Spring的@ControllerAdvice注解作用原理探究(转发)

在Spring里，我们可以使用@ControllerAdvice来声明一些全局性的东西，最常见的是结合@ExceptionHandler注解用于全局异常的处理。

@ControllerAdvice是在类上声明的注解，其用法主要有三点：

@ExceptionHandler注解标注的方法：用于捕获Controller中抛出的不同类型的异常，从而达到异常全局处理的目的；
@InitBinder注解标注的方法：用于请求中注册自定义参数的解析，从而达到自定义请求参数格式的目的；
@ModelAttribute注解标注的方法：表示此方法会在执行目标Controller方法之前执行。

看下具体用法：

// 这里@RestControllerAdvice等同于@ControllerAdvice + @ResponseBody
@RestControllerAdvice
public class GlobalHandler {
    private final Logger logger = LoggerFactory.getLogger(GlobalHandler.class);
    // 这里@ModelAttribute("loginUserInfo")标注的modelAttribute()方法表示会在Controller方法之前
    // 执行，返回当前登录用户的UserDetails对象
    @ModelAttribute("loginUserInfo")
    public UserDetails modelAttribute() {
        return (UserDetails) SecurityContextHolder.getContext().getAuthentication().getPrincipal();
    }
    // @InitBinder标注的initBinder()方法表示注册一个Date类型的类型转换器，用于将类似这样的2019-06-10
    // 日期格式的字符串转换成Date对象
    @InitBinder
    protected void initBinder(WebDataBinder binder) {
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
        dateFormat.setLenient(false);
        binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
    } 
    // 这里表示Controller抛出的MethodArgumentNotValidException异常由这个方法处理
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public Result exceptionHandler(MethodArgumentNotValidException e) {
        Result result = new Result(BizExceptionEnum.INVALID_REQ_PARAM.getErrorCode(),
                BizExceptionEnum.INVALID_REQ_PARAM.getErrorMsg());
        logger.error("req params error", e);
        return result;
    }
    // 这里表示Controller抛出的BizException异常由这个方法处理
    @ExceptionHandler(BizException.class)
    public Result exceptionHandler(BizException e) {
        BizExceptionEnum exceptionEnum = e.getBizExceptionEnum();
        Result result = new Result(exceptionEnum.getErrorCode(), exceptionEnum.getErrorMsg());
        logger.error("business error", e);
        return result;
    }
    // 这里就是通用的异常处理器了,所有预料之外的Exception异常都由这里处理
    @ExceptionHandler(Exception.class)
    public Result exceptionHandler(Exception e) {
        Result result = new Result(1000, "网络繁忙,请稍后再试");
        logger.error("application error", e);
        return result;
    }
 
}

@ExceptionHandler标注的多个方法分别表示只处理特定的异常。这里需要注意的是当Controller抛出的某个异常多个@ExceptionHandler标注的方法都适用时，Spring会选择最具体的异常处理方法来处理，也就是说@ExceptionHandler(Exception.class)这里标注的方法优先级最低，只有当其它方法都不适用时，才会来到这里处理。

下面我们看看Spring是怎么实现的，首先前端控制器DispatcherServlet对象在创建时会初始化一系列的对象：

public class DispatcherServlet extends FrameworkServlet {
    // ......
    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }
    // ......
}

对于@ControllerAdvice 注解，我们重点关注initHandlerAdapters(context)和initHandlerExceptionResolvers(context)这两个方法。

initHandlerAdapters(context)方法会取得所有实现了HandlerAdapter接口的bean并保存起来，其中就有一个类型为RequestMappingHandlerAdapter的bean，这个bean就是@RequestMapping注解能起作用的关键，这个bean在应用启动过程中会获取所有被@ControllerAdvice注解标注的bean对象做进一步处理，关键代码在这里：

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
        implements BeanFactoryAware, InitializingBean {
    // ......
    private void initControllerAdviceCache() {
                // ......
        List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
        AnnotationAwareOrderComparator.sort(adviceBeans);
 
        List<Object> requestResponseBodyAdviceBeans = new ArrayList<>();
 
        for (ControllerAdviceBean adviceBean : adviceBeans) {
            Class<?> beanType = adviceBean.getBeanType();
            if (beanType == null) {
                throw new IllegalStateException("Unresolvable type for ControllerAdviceBean: " + adviceBean);
            }
                        // 找到所有ModelAttribute标注的方法并缓存起来
            Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
            if (!attrMethods.isEmpty()) {
                this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
                if (logger.isInfoEnabled()) {
                    logger.info("Detected @ModelAttribute methods in " + adviceBean);
                }
            }
                        // 找到所有InitBinder标注的方法并缓存起来
            Set<Method> binderMethods = MethodIntrospector.selectMethods(beanType, INIT_BINDER_METHODS);
            if (!binderMethods.isEmpty()) {
                this.initBinderAdviceCache.put(adviceBean, binderMethods);
                if (logger.isInfoEnabled()) {
                    logger.info("Detected @InitBinder methods in " + adviceBean);
                }
            }
                        // ......
        }
    }
    // ......
}

来看DispatcherServlet的initHandlerExceptionResolvers(context)方法，方法会取得所有实现了HandlerExceptionResolver接口的bean并保存起来，其中就有一个类型为ExceptionHandlerExceptionResolver的bean，这个bean在应用启动过程中会获取所有被@ControllerAdvice注解标注的bean对象做进一步处理，关键代码在这里：　　

public class ExceptionHandlerExceptionResolver extends AbstractHandlerMethodExceptionResolver
        implements ApplicationContextAware, InitializingBean {
    // ......
    private void initExceptionHandlerAdviceCache() {
        // ......
        List<ControllerAdviceBean> adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
        AnnotationAwareOrderComparator.sort(adviceBeans);
 
        for (ControllerAdviceBean adviceBean : adviceBeans) {
            ExceptionHandlerMethodResolver resolver = new ExceptionHandlerMethodResolver(adviceBean.getBeanType());
            if (resolver.hasExceptionMappings()) {
                // 找到所有ExceptionHandler标注的方法并保存成一个ExceptionHandlerMethodResolver类型的对象缓存起来
                this.exceptionHandlerAdviceCache.put(adviceBean, resolver);
                if (logger.isInfoEnabled()) {
                    logger.info("Detected @ExceptionHandler methods in " + adviceBean);
                }
            }
            // ......
        }
    }
    // ......
}

当Controller抛出异常时，DispatcherServlet通过ExceptionHandlerExceptionResolver来解析异常，而ExceptionHandlerExceptionResolver又通过ExceptionHandlerMethodResolver 来解析异常， ExceptionHandlerMethodResolver 最终解析异常找到适用的@ExceptionHandler标注的方法是这里：　　

public class ExceptionHandlerMethodResolver {
    // ......
    private Method getMappedMethod(Class<? extends Throwable> exceptionType) {
        List<Class<? extends Throwable>> matches = new ArrayList<Class<? extends Throwable>>();
        // 找到所有适用于Controller抛出异常的处理方法,例如Controller抛出的异常
        // 是BizException(继承自RuntimeException),那么@ExceptionHandler(BizException.class)和
        // @ExceptionHandler(Exception.class)标注的方法都适用此异常
        for (Class<? extends Throwable> mappedException : this.mappedMethods.keySet()) {
            if (mappedException.isAssignableFrom(exceptionType)) {
                matches.add(mappedException);
            }
        }
        if (!matches.isEmpty()) {
                // 这里通过排序找到最适用的方法,排序的规则依据抛出异常相对于声明异常的深度,例如
            // Controller抛出的异常是BizException(继承自RuntimeException),那么BizException
            // 相对于@ExceptionHandler(BizException.class)声明的BizException.class其深度是0,
            // 相对于@ExceptionHandler(Exception.class)声明的Exception.class其深度是2,所以
            // @ExceptionHandler(BizException.class)标注的方法会排在前面
            Collections.sort(matches, new ExceptionDepthComparator(exceptionType));
            return this.mappedMethods.get(matches.get(0));
        }
        else {
            return null;
        }
    }
    // ......
}