SpringFramework之ControllerAdvice注解怎么用
这篇文章主要介绍了SpringFramework之ControllerAdvice注解怎么用，具有一定借鉴价值，感兴趣的朋友可以参考下，希望大家阅读完这篇文章之后大有收获，下面让小编带着大家一起了解一下。
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SpringFramework版本5.0.9.release。
我们会通过@ControllerAdvice和@ExceptionHandler来处理异常，Springmvc是如何进行处理的呢?
ControllerAdviceBean有个重要的方法findAnnotatedBeans，如下List-1
List-1
public class ControllerAdviceBean implements Ordered {
...
public static List findAnnotatedBeans(ApplicationContext applicationContext) {
List beans = new ArrayList();
String[] var2 = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(applicationContext, Object.class);
int var3 = var2.length;
for(int var4 = 0; var4 < var3; ++var4) {
String name = var2[var4];
if (applicationContext.findAnnotationOnBean(name, ControllerAdvice.class) != null) {
beans.add(new ControllerAdviceBean(name, applicationContext));
}
}
return beans;
}
... 如List-1所示，从applicationContext中获取所有的ControllerAdvice注解的Bean，之后封装到ControllerAdviceBean中。
来看下ExceptionHandlerExceptionResolver，它的类继承图如下图1所示:
图1
ExceptionHandlerExceptionResolver是HandlerExceptionResolver，所以在Springmvc的doDispatch中会调用它。实现了InitializingBean，所以有afterPropertiesSet方法，如下List-2所示:
List-2
@Override
public void afterPropertiesSet() {
// Do this first, it may add ResponseBodyAdvice beans
initExceptionHandlerAdviceCache();
...
}
private void initExceptionHandlerAdviceCache() {
...
List adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
AnnotationAwareOrderComparator.sort(adviceBeans);
for (ControllerAdviceBean adviceBean : adviceBeans) {
Class beanType = adviceBean.getBeanType();
if (beanType == null) {
throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
}
ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(beanType);
if (resolver.hasExceptionMappings()) {
this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
if (logger.isInfoEnabled()) {
logger.info("Detected @ExceptionHandler methods in " + adviceBean);
}
}
if (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
this.responseBodyAdvice.add(adviceBean);
if (logger.isInfoEnabled()) {
logger.info("Detected ResponseBodyAdvice implementation in " + adviceBean);
}
}
}
} List-2中，initExceptionHandlerAdviceCache方法调用List-1中ControllerAdviceBean的findAnnotatedBeans方法，获取所有ControllerAdvice的bean，之后排序，所以当有多个ControllerAdivce注解的类且需要排序时，可以实现spring的Order接口来实现。
之后遍历ControllerAdviceBean，之后获取Bean的class类，传入到ExceptionHandlerMethodResolver的构造方法中，如下List-3所示:
List-3
public class ExceptionHandlerMethodResolver {
...
public static final MethodFilter EXCEPTION_HANDLER_METHODS = method ->
(AnnotationUtils.findAnnotation(method, ExceptionHandler.class) != null);
...
private final Map, Method> mappedMethods = new HashMap<>(16);
...
public ExceptionHandlerMethodResolver(Class handlerType) {
for (Method method : MethodIntrospector.selectMethods(handlerType, EXCEPTION_HANDLER_METHODS)) {
for (Class exceptionType : detectExceptionMappings(method)) {
addExceptionMapping(exceptionType, method);
}
}
}
...
private List> detectExceptionMappings(Method method) {
List> result = new ArrayList<>();
detectAnnotationExceptionMappings(method, result);
if (result.isEmpty()) {
for (Class paramType : method.getParameterTypes()) {
if (Throwable.class.isAssignableFrom(paramType)) {
result.add((Class) paramType);
}
}
}
if (result.isEmpty()) {
throw new IllegalStateException("No exception types mapped to " + method);
}
return result;
}
protected void detectAnnotationExceptionMappings(Method method, List> result) {
ExceptionHandler ann = AnnotationUtils.findAnnotation(method, ExceptionHandler.class);
Assert.state(ann != null, "No ExceptionHandler annotation");
result.addAll(Arrays.asList(ann.value()));
}
private void addExceptionMapping(Class 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 + "}");
}
} 找到方法上有ExceptionHandler注解的方法。
detectExceptionMappings方法获取ExceptionHandler的value值，如果我们没有设置ExceptionHandler的value，那么遍历方法的参数，如果参数是Throwable的子类，就将改类型放入result中，所以由此可知道，我们可以不设置ExceptionHandler的value，只需要将方法的参数设置为Throwable的子类即可，spring会自动识别。
addExceptionMapping方法将结果放入mappedMethods这个map中，key是Throwable，而value则是method。
再回到List-2中，initExceptionHandlerAdviceCache方法中，将构造好的ControllerAdviceBean和ExceptionHandlerMethodResolver放入到exceptionHandlerAdviceCache(是个map)中。
HandlerExceptionResolver是个接口，如下List-4所示:
List-4
public interface HandlerExceptionResolver {
@Nullable
ModelAndView resolveException(
HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex);
}AbstractHandlerExceptionResolver.resolveException->AbstractHandlerMethodExceptionResolver.doResolveException->ExceptionHandlerExceptionResolver.doResolveHandlerMethodException。
接下来，来看Springmvc中是如何处理我们的ControllerAdvice的。
DispatcherServlet中，doDispatch()->processDispatchResult()->processHandlerException()，如下List-5所示，会遍历HandlerExceptionResovler来处理。
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
@Nullable Object handler, Exception ex) throws Exception {
ModelAndView exMv = null;
if (this.handlerExceptionResolvers != null) {
for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
if (exMv != null) {
break;
}
}
}ExceptionHandlerExceptionResolver是如何加入到Springmvc中handlerExceptionResolvers的，是因为DispatcherServlet.properties中HandlerExceptionResolver的值有ExceptionHandlerExceptionResolver，所以会被Spring自动加入进去。
Spring通过上面的方式，将捕获到的异常交给ExceptionHandlerExceptionResolver.doResolveHandlerMethodException来处理，通过多次转换，最终调用我们设置带有ExceptionHandler注解的方法。
通过源码分析，带有ControllerAdvice和ExceptionHandler注解的拦截处理的执行先于HandlerInterceptor的afterCompletion。
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