本文讲讲mvc的异常处理机制，方便查阅以及编写合理的异常响应方式
入口例子
很简单，根据之前的文章，我们只需要复写WebMvcConfigurer接口的异常添加方法即可，如下
1.创建简单的异常处理类，本例针对绑定异常
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.实现WebMvcConfigurer接口后复写其中的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());
}
}
上述简单的代码便会对系统抛出的BindException异常进行针对性的处理，从而返回合乎格式的响应体。当然这只是一小部分，笔者可以稍微从源码的角度来分析下spring的异常机制
源码层
查阅过DispatcherServlet源码的都知道，当出现异常的时候，则会尝试调用HandlerExceptionResolver解析器去根据异常进行视图渲染或者直接返回对应的错误信息。笔者按步骤来进行简单分析，从WebMvcConfigurationSupport入手
1.异常解析器注册
@Bean
public HandlerExceptionResolver handlerExceptionResolver() {
List<HandlerExceptionResolver> exceptionResolvers = new ArrayList<>();
// 优先加载用户自定义的异常解析器，也可通过WebMvcConfigurer来复写
configureHandlerExceptionResolvers(exceptionResolvers);
// 当用户没有复写上述方法后，采取默认的异常解析器
if (exceptionResolvers.isEmpty()) {
addDefaultHandlerExceptionResolvers(exceptionResolvers);
}
// 扩增异常解析器，可见上文中的例子
extendHandlerExceptionResolvers(exceptionResolvers);
HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite();
composite.setOrder(0);
composite.setExceptionResolvers(exceptionResolvers);
return composite;
}
2.直接看下spring内置的默认异常解析器吧，参考addDefaultHandlerExceptionResolvers()方法
protected final void addDefaultHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers) {
// 1.异常的方法处理，跟@RequestMapping注解的方法调用类似
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.携带@ResponseStatus注解的解析器
ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();
responseStatusResolver.setMessageSource(this.applicationContext);
exceptionResolvers.add(responseStatusResolver);
// 3.默认的异常解析器，针对spring的内置异常作下简单的response
exceptionResolvers.add(new DefaultHandlerExceptionResolver());
}
笔者主要关注ExceptionHandlerExceptionResolver和ResponseStatusExceptionResolver解析器，那就分块来简单的讲解把
ExceptionHandlerExceptionResolver
初始化状态的代码就不罗列了，读者直接阅读源码就知道，笔者此处作下初始化的总结
寻找所有的携带
@ControllerAdvice注解的bean，包装成ExceptionHandlerMethodResolver方法解析器，由此来从中挑选出携带@ExceptionHandler注解的方法集合对第一条中所得的方法集合，读取其中
@ExceptionHandler注解的值(Throwable实现类)；无则读取对应方法实现了Throwable异常接口的参数集合。即得出exceptionTypes集合对上述的exceptionTypes集合依次与对应的method形成映射，即方便针对指定的异常可以调用相应的方法来返回结果
对上述满足条件的ControllerAdvice ，结合ExceptionHandlerMethodResolver装入exceptionHandlerAdviceCache属性map中
封装参数解析器集合与返回值解析器集合，和处理
@RequestMapping的操作一样
具体的解析过程，笔者此处点一下，方便与上文对照着看，直接看关键的getExceptionHandlerMethod()方法
protected ServletInvocableHandlerMethod getExceptionHandlerMethod(
@Nullable HandlerMethod handlerMethod, Exception exception) {
Class<?> handlerType = null;
if (handlerMethod != null) {
// 获取出现异常类方法的所在类
handlerType = handlerMethod.getBeanType();
// 优先判断如果此类直接返回的是异常类，则尝试寻找解析器
ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);
if (resolver == null) {
// 查找异常所在类是否有符合的@ExceptionHandler语法方法
resolver = new ExceptionHandlerMethodResolver(handlerType);
this.exceptionHandlerCache.put(handlerType, resolver);
}
// 得到映射的方法
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());
}
}
// 进入@ControlleAdvice的语法环境了，判断抛异常的所在类，ControllerAdvice是否支持
for (Map.Entry<ControllerAdviceBean, ExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
ControllerAdviceBean advice = entry.getKey();
// 如果@ControllerAdvice注解无任何的属性配置，则默认是支持的
if (advice.isApplicableToBeanType(handlerType)) {
ExceptionHandlerMethodResolver resolver = entry.getValue();
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(advice.resolveBean(), method);
}
}
}
return null;
}
最终就是根据Exception的类型找寻符合条件的method，然后按照@RequestMapping注解的处理方式得到相应的视图对象供视图解析器去渲染
ResponseStatusExceptionResolver
针对携带@ResponseStatus注解的异常类来返回响应体的，简单的看下代码吧
@Override
@Nullable
protected ModelAndView doResolveException(
HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {
try {
// 直接返回的是ResponseStatusException类型的异常则直接处理
if (ex instanceof ResponseStatusException) {
return resolveResponseStatusException((ResponseStatusException) ex, request, response, handler);
}
// 读取异常类上携带的@ResponseStatus注解，有则返回结果
ResponseStatus status = AnnotatedElementUtils.findMergedAnnotation(ex.getClass(), ResponseStatus.class);
if (status != null) {
return resolveResponseStatus(status, request, response, handler, ex);
}
// 递归调用下
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);
}
// 无符合条件的，直接返回null，调用下一个异常解析器
return null;
}
最终调用的也就是HttpServletResponse#sendError(int statusCode,String reason)方法直接返回结果
DispatcherServlet异常处理逻辑
此处还是贴下重要的代码片段，加深印象，直接查阅processHandlerException()方法
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
@Nullable Object handler, Exception ex) throws Exception {
....
if (this.handlerExceptionResolvers != null) {
// 对异常解析器集合进行遍历
for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
// ModelAndView对象不为null则直接跳出，否则采取下一个异常解析器
if (exMv != null) {
break;
}
}
}
}
....
温馨提示:
- 根据上述代码的逻辑可见，用户在自定义相应的异常解析器时，需要注意如果满足解析指定的异常，则最后返回不为null的视图对象(return new ModelAndView())，以免其跑至下一个异常解析器，影响服务执行结果。
- 遍历的异常解析器顺序此处提一下，其采取的是简单的ArrayList集合来保持顺序，所以用户如果想自己的异常解析器保持较高的优先级，则可以采取List接口的add(int index, T value)方法添加或者直接实现HandlerExceptionResolver并设置order属性来保持即可
结语
了解异常解析器的加载机制以及运行逻辑，方便我们写出合乎spring逻辑的代码，以此保证代码的整洁性。