在进入DispatcherServlet的时候，spring容器会得到一个HttpServletRequest对象，但是我们在使用controller层方法时，经常会使用RequestBody，RequestHeader等注解将该对象封装成更便于操作的对象类型。那么在我们使用RequestBody等注解的时候，中间经历了怎样的转换逻辑。今天就来解析一下。
参数解析逻辑
首先找到代码封装的位置，进入InvocableHandlerMethod的invokeForRequest方法(调用链可以自行找一下，idea的debug功能很方便)。
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
//这行就是获取参数的逻辑
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Arguments: " + Arrays.toString(args));
}
//执行controller方法
return doInvoke(args);
}
protected Object[] getMethodArgumentValues(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
MethodParameter[] parameters = getMethodParameters();
Object[] args = new Object[parameters.length];
...
if (this.argumentResolvers.supportsParameter(parameter)) {
try {
//逐个解析方法参数
args[i] = this.argumentResolvers.resolveArgument(
parameter, mavContainer, request, this.dataBinderFactory);
continue;
}
...
return args;
}
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
//获取对应的参数解析器
HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
if (resolver == null) {
throw new IllegalArgumentException("Unknown parameter type [" + parameter.getParameterType().getName() + "]");
}
//执行解析逻辑
return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
}
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
if (result == null) {
for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
//遍历解析器，supportsParameter方法返回true就可以
if (methodArgumentResolver.supportsParameter(parameter)) {
result = methodArgumentResolver;
this.argumentResolverCache.put(parameter, result);
break;
}
}
}
return result;
}
整个解析的逻辑还比较清晰:
1、 获取方法需要的参数类型，逐个解析
2、 调用解析器的supportsParameter方法，判断是否支持参数类型
3、 调用解析器的resolveArgument方法，解析参数
参数解析示范
在这个就以最常用的RequestHeader,和RequestBody来分析一下解析的流程。
首先是RequestHeader
RequestHeader 对应的解析器为RequestHeaderMethodArgumentResolver。
先查看supportsParameter方法:
public boolean supportsParameter(MethodParameter parameter) {
//判断参数是否为RequestHeader注解标记，并且类型不为map
return (parameter.hasParameterAnnotation(RequestHeader.class) &&
!Map.class.isAssignableFrom(parameter.nestedIfOptional().getNestedParameterType()));
}
接下来查看resolveArgument方法，这个方法在他的父类中:
public final Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
//获取注解信息
NamedValueInfo namedValueInfo = getNamedValueInfo(parameter);
MethodParameter nestedParameter = parameter.nestedIfOptional();
//获取header的name
Object resolvedName = resolveStringValue(namedValueInfo.name);
if (resolvedName == null) {
...
}
//通过name查找
Object arg = resolveName(resolvedName.toString(), nestedParameter, webRequest);
if (arg == null) {
...
}
if (binderFactory != null) {
WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
try {
//处理类型转换之类的问题
arg = binder.convertIfNecessary(arg, parameter.getParameterType(), parameter);
}
...
}
handleResolvedValue(arg, namedValueInfo.name, parameter, mavContainer, webRequest);
return arg;
}
逻辑很清晰，关键代码就在resolveName方法里了，进入该方法:
protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
String[] headerValues = request.getHeaderValues(name);
if (headerValues != null) {
return (headerValues.length == 1 ? headerValues[0] : headerValues);
}
else {
return null;
}
}
这边就可以清晰的看到获取的过程了。
接下来看看RequestBody
RequestBody注解对应的解析器为RequestResponseBodyMethodProcessor
首先还是查看supportsParameter方法:
public boolean supportsParameter(MethodParameter parameter) {
//判断是否有RequestBody注解
return parameter.hasParameterAnnotation(RequestBody.class);
}
接下来查看resolveArgument方法:
public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer,
NativeWebRequest webRequest, @Nullable WebDataBinderFactory binderFactory) throws Exception {
parameter = parameter.nestedIfOptional();
//解析传入请求的请求体
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) {
//如果有需要的话校验相关参数
validateIfApplicable(binder, parameter);
...
}
...
}
return adaptArgumentIfNecessary(arg, parameter);
}
首先解析请求体，其次校验结果，这样就是我们拿到的RequestBody对象了。这个地方解析和校验相关的逻辑，我们是可以做点文章的具体可以查看:SpringBoot踩坑日记-一个非空校验引发的bug
讲到这儿可以发现参数解析的逻辑大同小异，那么springboot到底内置了多少解析器呢？
在DispatcherServlet中，根据mapping获取到了相应的方法后，还需要找到相应的HandlerAdapter去执行。当我们使用RequestMapping注解时，使用的就是RequestMappingHandlerAdapter这个类。查看该类的afterPropertiesSet方法()。
public void afterPropertiesSet() {
// Do this first, it may add ResponseBody advice beans
initControllerAdviceCache();
if (this.argumentResolvers == null) {
//获取解析器的方法就在这里
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
...
}
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
List<HandlerMethodArgumentResolver> resolvers = new ArrayList<>();
// Annotation-based argument resolution
resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), false));
resolvers.add(new RequestParamMapMethodArgumentResolver());
resolvers.add(new PathVariableMethodArgumentResolver());
resolvers.add(new PathVariableMapMethodArgumentResolver());
resolvers.add(new MatrixVariableMethodArgumentResolver());
resolvers.add(new MatrixVariableMapMethodArgumentResolver());
resolvers.add(new ServletModelAttributeMethodProcessor(false));
resolvers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
resolvers.add(new RequestPartMethodArgumentResolver(getMessageConverters(), this.requestResponseBodyAdvice));
resolvers.add(new RequestHeaderMethodArgumentResolver(getBeanFactory()));
resolvers.add(new RequestHeaderMapMethodArgumentResolver());
resolvers.add(new ServletCookieValueMethodArgumentResolver(getBeanFactory()));
resolvers.add(new ExpressionValueMethodArgumentResolver(getBeanFactory()));
resolvers.add(new SessionAttributeMethodArgumentResolver());
resolvers.add(new RequestAttributeMethodArgumentResolver());
// Type-based argument resolution
resolvers.add(new ServletRequestMethodArgumentResolver());
resolvers.add(new ServletResponseMethodArgumentResolver());
resolvers.add(new HttpEntityMethodProcessor(getMessageConverters(), this.requestResponseBodyAdvice));
resolvers.add(new RedirectAttributesMethodArgumentResolver());
resolvers.add(new ModelMethodProcessor());
resolvers.add(new MapMethodProcessor());
resolvers.add(new ErrorsMethodArgumentResolver());
resolvers.add(new SessionStatusMethodArgumentResolver());
resolvers.add(new UriComponentsBuilderMethodArgumentResolver());
// Custom arguments
if (getCustomArgumentResolvers() != null) {
resolvers.addAll(getCustomArgumentResolvers());
}
// Catch-all
resolvers.add(new RequestParamMethodArgumentResolver(getBeanFactory(), true));
resolvers.add(new ServletModelAttributeMethodProcessor(true));
return resolvers;
}
在getDefaultArgumentResolvers方法中，我们可以看到springboot内置了很多的参数解析器。我们上面说到的RequestResponseBodyMethodProcessor，RequestHeaderMethodArgumentResolver就是在这边注入进容器的。