本文分享了Spring MVC自定义异常解析及在源码中的处理过程
1. HandlerExceptionResolver
Spring MVC定义了HandlerExceptionResolver接口，实现该接口的类负责解析异常。
public interface HandlerExceptionResolver {
@Nullable
ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response,
@Nullable Object handler, Exception ex);
}2. 自定义异常解析
实现HandlerExceptionResolver接口并启用即可自定义异常解析器。Spring MVC中的AbstractHandlerExceptionResolver实现了该接口，所以继承此类即可自定义异常处理逻辑。
@Component
public class GlobalHandlerExceptionResolver extends AbstractHandlerExceptionResolver {
private final HandlerExceptionResolver defaultExceptionResolver;
public GlobalHandlerExceptionResolver() {
defaultExceptionResolver = new DefaultHandlerExceptionResolver();
}
@Override
protected ModelAndView doResolveException(HttpServletRequest request,
HttpServletResponse response, Object handler, Exception ex) {
Exception throwable = ex;
ModelAndView modelAndView = new ModelAndView()
if (throwable instanceof NullPointerException) {
//发生NullPointerException时，response写入操作失败等自定义逻辑
}
//其他的异常交还给spring的异常解析器解析
else{
modelAndView=this.defaultExceptionResolver.resolveException(request, response, handler, ex);
}
return modelAndView;
}
}3. 初始化过程
查看AbstractHandlerExceptionResolver的子类和DispatcherServlet.properties，Spring MVC默认配置异常解析器有：ExceptionHandlerExceptionResolver,DefaultHandlerExceptionResolver,ResponseStatusExceptionResolver。
在DispatcherServlet中初始化了异常解析器（initHandlerExceptionResolvers），如下：
DispatcherServlet：
private boolean detectAllHandlerExceptionResolvers = true;
private void initHandlerExceptionResolvers(ApplicationContext context) {
this.handlerExceptionResolvers = null;
//默认
if (this.detectAllHandlerExceptionResolvers) {
// 寻找HandlerExceptionResolvers，包括父容器
Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
.beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
// sorted order排序
AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
}
}
else {
try {
//寻找名字叫handlerExceptionResolver的类
HandlerExceptionResolver her =
context.getBean(HANDLER_EXCEPTION_RESOLVER_BEAN_NAME, HandlerExceptionResolver.class);
this.handlerExceptionResolvers = Collections.singletonList(her);
}
catch (NoSuchBeanDefinitionException ex) {
// Ignore, no HandlerExceptionResolver is fine too.
}
}
// 如果没有则创建默认解析器，至少找到一个HandlerExceptionResolvers被注册
if (this.handlerExceptionResolvers == null) {
this.handlerExceptionResolvers = getDefaultStrategies(context, HandlerExceptionResolver.class);
if (logger.isTraceEnabled()) {
logger.trace("No HandlerExceptionResolvers declared in servlet '" + getServletName() +
"': using default strategies from DispatcherServlet.properties");
}
}
}默认创建过程：getDefaultStrategies(context, HandlerExceptionResolver.class)
protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) {
String key = strategyInterface.getName();
//从DispatcherServlet.properties读取默认配置
//DispatcherServlet.properties中的配置为ExceptionHandlerExceptionResolver，ResponseStatusExceptionResolver，DefaultHandlerExceptionResolver
String value = defaultStrategies.getProperty(key);
if (value != null) {
String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
List<T> strategies = new ArrayList<>(classNames.length);
for (String className : classNames) {
try {
Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
Object strategy = createDefaultStrategy(context, clazz);
strategies.add((T) strategy);
}
catch (ClassNotFoundException ex) {
throw new BeanInitializationException(
"Could not find DispatcherServlet's default strategy class [" + className +
"] for interface [" + key + "]", ex);
}
catch (LinkageError err) {
throw new BeanInitializationException(
"Unresolvable class definition for DispatcherServlet's default strategy class [" +
className + "] for interface [" + key + "]", err);
}
}
return strategies;
}
else {
return new LinkedList<>();
}
}4. 处理异常
DispatcherServlet通过doDispatch处理一个请求。在doDispatch最后调用processDispatchResult进行页面渲染，返回正确结果或者异常结果。
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
boolean errorView = false;
//如果有异常，如果为ModelAndViewDefiningException异常直接返回，其他异常通过processHandlerException构建view
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("No view rendering, null ModelAndView returned.");
}
}
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
return;
}
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}processHandlerException:
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
@Nullable Object handler, Exception ex) throws Exception {
//设置content types
// Success and error responses may use different content types
request.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);
//找异常解析器
//Check registered HandlerExceptionResolvers...
//调用解析器resolveException方法
ModelAndView exMv = null;
if (this.handlerExceptionResolvers != null) {
for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
//异常解析器中的resolveException
exMv = resolver.resolveException(request, response, handler, ex);
if (exMv != null) {
break;
}
}
}
if (exMv != null) {
if (exMv.isEmpty()) {
request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
return null;
}
// We might still need view name translation for a plain error model...
// 有view返回，没有则设置默认view
if (!exMv.hasView()) {
String defaultViewName = getDefaultViewName(request);
if (defaultViewName != null) {
exMv.setViewName(defaultViewName);
}
}
if (logger.isTraceEnabled()) {
logger.trace("Using resolved error view: " + exMv, ex);
}
else if (logger.isDebugEnabled()) {
logger.debug("Using resolved error view: " + exMv);
}
//设置request
WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
return exMv;
}
throw ex;
}下篇文章将分享默认异常解析器的处理过程resolveException。