springboot情操陶冶-web配置(五)
阿新 • • 發佈：2018-12-12
本文講講mvc的異常處理機制，方便查閱以及編寫合理的異常響應方式
入口例子
很簡單，根據之前的文章，我們只需要複寫WebMvcConfigurer介面的異常新增方法即可，如下
1.建立簡單的異常處理類，本例針對繫結異常
package com.example.demo.web.validation; import com.example.demo.web.model.ResEntity; import com.google.gson.Gson; import org.slf4j.Logger; import org.slf4j.LoggerFactory; import org.springframework.validation.BindException; import org.springframework.validation.ObjectError; import org.springframework.web.servlet.ModelAndView; import org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; /** * @author nanco * ------------- * resolve bindexception * ------------- * @create 18/9/9 */ public class SimpleExceptionResolver extends AbstractHandlerExceptionResolver { private static final Logger EXCEPTION_LOG = LoggerFactory.getLogger(SimpleExceptionResolver.class); private final Map<String, List<String>> errorResultMap = new HashMap<>(2); private final String ERROR_KEY = "error_result"; private Gson gson = new Gson(); @Override protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) { // only process BindException,unless return null to allow the next handler understanding the exception if (BindException.class.isInstance(ex)) { ResEntity resEntity = new ResEntity(); try { BindException bindException = BindException.class.cast(ex); List<ObjectError> allErrors = bindException.getAllErrors(); List<String> resMessages = new ArrayList<>(allErrors.size()); allErrors.stream().forEach(error -> { resMessages.add(error.getDefaultMessage()); }); errorResultMap.put(ERROR_KEY, resMessages); resEntity.setData(errorResultMap); response.getOutputStream().write(gson.toJson(resEntity).getBytes()); } catch (IOException e) { EXCEPTION_LOG.error("process BindException fail.", e); } return new ModelAndView(); } return null; } }
2.實現WebMvcConfigurer介面後複寫其中的extendHandlerExceptionResolvers()方法
package com.example.demo.web.config; import com.example.demo.web.validation.SimpleExceptionResolver; import org.springframework.context.annotation.Configuration; import org.springframework.http.converter.HttpMessageConverter; import org.springframework.web.servlet.HandlerExceptionResolver; import org.springframework.web.servlet.config.annotation.InterceptorRegistry; import org.springframework.web.servlet.config.annotation.WebMvcConfigurer; import java.util.List; /** * @author nanco * ------------- * color the mvc config * ------------- * @create 2018/9/5 **/ @Configuration public class BootWebMvcConfigurer implements WebMvcConfigurer { @Override public void addInterceptors(InterceptorRegistry registry) { } @Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { } @Override public void extendHandlerExceptionResolvers(List<HandlerExceptionResolver> resolvers) { // response first resolvers.add(0, new SimpleExceptionResolver()); } }
上述簡單的程式碼便會對系統丟擲的BindException異常進行鍼對性的處理，從而返回合乎格式的響應體。當然這只是一小部分，筆者可以稍微從原始碼的角度來分析下spring的異常機制
原始碼層
查閱過DispatcherServlet原始碼的都知道，當出現異常的時候，則會嘗試呼叫HandlerExceptionResolver解析器去根據異常進行檢視渲染或者直接返回對應的錯誤資訊。筆者按步驟來進行簡單分析，從WebMvcConfigurationSupport入手
1.異常解析器註冊
@Bean public HandlerExceptionResolver handlerExceptionResolver() { List<HandlerExceptionResolver> exceptionResolvers = new ArrayList<>(); // 優先載入使用者自定義的異常解析器，也可通過WebMvcConfigurer來複寫 configureHandlerExceptionResolvers(exceptionResolvers); // 當用戶沒有複寫上述方法後，採取預設的異常解析器 if (exceptionResolvers.isEmpty()) { addDefaultHandlerExceptionResolvers(exceptionResolvers); } // 擴增異常解析器，可見上文中的例子 extendHandlerExceptionResolvers(exceptionResolvers); HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite(); composite.setOrder(0); composite.setExceptionResolvers(exceptionResolvers); return composite; }
2.直接看下spring內建的預設異常解析器吧，參考addDefaultHandlerExceptionResolvers()方法
protected final void addDefaultHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers) {
// 1.異常的方法處理，跟@RequestMapping註解的方法呼叫類似
ExceptionHandlerExceptionResolver exceptionHandlerResolver = createExceptionHandlerExceptionResolver();
exceptionHandlerResolver.setContentNegotiationManager(mvcContentNegotiationManager());
exceptionHandlerResolver.setMessageConverters(getMessageConverters());
exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());
exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());
if (jackson2Present) {
exceptionHandlerResolver.setResponseBodyAdvice(
Collections.singletonList(new JsonViewResponseBodyAdvice()));
}
if (this.applicationContext != null) {
exceptionHandlerResolver.setApplicationContext(this.applicationContext);
}
exceptionHandlerResolver.afterPropertiesSet();
exceptionResolvers.add(exceptionHandlerResolver);
// 2.攜帶@ResponseStatus註解的解析器
ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();
responseStatusResolver.setMessageSource(this.applicationContext);
exceptionResolvers.add(responseStatusResolver);
// 3.預設的異常解析器，針對spring的內建異常作下簡單的response
exceptionResolvers.add(new DefaultHandlerExceptionResolver());
}筆者主要關注ExceptionHandlerExceptionResolver和ResponseStatusExceptionResolver解析器，那就分塊來簡單的講解把
ExceptionHandlerExceptionResolver
初始化狀態的程式碼就不羅列了，讀者直接閱讀原始碼就知道，筆者此處作下初始化的總結
尋找所有的攜帶
@ControllerAdvice註解的bean，包裝成ExceptionHandlerMethodResolver方法解析器，由此來從中挑選出攜帶@ExceptionHandler註解的方法集合對第一條中所得的方法集合，讀取其中
@ExceptionHandler註解的值(Throwable實現類)；無則讀取對應方法實現了Throwable異常介面的引數集合。即得出exceptionTypes集合對上述的exceptionTypes集合依次與對應的method形成對映，即方便針對指定的異常可以呼叫相應的方法來返回結果
對上述滿足條件的ControllerAdvice ，結合ExceptionHandlerMethodResolver裝入exceptionHandlerAdviceCache屬性map中
封裝引數解析器集合與返回值解析器集合，和處理
@RequestMapping的操作一樣
具體的解析過程，筆者此處點一下，方便與上文對照著看，直接看關鍵的getExceptionHandlerMethod()方法
protected ServletInvocableHandlerMethod getExceptionHandlerMethod(
@Nullable HandlerMethod handlerMethod, Exception exception) {
Class<?> handlerType = null;
if (handlerMethod != null) {
// 獲取出現異常類方法的所在類
handlerType = handlerMethod.getBeanType();
// 優先判斷如果此類直接返回的是異常類，則嘗試尋找解析器
ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);
if (resolver == null) {
resolver = new ExceptionHandlerMethodResolver(handlerType);
this.exceptionHandlerCache.put(handlerType, resolver);
}
// 得到對映的方法
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);
}
// For advice applicability check below (involving base packages, assignable types
// and annotation presence), use target class instead of interface-based proxy.
if (Proxy.isProxyClass(handlerType)) {
handlerType = AopUtils.getTargetClass(handlerMethod.getBean());
}
}
// 進入@ControlleAdvice的語法環境了，判斷拋異常的所在類，ControllerAdvice是否支援
for (Map.Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
ControllerAdviceBean advice = entry.getKey();
if (advice.isApplicableToBeanType(handlerType)) {
ExceptionHandlerMethodResolver resolver = entry.getValue();
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(advice.resolveBean(), method);
}
}
}
return null;
}最終就是根據Exception的型別找尋符合條件的method，然後按照@RequestMapping註解的處理方式得到相應的檢視物件供檢視解析器去渲染
ResponseStatusExceptionResolver
針對攜帶@ResponseStatus註解的異常類來返回響應體的，簡單的看下程式碼吧
@Override
@Nullable
protected ModelAndView doResolveException(
HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {
try {
// 直接返回的是ResponseStatusException型別的異常則直接處理
if (ex instanceof ResponseStatusException) {
return resolveResponseStatusException((ResponseStatusException) ex, request, response, handler);
}
// 讀取異常類上攜帶的@ResponseStatus註解，有則返回結果
ResponseStatus status = AnnotatedElementUtils.findMergedAnnotation(ex.getClass(), ResponseStatus.class);
if (status != null) {
return resolveResponseStatus(status, request, response, handler, ex);
}
// 遞迴呼叫下
if (ex.getCause() instanceof Exception) {
ex = (Exception) ex.getCause();
return doResolveException(request, response, handler, ex);
}
}
catch (Exception resolveEx) {
logger.warn("ResponseStatus handling resulted in exception", resolveEx);
}
// 無符合條件的，直接返回null，呼叫下一個異常解析器
return null;
}最終呼叫的也就是HttpServletResponse#sendError(int statusCode,String reason)方法直接返回結果
DispatcherServlet異常處理邏輯
此處還是貼下重要的程式碼片段，加深印象，直接查閱processHandlerException()方法
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
@Nullable Object handler, Exception ex) throws Exception {
....
if (this.handlerExceptionResolvers != null) {
// 對異常解析器集合進行遍歷
for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
// ModelAndView物件不為null則直接跳出，否則採取下一個異常解析器
if (exMv != null) {
break;
}
}
}
}
....溫馨提示:
- 根據上述程式碼的邏輯可見，使用者在自定義相應的異常解析器時，需要注意如果滿足解析指定的異常，則最後返回不為null的檢視物件(return new ModelAndView())，以免其跑至下一個異常解析器，影響服務執行結果。
- 遍歷的異常解析器順序此處提一下，其採取的是簡單的ArrayList集合來保持順序，所以使用者如果想自己的異常解析器保持較高的優先順序，則可以採取List介面的add(int index, T value)方法新增或者直接實現HandlerExceptionResolver並設定order屬性來保持即可
結語
瞭解異常解析器的載入機制以及執行邏輯，方便我們寫出合乎spring邏輯的程式碼，以此保證程式碼的整潔性。