Springboot之HandlerMethodArgumentResolver源码解析
Springboot之HandlerMethodArgumentResolver源码解析
Springboot版本2.0.5.release。
前一篇博客讲解了如何用自定义HandlerMethodArgumentResolver进行参数解析，现在来看看springmvc是如何实现这个的。
来看下RequestMappingHandlerAdapter，
图1
HandlerMethodArgumentResolver是个接口，只有俩个方法，supportsParameter返回true标示支持解析，之后调用resolveArgument将结果作为参数传入，如下List-1所示。
List-1
public interface HandlerMethodArgumentResolver {
boolean supportsParameter(MethodParameter var1);
@Nullable
Object resolveArgument(MethodParameter var1, @Nullable ModelAndViewContainer var2, NativeWebRequest var3, @Nullable WebDataBinderFactory var4) throws Exception;
}RequestMappingHandlerAdapter中有个属性argumentResolvers，是HandlerMethodArgumentResolverComposite，由类名称可以看出使用了组合模式。RequestMappingHandlerAdapter的getDefaultArgumentResolvers，这些是默认的参数处理器。
List-2
private List<HandlerMethodArgumentResolver> getDefaultArgumentResolvers() {
List<HandlerMethodArgumentResolver> resolvers = new ArrayList<HandlerMethodArgumentResolver>();
// 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;
}如下图2所示：
图2
DispatcherServlet的doDispatch方法中，间接调用ServletInvocableHandlerMethod的invokeAndHandle方法，之后会逐个调用HandlerMethodArgumentResolver。所以我们自定义的HandlerMethodArgumentResolver在图2中的步骤9和步骤10处处理。如下List-3所示，遍历所有的参数，对每个参数调用HandlerMethodArgumentResolverComposite的support和resolve方法，HandlerMethodArgumentResolverComposite再委托给各个HandlerMethodArgumentResolver处理。
List-3
private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
MethodParameter[] parameters = getMethodParameters();
Object[] args = new Object[parameters.length];
for (int i = 0; i < parameters.length; i++) {
MethodParameter parameter = parameters[i];
parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
args[i] = resolveProvidedArgument(parameter, providedArgs);
if (args[i] != null) {
continue;
}
if (this.argumentResolvers.supportsParameter(parameter)) {
try {
args[i] = this.argumentResolvers.resolveArgument(
parameter, mavContainer, request, this.dataBinderFactory);
continue;
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
}
throw ex;
}
}
if (args[i] == null) {
throw new IllegalStateException("Could not resolve method parameter at index " +
parameter.getParameterIndex() + " in " + parameter.getMethod().toGenericString() +
": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));
}
}
return args;
}值得注意的是图2中的步骤12，HandlerMethodReturnValueHandler可以改变controller返还的值。
Reference
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