调试示例基于注解@RequestBody，请求的入参是json格式的请求，本文主要分析spring解析请求参数的过程。
InvocableHandlerMethod的getMethodArgumentValues方法，会解析请求参数。
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
MethodParameter[] parameters = getMethodParameters();
if (ObjectUtils.isEmpty(parameters)) {
return EMPTY_ARGS;
}
Object[] args = new Object[parameters.length];
for (int i = 0; i < parameters.length; i++) {
MethodParameter parameter = parameters[i];
parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
args[i] = findProvidedArgument(parameter, providedArgs);
if (args[i] != null) {
continue;
}
if (!this.resolvers.supportsParameter(parameter)) {
throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
}
try {
args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
}
catch (Exception ex) {
// Leave stack trace for later, exception may actually be resolved and handled...
if (logger.isDebugEnabled()) {
String exMsg = ex.getMessage();
if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {
logger.debug(formatArgumentError(parameter, exMsg));
}
}
throw ex;
}
}
return args;
}
在上面的代码中：args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);完成对参数的解析。实际是HandlerMethodArgumentResolverComposite对象进行解析的：
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
if (resolver == null) {
throw new IllegalArgumentException("Unsupported parameter type [" +
parameter.getParameterType().getName() + "]. supportsParameter should be called first.");
}
return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}
下面这个方法获取解析参数的解析器：
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
if (result == null) {
for (HandlerMethodArgumentResolver resolver : this.argumentResolvers) {
if (resolver.supportsParameter(parameter)) {
result = resolver;
this.argumentResolverCache.put(parameter, result);
break;
}
}
}
return result;
}
参数MethodParameter包含了处理请求的我们定义的Controller的Method信息，也就是请求方法的完整路径和参数类型信息都在这。据此可以找到解析器。找到之后进入解析器的解析参数方法，也就是类HandlerMethodArgumentResolver的resolveArgument(parameter, mavContainer, webRequest, binderFactory);方法。
@Override
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
parameter = parameter.nestedIfOptional();
Object arg = readWithMessageConverters(webRequest, parameter, parameter.getNestedGenericParameterType());
String name = Conventions.getVariableNameForParameter(parameter);
if (binderFactory != null) {
WebDataBinder binder = binderFactory.createBinder(webRequest, arg, name);
if (arg != null) {
validateIfApplicable(binder, parameter);
if (binder.getBindingResult().hasErrors() && isBindExceptionRequired(binder, parameter)) {
throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());
}
}
if (mavContainer != null) {
mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
}
}
return adaptArgumentIfNecessary(arg, parameter);
}
上述方法主要分以下几步：
首先要冲请求中读取参数值，这需要根据处理请求的controller的方法参数类型，来确定绑定的类型,方法如下：
protected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter,
Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
HttpServletRequest servletRequest = webRequest.getNativeRequest(HttpServletRequest.class);
Assert.state(servletRequest != null, "No HttpServletRequest");
ServletServerHttpRequest inputMessage = new ServletServerHttpRequest(servletRequest);
Object arg = readWithMessageConverters(inputMessage, parameter, paramType);
if (arg == null && checkRequired(parameter)) {
throw new HttpMessageNotReadableException("Required request body is missing: " +
parameter.getExecutable().toGenericString(), inputMessage);
}
return arg;
}
里面的readWithMessageCoverters调用的是父类AbstractMessageConverterMethodArgumentResolver中的方法：
@Nullable
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 = null;
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);
}
finally {
if (message != null && message.hasBody()) {
closeStreamIfNecessary(message.getBody());
}
}
if (body == NO_VALUE) {
if (httpMethod == null || !SUPPORTED_METHODS.contains(httpMethod) ||
(noContentType && !message.hasBody())) {
return null;
}
throw new HttpMediaTypeNotSupportedException(contentType,
getSupportedMediaTypes(targetClass != null ? targetClass : Object.class));
}
MediaType selectedContentType = contentType;
Object theBody = body;
LogFormatUtils.traceDebug(logger, traceOn -> {
String formatted = LogFormatUtils.formatValue(theBody, !traceOn);
return "Read \"" + selectedContentType + "\" to [" + formatted + "]";
});
return body;
}
可以看到这里进行了真正的请求参数读取过程。上述方法首先读取请求参数的类型是什么比如是json还是字符串。然后将请求转化为EmptyBodyCheckingHttpInputMessage对象。在EmptyBodyCheckingHttpInputMessage的构造函数中会读取请求体中的数据。
public EmptyBodyCheckingHttpInputMessage(HttpInputMessage inputMessage) throws IOException {
this.headers = inputMessage.getHeaders();
InputStream inputStream = inputMessage.getBody();
if (inputStream.markSupported()) {
inputStream.mark(1);
this.body = (inputStream.read() != -1 ? inputStream : null);
inputStream.reset();
}
else {
PushbackInputStream pushbackInputStream = new PushbackInputStream(inputStream);
int b = pushbackInputStream.read();
if (b == -1) {
this.body = null;
}
else {
this.body = pushbackInputStream;
pushbackInputStream.unread(b);
}
}
}
对于json类型请求参数，实际解析器是AbstractJackson2HttpMessageConverter的子类完成，在AbstractJackson2HttpMessageConverter的read方法中，如下：
@Override
public Object read(Type type, @Nullable Class<?> contextClass, HttpInputMessage inputMessage)
throws IOException, HttpMessageNotReadableException {
JavaType javaType = getJavaType(type, contextClass);
return readJavaType(javaType, inputMessage);
}
readJavaType方法完成将请求体读取到java对象中的过程。上述javaType对象解析出来之后就是请求体绑定的java类型。
readJavaType方法如下：
private Object readJavaType(JavaType javaType, HttpInputMessage inputMessage) throws IOException {
MediaType contentType = inputMessage.getHeaders().getContentType();
Charset charset = getCharset(contentType);
ObjectMapper objectMapper = selectObjectMapper(javaType.getRawClass(), contentType);
Assert.state(objectMapper != null, "No ObjectMapper for " + javaType);
boolean isUnicode = ENCODINGS.containsKey(charset.name()) ||
"UTF-16".equals(charset.name()) ||
"UTF-32".equals(charset.name());
try {
InputStream inputStream = StreamUtils.nonClosing(inputMessage.getBody());
if (inputMessage instanceof MappingJacksonInputMessage) {
Class<?> deserializationView = ((MappingJacksonInputMessage) inputMessage).getDeserializationView();
if (deserializationView != null) {
ObjectReader objectReader = objectMapper.readerWithView(deserializationView).forType(javaType);
if (isUnicode) {
return objectReader.readValue(inputStream);
}
else {
Reader reader = new InputStreamReader(inputStream, charset);
return objectReader.readValue(reader);
}
}
}
if (isUnicode) {
return objectMapper.readValue(inputStream, javaType);
}
else {
Reader reader = new InputStreamReader(inputStream, charset);
return objectMapper.readValue(reader, javaType);
}
}
catch (InvalidDefinitionException ex) {
throw new HttpMessageConversionException("Type definition error: " + ex.getType(), ex);
}
catch (JsonProcessingException ex) {
throw new HttpMessageNotReadableException("JSON parse error: " + ex.getOriginalMessage(), ex, inputMessage);
}
}
将数据转换成java对象，这里由于请求体是json，使用的是com.fasterxml.jackson.databind包下的ObjectMapper，读取数据到java对象。调用return objectMapper.readValue(inputStream, javaType);方法读取输入流到objectMapper，其中第二个参数是指定提取依据什么类型来提取数据。
到此就将请求数据读取成java对象了。
回到RequestResponseBodyMethodProcessor的resolveArgument方法。代码134行，获取参数名称，然后看是否有binderFactory，如果有会调用它的相关逻辑。主要是在validateIfApplicable完成的：
protected void validateIfApplicable(WebDataBinder binder, MethodParameter parameter) {
Annotation[] annotations = parameter.getParameterAnnotations();
for (Annotation ann : annotations) {
Object[] validationHints = ValidationAnnotationUtils.determineValidationHints(ann);
if (validationHints != null) {
binder.validate(validationHints);
break;
}
}
}
上述方法，会读取方法参数上是否有注解，有的话看下该注解是否要求对参数进行校验，即binder.validate(validationHints);。进入DataBinder的如下方法：
public void validate(Object... validationHints) {
Object target = getTarget();
Assert.state(target != null, "No target to validate");
BindingResult bindingResult = getBindingResult();
// Call each validator with the same binding result
for (Validator validator : getValidators()) {
if (!ObjectUtils.isEmpty(validationHints) && validator instanceof SmartValidator) {
((SmartValidator) validator).validate(target, bindingResult, validationHints);
}
else if (validator != null) {
validator.validate(target, bindingResult);
}
}
}
校验还需要通过SpringValidatorAdapter来做适配：
@Override
public void validate(Object target, Errors errors) {
if (this.targetValidator != null) {
processConstraintViolations(this.targetValidator.validate(target), errors);
}
}
其中targetValidator对象就是我们编写的校验器