前言
上一篇文章介绍了HandlerAdapter和HttpMessageConverter，这里介绍SpringWeb的另一个重要的接口HandlerExceptionResolver。，并通过自定义注解@MyControllerAdvice和@MyExceptionHandler，来实现异常的拦截。
ControllerAdvice和ExceptionHandler
DispatcherServlet会读取ApplicationContext中是实现了HandlerExceptionResolver接口的bean，并在出现异常时调用对应的方法处理异常，入口是在doDispatch方法中，这里有两次try-catch，最内的异常捕获后交给了processDispatchResult来处理。
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
...
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
...
}
catch (Exception ex) {
dispatchException = ex;
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Error err) {
triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
}
finally {
...
}
}
HandlerExceptionResolver主要的一个实现类是ExceptionHandlerExceptionResolver，这个类扫描了ControllerAdvice和ExceptionHandler定义的类和方法来提供异常处理。
ControllerAdvice是围绕Controller提供嵌入操作，下面的类的注释介绍了它的作用，这里主要用到了ExceptionHandler。
It is typically used to define {@link ExceptionHandler @ExceptionHandler},
{@link InitBinder @InitBinder}, and {@link ModelAttribute @ModelAttribute}
methods that apply to all {@link RequestMapping @RequestMapping} methods
ControllerAdviceBean负责加载@ControllerAdvice注释的类，稍后会自定义这个类来实现自定义功能。
ExceptionHandler是注解在处理特定异常的方法上，由ExceptionHandlerMethodResolver来扫描加载。
自定义ExceptionHandler
接下来就是自定义@MyControllerAdvice和@MyExceptionHandler来实现异常拦截。完整的代码还是放在Github上了。
- 自定义注解，MyControllerAdvice加上@Component，可以被Spring加载扫描到。
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface MyControllerAdvice {
String[] value() default {};
}
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface MyExceptionHandler {
Class<? extends Throwable>[] value() default {};
}
- 定义Controller
@Controller
public static class MyController {
@RequestMapping
@ResponseBody
public Object index() {
throw new RuntimeException("hello world");
}
}
- 定义Exception Handler，这里接收到异常后返回一个map。
@MyControllerAdvice
public static class MyExceptionHandlerClass {
@MyExceptionHandler(RuntimeException.class)
@ResponseBody
public Map<String, String> handleException(RuntimeException runtimeException) {
Map<String, String> data = new HashMap<String, String>();
data.put("msg", runtimeException.getMessage());
return data;
}
}
- 初始化并配置MyExceptionHandlerExceptionResolver。WebMvcConfigurationSupport最终导出的是HandlerExceptionResolverComposite。这个类会集成配置的HandlerExceptionResolver。所以这里继承了WebMvcConfigurationSupport，并在HandlerExceptionResolver列表中加入了自定义的MyExceptionHandlerExceptionResolver。
@Configuration
public static class MyWebMvcConfigurationSupport extends WebMvcConfigurationSupport {
public void configureHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers) {
MyExceptionHandlerExceptionResolver exceptionHandlerExceptionResolver = new MyExceptionHandlerExceptionResolver();
exceptionHandlerExceptionResolver.setContentNegotiationManager(mvcContentNegotiationManager());
exceptionHandlerExceptionResolver.setMessageConverters(getMessageConverters());
exceptionHandlerExceptionResolver.setApplicationContext(getApplicationContext());
exceptionHandlerExceptionResolver.afterPropertiesSet();
exceptionResolvers.add(exceptionHandlerExceptionResolver);
}
}
- 自定义ExceptionHandlerExceptionResolver，继承了ExceptionHandlerExceptionResolver，重用了大部分逻辑，
同时为了加载自定义的注解，使用了MyControllerAdviceBean和MyExceptionHandlerMethodResolver两个新的类，用来加载自定义的注解，并保存在exceptionHandlerCache和exceptionHandlerAdviceCache中。在使用的时候，在根据Exception的类型来取得对应的方法。
public static class MyExceptionHandlerExceptionResolver extends ExceptionHandlerExceptionResolver {
private final Map<Class<?>, MyExceptionHandlerMethodResolver> exceptionHandlerCache = new ConcurrentHashMap<Class<?>, MyExceptionHandlerMethodResolver>(64);
private final Map<MyControllerAdviceBean, MyExceptionHandlerMethodResolver> exceptionHandlerAdviceCache = new LinkedHashMap<MyControllerAdviceBean, MyExceptionHandlerMethodResolver>();
@Override
public void afterPropertiesSet() {
initExceptionHandlerAdviceCache();
super.afterPropertiesSet();
}
private void initExceptionHandlerAdviceCache() {
if (getApplicationContext() == null) {
return;
}
List<MyControllerAdviceBean> adviceBeans = MyControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
AnnotationAwareOrderComparator.sort(adviceBeans);
for (MyControllerAdviceBean adviceBean : adviceBeans) {
MyExceptionHandlerMethodResolver resolver = new MyExceptionHandlerMethodResolver(adviceBean.getBeanType());
if (resolver.hasExceptionMappings()) {
this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
}
}
}
public ServletInvocableHandlerMethod getExceptionHandlerMethod(HandlerMethod handlerMethod, Exception exception) {
Class<?> handlerType = (handlerMethod != null ? handlerMethod.getBeanType() : null);
if (handlerMethod != null) {
MyExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);
if (resolver == null) {
resolver = new MyExceptionHandlerMethodResolver(handlerType);
this.exceptionHandlerCache.put(handlerType, resolver);
}
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);
}
}
for (Map.Entry<MyControllerAdviceBean, MyExceptionHandlerMethodResolver> entry : this.exceptionHandlerAdviceCache.entrySet()) {
MyExceptionHandlerMethodResolver resolver = entry.getValue();
Method method = resolver.resolveMethod(exception);
if (method != null) {
return new ServletInvocableHandlerMethod(entry.getKey().resolveBean(), method);
}
}
return null;
}
}
- 自定义ExceptionHandlerMethodResolver，这里重用了ExceptionHandlerMethodResolver的大部分内容。只是替换了注解的类型。
public static class MyExceptionHandlerMethodResolver {
public static final ReflectionUtils.MethodFilter EXCEPTION_HANDLER_METHODS = new ReflectionUtils.MethodFilter() {
public boolean matches(Method method) {
return (AnnotationUtils.findAnnotation(method, MyExceptionHandler.class) != null);
}
};
private static final Method NO_METHOD_FOUND = ClassUtils.getMethodIfAvailable(System.class, "currentTimeMillis");
private final Map<Class<? extends Throwable>, Method> mappedMethods = new ConcurrentHashMap<Class<? extends Throwable>, Method>(16);
private final Map<Class<? extends Throwable>, Method> exceptionLookupCache = new ConcurrentHashMap<Class<? extends Throwable>, Method>(16);
public MyExceptionHandlerMethodResolver(Class<?> handlerType) {
for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
for (Class<? extends Throwable> exceptionType : detectExceptionMappings(method)) {
addExceptionMapping(exceptionType, method);
}
}
}
@SuppressWarnings("unchecked")
private List<Class<? extends Throwable>> detectExceptionMappings(Method method) {
List<Class<? extends Throwable>> result = new ArrayList<Class<? extends Throwable>>();
detectAnnotationExceptionMappings(method, result);
if (result.isEmpty()) {
for (Class<?> paramType : method.getParameterTypes()) {
if (Throwable.class.isAssignableFrom(paramType)) {
result.add((Class<? extends Throwable>) paramType);
}
}
}
Assert.notEmpty(result, "No exception types mapped to {" + method + "}");
return result;
}
public void detectAnnotationExceptionMappings(Method method, List<Class<? extends Throwable>> result) {
MyExceptionHandler annot = AnnotationUtils.findAnnotation(method, MyExceptionHandler.class);
result.addAll(Arrays.asList(annot.value()));
}
private void addExceptionMapping(Class<? extends Throwable> exceptionType, Method method) {
Method oldMethod = this.mappedMethods.put(exceptionType, method);
if (oldMethod != null && !oldMethod.equals(method)) {
throw new IllegalStateException("Ambiguous @ExceptionHandler method mapped for [" +
exceptionType + "]: {" + oldMethod + ", " + method + "}");
}
}
public boolean hasExceptionMappings() {
return !this.mappedMethods.isEmpty();
}
public Method resolveMethod(Exception exception) {
return resolveMethodByExceptionType(exception.getClass());
}
public Method resolveMethodByExceptionType(Class<? extends Exception> exceptionType) {
Method method = this.exceptionLookupCache.get(exceptionType);
if (method == null) {
method = getMappedMethod(exceptionType);
this.exceptionLookupCache.put(exceptionType, (method != null ? method : NO_METHOD_FOUND));
}
return (method != NO_METHOD_FOUND ? method : null);
}
private Method getMappedMethod(Class<? extends Exception> exceptionType) {
List<Class<? extends Throwable>> matches = new ArrayList<Class<? extends Throwable>>();
for (Class<? extends Throwable> mappedException : this.mappedMethods.keySet()) {
if (mappedException.isAssignableFrom(exceptionType)) {
matches.add(mappedException);
}
}
if (!matches.isEmpty()) {
Collections.sort(matches, new ExceptionDepthComparator(exceptionType));
return this.mappedMethods.get(matches.get(0));
} else {
return null;
}
}
}
- 自定义ControllerAdviceBean，这里简化了ControllerAdviceBean的使用属性，本来还有package和类型的过滤。findAnnotatedBeans方法是扫描注解的方法。
public static class MyControllerAdviceBean implements Ordered {
private final Object bean;
private final BeanFactory beanFactory;
private final int order;
private MyControllerAdviceBean(Object bean, BeanFactory beanFactory) {
this.bean = bean;
this.beanFactory = beanFactory;
this.order = initOrderFromBean(bean);
}
public int getOrder() {
return this.order;
}
public Class<?> getBeanType() {
Class<?> clazz = (this.bean instanceof String ?
this.beanFactory.getType((String) this.bean) : this.bean.getClass());
return ClassUtils.getUserClass(clazz);
}
public Object resolveBean() {
return (this.bean instanceof String ? this.beanFactory.getBean((String) this.bean) : this.bean);
}
@Override
public boolean equals(Object other) {
if (this == other) {
return true;
}
if (!(other instanceof MyControllerAdviceBean)) {
return false;
}
MyControllerAdviceBean otherAdvice = (MyControllerAdviceBean) other;
return (this.bean.equals(otherAdvice.bean) && this.beanFactory == otherAdvice.beanFactory);
}
@Override
public int hashCode() {
return this.bean.hashCode();
}
@Override
public String toString() {
return this.bean.toString();
}
public static List<MyControllerAdviceBean> findAnnotatedBeans(ApplicationContext applicationContext) {
List<MyControllerAdviceBean> beans = new ArrayList<MyControllerAdviceBean>();
for (String name : BeanFactoryUtils.beanNamesForTypeIncludingAncestors(applicationContext, Object.class)) {
if (applicationContext.findAnnotationOnBean(name, MyControllerAdvice.class) != null) {
beans.add(new MyControllerAdviceBean(name, applicationContext));
}
}
return beans;
}
private static int initOrderFromBean(Object bean) {
return (bean instanceof Ordered ? ((Ordered) bean).getOrder() : initOrderFromBeanType(bean.getClass()));
}
private static int initOrderFromBeanType(Class<?> beanType) {
return OrderUtils.getOrder(beanType, Ordered.LOWEST_PRECEDENCE);
}
}
- 程序的入口
public static void main(String[] args) throws ServletException, IOException {
MockServletContext mockServletContext = new MockServletContext();
MockServletConfig mockServletConfig = new MockServletConfig(mockServletContext);
AnnotationConfigWebApplicationContext annotationConfigWebApplicationContext = new AnnotationConfigWebApplicationContext();
annotationConfigWebApplicationContext.setServletConfig(mockServletConfig);
annotationConfigWebApplicationContext.register(CustomizeExceptionHandlerTest.class);
DispatcherServlet dispatcherServlet = new DispatcherServlet(annotationConfigWebApplicationContext);
dispatcherServlet.init(mockServletConfig);
MockHttpServletResponse response = new MockHttpServletResponse();
MockHttpServletRequest request = new MockHttpServletRequest("GET", "/");
request.addHeader("Accept", "application/json");
request.setContent(("result,hello world;date," + Calendar.getInstance().getTimeInMillis()).getBytes());
dispatcherServlet.service(request, response);
System.out.println(new String(response.getContentAsByteArray()));
}
运行程序就会发现捕获了RuntimeException，返回了对应的map。
结语
这里主要介绍了HandlerExceptionResolver的使用和自定义，目前为止已经介绍了HandlerMapping，HandlerAdapter，
HandlerExceptionResolver三个主要的SpringWeb的内部实现的接口，已经涵盖了DispatcherServlet的基本功能内部实现。
