Cannot deserialize instance of `com.xxx. entity class` out of START_ARRAY token at [Source: (PushbackInpu

1. Reproduce the error

I finished writing the interface today 页面按钮排序, as shown in the following code:

@ApiOperationSupport(author = "super先生", order = 8)
@ApiOperation(value = "页面按钮排序")
@PostMapping("/sort/pageButton")
public ReturnResult sortPageButton(
    @Validated @RequestBody SortPageButtonDto sortPageButtonDto,
    BindingResult bindingResult) {
    
    
  BindingParamUtil.checkParam(bindingResult);
  return appPageButtonService.sortPageButton(sortPageButtonDto);
}

When using postmanthe test, the following error is reported:

At this point, immediately check the error message reported by the console, as shown in the following code:

org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize instance of `com.xxx.SortPageButtonDto` out of START_ARRAY token; nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `com.xxx.SortPageButtonDto` out of START_ARRAY token
 at [Source: (PushbackInputStream); line: 1, column: 1]
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:389)
	at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:342)
	at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver.readWithMessageConverters(AbstractMessageConverterMethodArgumentResolver.java:185)
	at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.readWithMessageConverters(RequestResponseBodyMethodProcessor.java:158)
	at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.resolveArgument(RequestResponseBodyMethodProcessor.java:131)
	at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:121)
	at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:170)
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:137)
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894)
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808)
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1063)
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963)
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
	at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:909)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:652)
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:733)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
	at com.github.xiaoymin.knife4j.spring.filter.SecurityBasicAuthFilter.doFilter(SecurityBasicAuthFilter.java:87)
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189)
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162)
 ......

Since there are many error messages, we extract the main error messages, namelyCannot deserialize instance of 'com.xxx.SortPageButtonDto' out of START_ARRAY token at [Source: (PushbackInputStream); line: 1, column: 1]

2. Analysis errors

It is catching up with the recent ChatGPTfire, and I use it to help me analyze errors, as shown in the following figure:

ChatGPTNothing is returned, that is, I can't understand my problem. It seems that I can only troubleshoot errors by myself.

Look at the error message again Cannot deserialize instance of 'com.xxx.SortPageButtonDto' out of START_ARRAY tokenand simply translate it into Chinese无法反序列化SortPageButtonDto类的实例。

Why can't an instance of a class be deserialized SortPageButtonDto?

First, go to the troubleshooting SortPageButtonDtoclass, as shown in the following code:

@AllArgsConstructor
@NoArgsConstructor
@Data
@ApiModel(description = "页面按钮排序请求类")
public class SortPageButtonDto {
    
    

  @Size(min = 1)
  @ApiModelProperty(name = "sortPageButtons", value = "页面按钮请求参数", required = true)
  private List<SortPageButton> sortPageButtons;

  @ApiModel(description = "查询条件请求类")
  @Data
  public class SortPageButton {
    
    
    @NotNull(message = "id不能为空")
    @ApiModelProperty(name = "id", value = "页面按钮id", required = true, example = "1")
    private Long id;

    @NotNull(message = "排序值不能为空")
    @ApiModelProperty(name = "seq", value = "排序值", required = true, example = "1")
    private Long seq;
  }
}

This classSortPageButtonDto is used to receive parameters in the array format passed by the front end, as shown in the following code:

[
    {
    
    
        "id": 1,
        "seq": 20
    },
    {
    
    
        "id": 2,
        "seq": 21
    }
]

Therefore, I SortPageButtonDtodefine an SortPageButtoninner class in the class. SortPageButtonThe class is used as an SortPageButtonDtoattribute of the class to receive the array passed by the front end.

There is no problem in writing this way, but the above error is reported, so I searched for information everywhere.

According to some information, jsonthere is a problem with the above format and needs to be modified to the following format:

{
    
    
    "sortPageButtonDto": [
        {
    
    
            "id": 1,
            "seq": 20
        },
        {
    
    
            "id": 2,
            "seq": 21
        }
    ]
}

After modifying to the above format, use the postmancalling 页面按钮排序interface again. Although no error is reported, the backend cannot receive the parameter, that is, nullthe value, as shown in the following figure:

Since the above method does not work, use the following method to solve it.

3. Solve the problem

Try to modify SortPageButtonDtothe class, delete SortPageButtonthe inner class, as shown in the following code:

@AllArgsConstructor
@NoArgsConstructor
@Data
@ApiModel(description = "页面按钮排序请求类")
public class SortPageButtonDto {
    
    

  @NotNull(message = "id不能为空")
  @ApiModelProperty(name = "id", value = "页面按钮id", required = true, example = "1")
  private Long id;

  @NotNull(message = "排序值不能为空")
  @ApiModelProperty(name = "seq", value = "排序值", required = true, example = "1")
  private Long seq;
}

At the same time, modify the interface code started above /sort/pageButtonas follows:

@ApiOperationSupport(author = "super先生", order = 8)
@ApiOperation(value = "页面按钮排序")
@PostMapping("/sort/pageButton")
public ReturnResult sortPageButton(
    @Validated @RequestBody List<SortPageButtonDto> sortPageButtonDto,
    BindingResult bindingResult) {
    
    
  BindingParamUtil.checkParam(bindingResult);
  return appPageButtonService.sortPageButton(sortPageButtonDto);
}

At this point, restart the project and use postmanthe test again, but still report the same error, as shown in the following figure:

Restore the request parameters in the format in the above figure jsonto the format of the initial request json, and send the request to the backend again, as shown in the figure below:

It can be clearly seen from the above figure that after the above modification, the backend can successfully receive the array parameters passed by the frontend.

After repeated attempts, I finally solved the error, turned off the computer with a sigh of relief, and went home from get off work.

4. Summary at the end of the paper

When this kind of error occurs, it is generally used by the backend 集合或数组to receive parameters from the frontend, as shown in the following code:

//集合接参
@Size(min = 1)
@ApiModelProperty(name = "sortPageButtons", value = "页面按钮请求参数", required = true)
private List<SortPageButton> sortPageButtons;

//数组接参
@Size(min = 1)
@ApiModelProperty(name = "sortPageButtons", value = "页面按钮请求参数", required = true)
private SortPageButton[] sortPageButtons;

However, due to irregular parameter passing, Jacksonthe actual parameter value cannot be obtained when parsing the data, thus reporting the above error.

Therefore, when solving this error, the front and back ends need to be modified together. Otherwise, if the backend is modified but the frontend is not modified, the above error will still be reported.