handler是如何完成线程切换的（项目自定义HandlerMethodArgumentResolver不生效原因与解法）
handler是如何完成线程切换的（项目自定义HandlerMethodArgumentResolver不生效原因与解法）List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers(); this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers); 而getDefaultArgumentResolvers这方法点开@Override public void afterPropertiesSet() { if (this.argumentResolvers == null) { List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers(); this
前言本文素材的来源自业务部门技术负责人一次代码走查引发的故事，技术负责人在某次走查成员的代码时，发现他们的业务控制层大量充斥着如下的代码
@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;
}
他们的tenantId和appId是作为元数据放在请求头，而业务model又需要tenantId和appId，于是他们团队的成员就写出了形如上的代码，虽然这样的代码是能满足业务要求，但是大面积如上的写法，都是重复性的代码，很不优雅。后面这个技术负责人项通过自定义HandlerMethodArgumentResolver的方式来优雅解决这问题，他的代码形如下
@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);
}
}
当他写下如下代码时，按他的想法应该是没问题才对，但是事实上这个HandlerMethodArgumentResolver却无法生效，他排查了很久，没啥头绪，于是就找我探讨了一下。本文就来聊一下该自定义HandlerMethodArgumentResolver不生效原因
为何自定义的HandlerMethodArgumentResolver不生效看过springmvc的源码或者背过springmvc相关八股文的朋友，可能会知道springmvc执行HandlerMethodArgumentResolver，主要是通过HandlerMethodArgumentResolverComposite这个聚合器来进行执行。而HandlerMethodArgumentResolverComposite这个聚合器是如何获取要执行的HandlerMethodArgumentResolver呢？我们可以直接查看源码
@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;
}
看到这个源码，我想老司机应该会有点头绪，HandlerMethodArgumentResolverComposite内部是会维护一个key为MethodParameter，值为HandlerMethodArgumentResolver的本地缓存，因此要取HandlerMethodArgumentResolver，就会通过MethodParameter来取。
接着我们在来思考一个问题，源码中的this.argumentResolvers的是什么时候放进去的，我们继续跟踪源码会发现，他是通过
public HandlerMethodArgumentResolverComposite addResolvers(@Nullable List<? extends HandlerMethodArgumentResolver> resolvers) {
if (resolvers != null) {
this.argumentResolvers.addAll(resolvers);
}
return this;
}
这个方法进行添加。而addResolvers又是什么时候被调用的，我们继续跟踪源码，会发现addResolvers，他是会RequestMappingHandlerAdapter的afterPropertiesSet方法中的被调用
@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);
}
从这个代码片段，我们可以看到HandlerMethodArgumentResolverComposite初始会添加一些默认的HandlerMethodArgumentResolver
List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
而getDefaultArgumentResolvers这方法点开
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;
}
就会发现一堆默认的HandlerMethodArgumentResolver，有经验的老司机看到这里，应该就知道为啥自定义HandlerMethodArgumentResolver会失效了吧。
自定义HandlerMethodArgumentResolver会失效的原因是当我们方法中有引入@RequestBody时，他的用到的HandlerMethodArgumentResolver是RequestResponseBodyMethodProcessor，而我们自定义的
HandlerMethodArgumentResolver是通过setCustomArgumentResolvers塞进去，而从源码我们可以看出，我们自定义的HandlerMethodArgumentResolver是放在默认的HandlerMethodArgumentResolver之后
当我们方法中同时存在@RequestBody和自定义HandlerMethodArgumentResolver，因为他们的Method相同，即MethodParameter一样，因此argumentResolverCache的key是一样的，从一开始的源码我们就可以得知，当key已经找到值时，它就直接返回了，因此当它找到@RequestBody的HandlerMethodArgumentResolver，它就不会再找自定义的HandlerMethodArgumentResolver，这就会导致我们自定义的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，此时方法就不存在解析@RequestBody的HandlerMethodArgumentResolver，因此就只剩我们自定义的HandlerMethodArgumentResolver必然会执行
2、方法二：提高我们自定义HandlerMethodArgumentResolver的执行顺序
具体做法如下
@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);
}
}
将自定义的HandlerMethodArgumentResolver放在解析@RequestBody的HandlerMethodArgumentResolver之前。调整后，我们测试一下
此时会发现已经有值填充进去了
本文主要讲解自定义HandlerMethodArgumentResolver不生效原因与解法，我们可以思考一个问题修改或者填充请求参数，除了利用HandlerMethodArgumentResolver之外，还有没有其他实现方式？下篇文章揭晓答案
