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详解Spring自定义消息格式转换器及底层源码分析

Spring全家桶实战案例

2023-08-26

导读：详解Spring自定义消息格式转换器及底层源码分析

环境：Springboot2.5.12

假设现在要实现这样的一个消息格式：

入参：

name:张三,age:20

接口接收对象Users

自定义消息转换器

public class CustomHttpMessageConverter extends AbstractHttpMessageConverter<Object> {
  private static Logger logger = LoggerFactory.getLogger(CustomHttpMessageConverter.class) ;    // 这里指明了只要接收参数是Users类型的都能进行转换  @Override  protected boolean supports(Class<?> clazz) {    return Users.class == clazz ;  }  // 读取内容进行内容的转换  @Override  protected Object readInternal(Class<? extends Object> clazz, HttpInputMessage inputMessage)      throws IOException, HttpMessageNotReadableException {    String content = inToString(inputMessage.getBody()) ;    String[] keys = content.split(",") ;    Users instance = null ;    try {      instance = (Users) clazz.newInstance();    } catch (Exception e1) {      e1.printStackTrace() ;    }    for (String key : keys) {      String[] vk = key.split(":") ;      try {        Field[] fields = clazz.getDeclaredFields() ;        for (Field f:fields) {          if (f.getName().equals(vk[0])) {            f.setAccessible(true) ;            Class<?> type = f.getType() ;            if (String.class == type) {              f.set(instance, vk[1]) ;            } else if (Integer.class == type) {              f.set(instance, Integer.parseInt(vk[1])) ;            }            break ;          }        }      } catch (Exception e) {        logger.error("错误：{}", e) ;      }    }    return instance ;  }
  // 如果将返回值以什么形式输出，这里就是调用了对象的toString方法。  @Override  protected void writeInternal(Object t, HttpOutputMessage outputMessage)      throws IOException, HttpMessageNotWritableException {    outputMessage.getBody().write(t.toString().getBytes()) ;  }    @Override  protected boolean canWrite(MediaType mediaType) {    if (mediaType == null || MediaType.ALL.equalsTypeAndSubtype(mediaType)) {      return true;    }    for (MediaType supportedMediaType : getSupportedMediaTypes()) {      if (supportedMediaType.isCompatibleWith(mediaType)) {        return true;      }    }    return false;  }    private String inToString(InputStream is) {    byte[] buf = new byte[10 * 1024] ;    int leng = -1 ;    StringBuilder sb = new StringBuilder() ;    try {      while ((leng = is.read(buf)) != -1) {        sb.append(new String(buf, 0, leng)) ;      }      return sb.toString() ;    } catch (IOException e) {      throw new RuntimeException(e) ;    }  }
}

配置消息转换器

@Configurationpublic class WebConfig implements WebMvcConfigurer {
  @Override  public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {    CustomHttpMessageConverter messageConvert = new CustomHttpMessageConverter() ;    List<MediaType> supportedMediaTypes = new ArrayList<>() ;    supportedMediaTypes.add(new MediaType("application", "fm")) ;    messageConvert.setSupportedMediaTypes(supportedMediaTypes) ;    converters.add(messageConvert) ;    WebMvcConfigurer.super.configureMessageConverters(converters);  }  }

在配置消息转换器时，指明了当前这个消息转换器能够接收的内容类型，也就是客户端请求时需要设定Content-Type为application/fm。

参数对象

public class Users {    private String name ;  private Integer age ;
    @Override  public String toString() {    return "【name = " + this.name + ", age = " + this.age + "】" ;  }  }

Controller接口

@RestController@RequestMapping("/message")public class MessageController {    @PostMapping("/save")  public Users save(@RequestBody Users user) {    System.out.println("接受到内容：" + user) ;    return user ;  }}

测试

请求：

响应

源码分析为何自定义消息转换器时要重写那几个方法：

由于我们的接口参数用@RequestBody 注解了，系统采用了

RequestResponseBodyMethodProcessor这个参数解析器进行参数的处理。

整个处理流程的入口是DispatcherServlet中的这行代码：

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

接着进入
RequestMappingHandlerAdapter#handleInternal方法中的这行代码：

mav = invokeHandlerMethod(request, response, handlerMethod);

接着进入
RequestMappingHandlerAdapter#invokeHandlerMethod方法的这行代码：

invocableMethod.invokeAndHandle(webRequest, mavContainer);

接着进入
ServletInvocableHandlerMethod#invokeAndHandle方法中的这行代码：

Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);

接着进入invokeForRequest方法

@Nullablepublic Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,    Object... providedArgs) throws Exception {
  Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);  if (logger.isTraceEnabled()) {    logger.trace("Arguments: " + Arrays.toString(args));  }  return doInvoke(args);}

接着进入getMethodArgumentValues方法

1、这里就开始判断有没有参数解析器可以处理，如果没有会抛出异常。

这里还会吧找到处理的参数解析器缓存起来

this.argumentResolverCache.put(parameter, result);

这行代码缓存了当前可以处理的解析器。

2、开始解析参数，直接从缓存中获取。因为上一步已经得到了解析器。

得到了解析器后：

进行入选中的方法，这个方法最终会进入父类
AbstractMessageConverterMethodArgumentResolver的如下方法：

protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,    Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
  MediaType contentType;  boolean noContentType = false;  try {    contentType = inputMessage.getHeaders().getContentType();  }  catch (InvalidMediaTypeException ex) {    throw new HttpMediaTypeNotSupportedException(ex.getMessage());  }  if (contentType == null) {    noContentType = true;    contentType = MediaType.APPLICATION_OCTET_STREAM;  }
  Class<?> contextClass = parameter.getContainingClass();  Class<T> targetClass = (targetType instanceof Class ? (Class<T>) targetType : null);  if (targetClass == null) {    ResolvableType resolvableType = ResolvableType.forMethodParameter(parameter);    targetClass = (Class<T>) resolvableType.resolve();  }
  HttpMethod httpMethod = (inputMessage instanceof HttpRequest ? ((HttpRequest) inputMessage).getMethod() : null);  Object body = NO_VALUE;
  EmptyBodyCheckingHttpInputMessage message;  try {    message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
    for (HttpMessageConverter<?> converter : this.messageConverters) {      Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();      GenericHttpMessageConverter<?> genericConverter =          (converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);      if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :          (targetClass != null && converter.canRead(targetClass, contentType))) {        if (message.hasBody()) {          HttpInputMessage msgToUse =              getAdvice().beforeBodyRead(message, parameter, targetType, converterType);          body = (genericConverter != null ? genericConverter.read(targetType, contextClass, msgToUse) :              ((HttpMessageConverter<T>) converter).read(targetClass, msgToUse));          body = getAdvice().afterBodyRead(body, msgToUse, parameter, targetType, converterType);        }        else {          body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);        }        break;      }    }  }  catch (IOException ex) {    throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);  }
  if (body == NO_VALUE) {    if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||        (noContentType && !message.hasBody())) {      return null;    }    throw new HttpMediaTypeNotSupportedException(contentType, this.allSupportedMediaTypes);  }
  MediaType selectedContentType = contentType;  Object theBody = body;  LogFormatUtils.traceDebug(logger, traceOn -> {    String formatted = LogFormatUtils.formatValue(theBody, !traceOn);    return "Read \"" + selectedContentType + "\" to [" + formatted + "]";  });
  return body;}