前端传JSON嵌套类，且属性名首字母大写时后端接收的一些坑

因为开发腾讯云业务，要写一个回调函数，规范文档上说会传一个json传给我

{
    "EventType": "ComposeMediaComplete",
    "ComposeMediaCompleteEvent": {
        "TaskId": "1256768367-ComposeMedia-f5ac8127b3b6b85cdc13f237c6005d8",
        "Status": "FINISH",
        "ErrCode": 0,
        "Message": "SUCCESS",
        "Input": {
            "Tracks": [{
                    "Type": "Video",
                    "TrackItems": [{
                        "Type": "Video",
                        "SourceMedia": "5285485487985271487",
                        "AudioOperations": [{
                            "Type": "Volume",
                            "VolumeParam": {
                                "Mute": 1
                            }
                        }]
                    }]
                },
                {
                    "Type": "Audio",
                    "TrackItems": [{
                            "Type": "Empty",
                            "EmptyItem": {
                                "Duration": 5
                            }
                        },
                        {
                            "Type": "Audio",
                            "AudioItem": {
                                "SourceMedia": "5285485487985271488",
                                "Duration": 15
                            }
                        },
                        {
                            "Type": "Audio",
                            "AudioItem": {
                                "SourceMedia": "5285485487985271489",
                                "SourceMediaStartTime": 2,
                                "Duration": 14
                            }
                        }
                    ]
                }
            ],
            "Output": {
                "FileName": "视频合成效果测试",
                "Container": "mp4"
            }
        },
        "Output": {
            "FileType": "mp4",
            "FileId": 5285485487985271490,
            "FileUrl": "http://125676836723.vod2.myqcloud.com/xxx/xxx/xxx.mp4"
        }
    }
}

json结构比较复杂，嵌套类+属性首字母大写
按照格式，我声明了一个DTO类

@Data
public class ComposeCallbackRequest {

    /**
     * 事件类型：默认为ComposeMediaComplete
     */
    private String EventType;

    private ComposeMediaCompleteEvent ComposeMediaCompleteEvent;


    @Data
    class ComposeMediaCompleteEvent{
        private String TaskId;
        private String Status;
        private Integer ErrCode;
        private String Message;
		
        private Output Output;
		//一些不涉及业务的类 略去不再声明
    }

    @Data
    class Output {
        private String FileType;
        private String FileId;
        private String FileUrl;
    }

}

经postman测试，后台收到属性均为null
原因与我声明属性名首字母大写有关：

这是由于JavaBean的规范导致的。一般JavaBean属性都是首字母小写，以驼峰命名格式命名，相应的 getter/setter 方法是 get/set 接上首字母大写的属性名。例如：属性名为clientId，其对应的getter/setter 方法是 getClientId/setClientId。

解决方法是在每个属性名上加@JsonProperty注解，例如@JsonProperty(“EventType”)

加上注解后，报了另一个错误信息：

Resolved [org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot construct instance of com.***.dto.request.ComposeCallbackRequest$Output (although at least one Creator exists): can only instantiate non-static inner class by using default, no-argument constructor; nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of com.***.dto.request.ComposeCallbackRequest$Output (although at least one Creator exists): can only instantiate non-static inner class by using default, no-argument constructor

重点信息：

can only instantiate non-static inner class //只能实例化非静态内部类

在内部类 Output 的声明前加了一个static，问题顺利解决。
放上最终代码：

@Data
public class ComposeCallbackRequest {

    /**
     * 事件类型：默认为ComposeMediaComplete
     */
    @JsonProperty("EventType")
    private String EventType;

    @JsonProperty("ComposeMediaCompleteEvent")
    private ComposeMediaCompleteEvent ComposeMediaCompleteEvent;


    @Data
    class ComposeMediaCompleteEvent{
        @JsonProperty("TaskId")
        private String TaskId;
        @JsonProperty("Status")
        private String Status;
        @JsonProperty("ErrCode")
        private Integer ErrCode;
        @JsonProperty("Message")
        private String Message;

        @JsonProperty("Output")
        private Output Output;

    }

    @Data
    static class Output {
        @JsonProperty("FileType")
        private String FileType;
        @JsonProperty("FileId")
        private String FileId;
        @JsonProperty("FileUrl")
        private String FileUrl;
    }

}