【JavaEE进阶】 @ControllerAdvice源码分析

不念不忘少年蓝@ 2024-04-17 10:50 61阅读 0赞

#### 文章目录 ####

*  ?前言
 *  ?@ControllerAdvice源码分析
 *   *  ?initHandlerAdapters(context)
     *  ?initHandlerExceptionResolvers(context)
 *  ⭕总结

## ?前言 ##

在前面的项目开发中，我们使用了统一数据返回和统一异常的功能

它们都是基于 @ControllerAdvice 注解来实现的,接下来我们将通过分析  
@ControllerAdvice 的源码,来了解他们的执行流程.

## ?@ControllerAdvice源码分析 ##

首先我们点击@ControllerAdvice，查看其实现源码

Target({
        ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Component
    public @interface ControllerAdvice {
        
        @AliasFor("basePackages")
        String[] value() default {
        };
        @AliasFor("value")
        String[] basePackages() default {
        };
        Class<?>[] basePackageClasses() default {
        };
        Class<?>[] assignableTypes() default {
        };
        Class<? extends Annotation>[] annotations() default {
        };
    }

从上述源码可以看出 @ControllerAdvice 派⽣于@Component组件,这也就是为什么没有五⼤注解,ControllerAdvice 就⽣效的原因.

下⾯我们看看Spring是怎么实现着两项功能的,我们还是从 DispatcherServlet 的代码开始分析.

DispatcherServlet 对象在创建时会初始化⼀系列的对象

public class DispatcherServlet extends FrameworkServlet {
        
        //...
        @Override
        protected void onRefresh(ApplicationContext context) {
        
            initStrategies(context);
        }
        /**
         * Initialize the strategy objects that this servlet uses.
         * <p>May be overridden in subclasses in order to initialize further
         strategy objects.
         */
        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) ###

initHandlerAdapters(context) 方法会取得所有实现了 HandlerAdapter 接⼝的bean并保存起来，其中有⼀个类型为 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,并做进⼀步处理，关键代码如下

public class RequestMappingHandlerAdapter extends AbstractHandlerMethodAdapter
            implements BeanFactoryAware, InitializingBean {
        
    //...
        /**
         * 添加ControllerAdvice bean的处理
         */
        private void initControllerAdviceCache() {
        
            if (getApplicationContext() == null) {
        
                return;
            }
    //获取所有所有被 @ControllerAdvice 注解标注的bean对象
            List<ControllerAdviceBean> adviceBeans =
                    ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
            List<Object> requestResponseBodyAdviceBeans = new ArrayList<>();
            for (ControllerAdviceBean adviceBean : adviceBeans) {
        
                Class<?> beanType = adviceBean.getBeanType();
                if (beanType == null) {
        
                    throw new IllegalStateException("Unresolvable type for
                            ControllerAdviceBean: " + adviceBean);
                }
                Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType,
                        MODEL_ATTRIBUTE_METHODS);
                if (!attrMethods.isEmpty()) {
        
                    this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
                }
                Set<Method> binderMethods =
                        MethodIntrospector.selectMethods(beanType, INIT_BINDER_METHODS);
                if (!binderMethods.isEmpty()) {
        
                    this.initBinderAdviceCache.put(adviceBean, binderMethods);
                }
                if (RequestBodyAdvice.class.isAssignableFrom(beanType) ||
                        ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
        
                    requestResponseBodyAdviceBeans.add(adviceBean);
                }
            }
            if (!requestResponseBodyAdviceBeans.isEmpty()) {
        
                this.requestResponseBodyAdvice.addAll(0,
                        requestResponseBodyAdviceBeans);
            }
            if (logger.isDebugEnabled()) {
        
                int modelSize = this.modelAttributeAdviceCache.size();
                int binderSize = this.initBinderAdviceCache.size();
                int reqCount = getBodyAdviceCount(RequestBodyAdvice.class);
                int resCount = getBodyAdviceCount(ResponseBodyAdvice.class);
                if (modelSize == 0 && binderSize == 0 && reqCount == 0 && resCount
                        == 0) {
        
                    logger.debug("ControllerAdvice beans: none");
                }
                else {
        
                    logger.debug("ControllerAdvice beans: " + modelSize + "
                    @ModelAttribute, " + binderSize +
                    " @InitBinder, " + reqCount + " RequestBodyAdvice, " +
                            resCount + " ResponseBodyAdvice");
                }
            }
        }
    //...
    }

这个方法在执行时会查找使用所有的 @ControllerAdvice 类，把 ResponseBodyAdvice 类放在容器中，当发⽣某个事件时，调用相应的Advice⽅法，就比如如返回数据前调用统⼀数据封装

### ?initHandlerExceptionResolvers(context) ###

DispatcherServlet 的 initHandlerExceptionResolvers(context) ⽅法，这个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,代码如下：

public class ExceptionHandlerExceptionResolver extends
            AbstractHandlerMethodExceptionResolver
            implements ApplicationContextAware, InitializingBean {
        
            //...
            private void initExceptionHandlerAdviceCache() {
        
                    if (getApplicationContext() == null) {
        
                            return;
                    }
    // 获取所有所有被 @ControllerAdvice 注解标注的bean对象
                    List<ControllerAdviceBean> adviceBeans =
                            ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());
                    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 (ResponseBodyAdvice.class.isAssignableFrom(beanType)) {
        
                                    this.responseBodyAdvice.add(adviceBean);
                            }
                    }
                    if (logger.isDebugEnabled()) {
        
                            int handlerSize = this.exceptionHandlerAdviceCache.size();
                            int adviceSize = this.responseBodyAdvice.size();
                            if (handlerSize == 0 && adviceSize == 0) {
        
                                    logger.debug("ControllerAdvice beans: none");
                            }
                            else {
        
                                    logger.debug("ControllerAdvice beans: " +
                                                    handlerSize + " @ExceptionHandler, " + adviceSize + "
                                            ResponseBodyAdvice");
                            }
                    }
            }
    //...
    }

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，如下代码所示：

public class ExceptionHandlerMethodResolver {
        
            //...
            private Method getMappedMethod(Class<? extends Throwable> exceptionType) {
        
                    List<Class<? extends Throwable>> matches = new ArrayList();
                    //根据异常类型, 查找匹配的异常处理⽅法
                    //⽐如NullPointerException会匹配两个异常处理⽅法:
                    //handler(Exception e) 和 handler(NullPointerException e)
                    for (Class<? extends Throwable> mappedException :
                            this.mappedMethods.keySet()) {
        
                            if (mappedException.isAssignableFrom(exceptionType)) {
        
                                    matches.add(mappedException);
                            }
                    }
                    //如果查找到多个匹配, 就进⾏排序, 找到最使⽤的⽅法. 排序的规则依据抛出异常相对于声明异常的深度
                    //⽐如抛出的是NullPointerException(继承于RuntimeException,RuntimeException⼜继承于Exception)
                    //相对于handler(NullPointerException e) 声明的NullPointerException深度为0,
                    //相对于handler(Exception e) 声明的Exception 深度 为2
                    //所以 handler(NullPointerException e)标注的⽅法会排在前⾯
                    if (!matches.isEmpty()) {
        
                            if (matches.size() > 1) {
        
                                    matches.sort(new ExceptionDepthComparator(exceptionType));
                            }
                            return this.mappedMethods.get(matches.get(0));
                    }
                    else {
        
                            return NO_MATCHING_EXCEPTION_HANDLER_METHOD;
                    }
            }
    //...
    }

## ⭕总结 ##

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