前言
本文素材的來源自業務部門技術負責人一次代碼走查引發的故事，技術負責人在某次走查成員的代碼時，發現他們的業務控制層大量充斥着如下的代碼
@PostMapping("add")
public User add(@RequestBody User user, HttpServletRequest request){
String tenantId = request.getHeader("x-tenantid");
String appId = request.getHeader("x-appid");
user.setAppId(appId);
user.setTenantId(tenantId);
return user;
}
他們的tenantId和appId是作為中繼資料放在請求頭，而業務model又需要tenantId和appId，于是他們團隊的成員就寫出了形如上的代碼，雖然這樣的代碼是能滿足業務要求，但是大面積如上的寫法，都是重複性的代碼，很不優雅。後面這個技術負責人項通過自定義HandlerMethodArgumentResolver的方式來優雅解決這問題，他的代碼形如下
@Data
public class MetaInfo {
private String tenantId;
private String appId;
}
public class MetaInfoHandlerMethodArgumentResolver implements HandlerMethodArgumentResolver {
private RequestResponseBodyMethodProcessor handlerMethodArgumentResolver;
public MetaInfoHandlerMethodArgumentResolver(RequestResponseBodyMethodProcessor handlerMethodArgumentResolver) {
this.handlerMethodArgumentResolver = handlerMethodArgumentResolver;
}
@Override
public boolean supportsParameter(MethodParameter parameter) {
return parameter.hasParameterAnnotation(RequestBody.class) && MetaInfo.class.isAssignableFrom(parameter.getParameterType());
}
@Override
public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer, NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {
MetaInfo metaInfo = (MetaInfo) handlerMethodArgumentResolver.resolveArgument(parameter,mavContainer,webRequest,binderFactory);
HttpServletRequest request = webRequest.getNativeRequest(HttpServletRequest.class);
metaInfo.setAppId(request.getHeader("x-appid"));
metaInfo.setTenantId(request.getHeader("x-tenantid"));
return metaInfo;
}
}
@Configuration
public class WebConfig implements WebMvcConfigurer {
@Autowired
private MetaInfoHandlerMethodArgumentResolver metaInfoHandlerMethodArgumentResolver;
@Bean
@ConditionalOnMissingBean
public MetaInfoHandlerMethodArgumentResolver metaInfoHandlerMethodArgumentResolver(List<HttpMessageConverter<?>> httpMessageConverters){
RequestResponseBodyMethodProcessor handlerMethodArgumentResolver = new RequestResponseBodyMethodProcessor(httpMessageConverters);
return new MetaInfoHandlerMethodArgumentResolver(handlerMethodArgumentResolver);
}
@Override
public void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {
resolvers.add(metaInfoHandlerMethodArgumentResolver);
}
}
當他寫下如下代碼時，按他的想法應該是沒問題才對，但是事實上這個HandlerMethodArgumentResolver卻無法生效，他排查了很久，沒啥頭緒，于是就找我探讨了一下。本文就來聊一下該自定義HandlerMethodArgumentResolver不生效原因
為何自定義的HandlerMethodArgumentResolver不生效
看過springmvc的源碼或者背過springmvc相關八股文的朋友，可能會知道springmvc執行HandlerMethodArgumentResolver，主要是通過HandlerMethodArgumentResolverComposite這個聚合器來進行執行。而HandlerMethodArgumentResolverComposite這個聚合器是如何擷取要執行的HandlerMethodArgumentResolver呢？我們可以直接檢視源碼
@Nullable
private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
HandlerMethodArgumentResolver result = (HandlerMethodArgumentResolver)this.argumentResolverCache.get(parameter);
if (result == null) {
Iterator var3 = this.argumentResolvers.iterator();
while(var3.hasNext()) {
HandlerMethodArgumentResolver resolver = (HandlerMethodArgumentResolver)var3.next();
if (resolver.supportsParameter(parameter)) {
result = resolver;
this.argumentResolverCache.put(parameter, resolver);
break;
}
}
}
return result;
}
看到這個源碼，我想老司機應該會有點頭緒，HandlerMethodArgumentResolverComposite内部是會維護一個key為MethodParameter，值為HandlerMethodArgumentResolver的本地緩存，是以要取HandlerMethodArgumentResolver，就會通過MethodParameter來取。
接着我們在來思考一個問題，源碼中的this.argumentResolvers的是什麼時候放進去的，我們繼續跟蹤源碼會發現，他是通過
public HandlerMethodArgumentResolverComposite addResolvers(@Nullable List<? extends HandlerMethodArgumentResolver> resolvers) {
if (resolvers != null) {
this.argumentResolvers.addAll(resolvers);
}
return this;
}
這個方法進行添加。而addResolvers又是什麼時候被調用的，我們繼續跟蹤源碼，會發現addResolvers，他是會RequestMappingHandlerAdapter的afterPropertiesSet方法中的被調用
@Override
public void afterPropertiesSet() {
if (this.argumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
if (this.initBinderArgumentResolvers == null) {
List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
}
從這個代碼片段，我們可以看到HandlerMethodArgumentResolverComposite初始會添加一些預設的HandlerMethodArgumentResolver
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
而getDefaultArgumentResolvers這方法點開
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;
}
就會發現一堆預設的HandlerMethodArgumentResolver，有經驗的老司機看到這裡，應該就知道為啥自定義HandlerMethodArgumentResolver會失效了吧。
自定義HandlerMethodArgumentResolver會失效的原因是當我們方法中有引入@RequestBody時，他的用到的HandlerMethodArgumentResolver是RequestResponseBodyMethodProcessor，而我們自定義的
HandlerMethodArgumentResolver是通過setCustomArgumentResolvers塞進去，而從源碼我們可以看出，我們自定義的HandlerMethodArgumentResolver是放在預設的HandlerMethodArgumentResolver之後
當我們方法中同時存在@RequestBody和自定義HandlerMethodArgumentResolver，因為他們的Method相同，即MethodParameter一樣，是以argumentResolverCache的key是一樣的，從一開始的源碼我們就可以得知，當key已經找到值時，它就直接傳回了，是以當它找到@RequestBody的HandlerMethodArgumentResolver，它就不會再找自定義的HandlerMethodArgumentResolver，這就會導緻我們自定義的HandlerMethodArgumentResolver不生效
HandlerMethodArgumentResolver不生效的解法
1、方法一：直接去掉方法中的@RequestBody
去掉方法中的@RequestBody，此時方法就不存在解析@RequestBody的HandlerMethodArgumentResolver，是以就隻剩我們自定義的HandlerMethodArgumentResolver必然會執行
2、方法二：提高我們自定義HandlerMethodArgumentResolver的執行順序
具體做法如下
@Configuration
public class HandlerMethodArgumentResolverAutoConfiguration implements InitializingBean{
@Autowired
private RequestMappingHandlerAdapter requestMappingHandlerAdapter;
@Override
public void afterPropertiesSet() throws Exception {
List<HandlerMethodArgumentResolver> argumentResolvers = requestMappingHandlerAdapter.getArgumentResolvers();
List<HandlerMethodArgumentResolver> customArgumentResolvers = new ArrayList<>();
for (HandlerMethodArgumentResolver argumentResolver : argumentResolvers) {
if(argumentResolver instanceof RequestResponseBodyMethodProcessor){
customArgumentResolvers.add(new MetaInfoHandlerMethodArgumentResolver(argumentResolver));
}
customArgumentResolvers.add(argumentResolver);
}
requestMappingHandlerAdapter.setArgumentResolvers(customArgumentResolvers);
}
}
将自定義的HandlerMethodArgumentResolver放在解析@RequestBody的HandlerMethodArgumentResolver之前。調整後，我們測試一下
此時會發現已經有值填充進去了
總結
本文主要講解自定義HandlerMethodArgumentResolver不生效原因與解法，我們可以思考一個問題修改或者填充請求參數，除了利用HandlerMethodArgumentResolver之外，還有沒有其他實作方式？下篇文章揭曉答案
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