(object  == null)
			return true;
		
		Class<?> clazz = object.getClass();
		
		if(Map.class.isAssignableFrom(clazz) || Collection.class.isAssignableFrom(clazz) || clazz.isArray())
			return true;
		
		try
		{
			Field field = clazz.getDeclaredField(name);
			
			JSONFieldExclude annotation = field.getAnnotation(JSONFieldExclude.class);
			
			if(annotation == null)
				return true;

			JSONFieldExclude.Exclude exclude = annotation.value();
			
			if(exclude == Exclude.ALWAYS)
				return false;
			else if(exclude == Exclude.NULL)
				return Objects.nonNull(value);
			else if(exclude == Exclude.ABSENT)
			{
				if(Objects.isNull(value))
					return false;
				
				if(value instanceof Optional<?> opt)
					return opt.isPresent();
			}
			else if(exclude == Exclude.EMPTY)
			{
				if(Objects.isNull(value))
					return false;
				
				if(value instanceof Optional<?> opt)
					return opt.isPresent();
				else if(value instanceof String str)
					return !str.isEmpty();
				else if(value instanceof Collection<?> col)
					return !col.isEmpty();
				else if(value instanceof Map<?, ?> map)
					return !map.isEmpty();
				else if(clazz.isArray())
					return Array.getLength(value) > 0;
			}
			else
			{
				throw new IllegalArgumentException("Unexpected value: " + exclude);
			}
		}
		catch(NoSuchFieldException | SecurityException e)
		{
			
		}
		
		return true;
	}

}

[wenshao]

请参考文档 https://github.com/alibaba/fastjson2/blob/main/docs/features_cn.md 中第5点的WriteNulls，如下方式使用

    public static class Bean {
        @JSONField(serializeFeatures = Feature.WriteNulls)
        public int id;
    }

[ldcsaa]

请参考文档 https://github.com/alibaba/fastjson2/blob/main/docs/features_cn.md 中第5点的WriteNulls，如下方式使用

    public static class Bean {
        @JSONField(serializeFeatures = Feature.WriteNulls)
        public int id;
    }

正好相反，我的需求是 null 值不输出~~