SpringBoot介面

SpringBoot介面如何對異常進行統一封裝，並統一返回呢？以上文的引數校驗為例，如何優雅的將引數校驗的錯誤資訊統一處理並封裝返回呢？@pdai

為什麼要優雅的處理異常

如果我們不統一的處理異常，經常會在controller層有大量的例外處理的程式碼，比如：

@Slf4j
@Api(value = "User Interfaces", tags = "User Interfaces")
@RestController
@RequestMapping("/user")
public class UserController {

    /**
     * http://localhost:8080/user/add .
     *
     * @param userParam user param
     * @return user
     */
    @ApiOperation("Add User")
    @ApiImplicitParam(name = "userParam", type = "body", dataTypeClass = UserParam.class, required = true)
    @PostMapping("add")
    public ResponseEntity<String> add(@Valid @RequestBody UserParam userParam) {
        // 每個介面充斥著大量的例外處理
        try {
            // do something
        } catch(Exception e) {
            return ResponseEntity.fail("error");
        }
        return ResponseEntity.ok("success");
    }
}

那怎麼實現統一的例外處理，特別是結合引數校驗等封裝？

實現案例

簡單展示通過@ControllerAdvice進行統一例外處理。

@ControllerAdvice異常統一處理

對於400引數錯誤異常

/**
 * Global exception handler.
 *
 * @author pdai
 */
@Slf4j
@RestControllerAdvice
public class GlobalExceptionHandler {

    /**
     * exception handler for bad request.
     *
     * @param e
     *            exception
     * @return ResponseResult
     */
    @ResponseBody
    @ResponseStatus(code = HttpStatus.BAD_REQUEST)
    @ExceptionHandler(value = { BindException.class, ValidationException.class, MethodArgumentNotValidException.class })
    public ResponseResult<ExceptionData> handleParameterVerificationException(@NonNull Exception e) {
        ExceptionData.ExceptionDataBuilder exceptionDataBuilder = ExceptionData.builder();
        log.warn("Exception: {}", e.getMessage());
        if (e instanceof BindException) {
            BindingResult bindingResult = ((MethodArgumentNotValidException) e).getBindingResult();
            bindingResult.getAllErrors().stream().map(DefaultMessageSourceResolvable::getDefaultMessage)
                    .forEach(exceptionDataBuilder::error);
        } else if (e instanceof ConstraintViolationException) {
            if (e.getMessage() != null) {
                exceptionDataBuilder.error(e.getMessage());
            }
        } else {
            exceptionDataBuilder.error("invalid parameter");
        }
        return ResponseResultEntity.fail(exceptionDataBuilder.build(), "invalid parameter");
    }

}

對於自定義異常

/**
 * handle business exception.
 *
 * @param businessException
 *            business exception
 * @return ResponseResult
 */
@ResponseBody
@ExceptionHandler(BusinessException.class)
public ResponseResult<BusinessException> processBusinessException(BusinessException businessException) {
    log.error(businessException.getLocalizedMessage(), businessException);
    // 這裡可以遮蔽掉後臺的異常棧資訊，直接返回"business error"
    return ResponseResultEntity.fail(businessException, businessException.getLocalizedMessage());
}

對於其它異常

/**
 * handle other exception.
 *
 * @param exception
 *            exception
 * @return ResponseResult
 */
@ResponseBody
@ExceptionHandler(Exception.class)
public ResponseResult<Exception> processException(Exception exception) {
    log.error(exception.getLocalizedMessage(), exception);
    // 這裡可以遮蔽掉後臺的異常棧資訊，直接返回"server error"
    return ResponseResultEntity.fail(exception, exception.getLocalizedMessage());
}

Controller介面

（介面中無需處理異常）

@Slf4j
@Api(value = "User Interfaces", tags = "User Interfaces")
@RestController
@RequestMapping("/user")
public class UserController {

    /**
     * http://localhost:8080/user/add .
     *
     * @param userParam user param
     * @return user
     */
    @ApiOperation("Add User")
    @ApiImplicitParam(name = "userParam", type = "body", dataTypeClass = UserParam.class, required = true)
    @PostMapping("add")
    public ResponseEntity<UserParam> add(@Valid @RequestBody UserParam userParam) {
        return ResponseEntity.ok(userParam);
    }
}

執行測試

這裡用postman測試下

進一步理解

我們再通過一些問題來幫助你更深入理解@ControllerAdvice。@pdai

@ControllerAdvice還可以怎麼用？

除了通過@ExceptionHandler註解用於全域性異常的處理之外，@ControllerAdvice還有兩個用法：

@InitBinder註解

用於請求中註冊自定義引數的解析，從而達到自定義請求引數格式的目的；

比如，在@ControllerAdvice註解的類中新增如下方法，來統一處理日期格式的格式化

@InitBinder
public void handleInitBinder(WebDataBinder dataBinder){
    dataBinder.registerCustomEditor(Date.class,
            new CustomDateEditor(new SimpleDateFormat("yyyy-MM-dd"), false));
}

Controller中傳入引數（string型別）自動轉化為Date型別

@GetMapping("testDate")
public Date processApi(Date date) {
    return date;
}

@ModelAttribute註解

用來預設全域性引數，比如最典型的使用Spring Security時將新增當前登入的使用者資訊（UserDetails)作為引數。

@ModelAttribute("currentUser")
public UserDetails modelAttribute() {
    return (UserDetails) SecurityContextHolder.getContext().getAuthentication().getPrincipal();
}

所有controller類中requestMapping方法都可以直接獲取並使用currentUser

@PostMapping("saveSomething")
public ResponseEntity<String> saveSomeObj(@ModelAttribute("currentUser") UserDetails operator) {
    // 儲存操作，並設定當前操作人員的ID（從UserDetails中獲得）
    return ResponseEntity.success("ok");
}

@ControllerAdvice是如何起作用的（原理）？

我們在Spring基礎 - SpringMVC案例和機制的基礎上來看@ControllerAdvice的原始碼實現。

DispatcherServlet中onRefresh方法是初始化ApplicationContext後的回撥方法，它會呼叫initStrategies方法，主要更新一些servlet需要使用的物件，包括國際化處理，requestMapping，檢視解析等等。

/**
    * This implementation calls {@link #initStrategies}.
    */
@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); // 檔案上傳
    initLocaleResolver(context); // i18n國際化
    initThemeResolver(context); // 主題
    initHandlerMappings(context); // requestMapping
    initHandlerAdapters(context); // adapters
    initHandlerExceptionResolvers(context); // 例外處理
    initRequestToViewNameTranslator(context);
    initViewResolvers(context);
    initFlashMapManager(context);
}

從上述程式碼看，如果要提供@ControllerAdvice提供的三種註解功能，從設計和實現的角度肯定是實現的程式碼需要放在initStrategies方法中。

@ModelAttribute和@InitBinder處理

具體來看，如果你是設計者，很顯然容易想到：對於@ModelAttribute提供的引數預置和@InitBinder註解提供的預處理方法應該是放在一個方法中的，因為它們都是在進入requestMapping方法前做的操作。

如下方法是獲取所有的HandlerAdapter，無非就是從BeanFactory中獲取（BeanFactory相關知識請參考 Spring進階- Spring IOC實現原理詳解之IOC體系結構設計)

private void initHandlerAdapters(ApplicationContext context) {
    this.handlerAdapters = null;

    if (this.detectAllHandlerAdapters) {
        // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.
        Map<String, HandlerAdapter> matchingBeans =
                BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.handlerAdapters = new ArrayList<>(matchingBeans.values());
            // We keep HandlerAdapters in sorted order.
            AnnotationAwareOrderComparator.sort(this.handlerAdapters);
        }
    }
    else {
        try {
            HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
            this.handlerAdapters = Collections.singletonList(ha);
        }
        catch (NoSuchBeanDefinitionException ex) {
            // Ignore, we'll add a default HandlerAdapter later.
        }
    }

    // Ensure we have at least some HandlerAdapters, by registering
    // default HandlerAdapters if no other adapters are found.
    if (this.handlerAdapters == null) {
        this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
        if (logger.isTraceEnabled()) {
            logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +
                    "': using default strategies from DispatcherServlet.properties");
        }
    }
}

我們要處理的是requestMapping的handlerResolver，作為設計者，就很容易出如下的結構

在RequestMappingHandlerAdapter中的afterPropertiesSet去處理advice

@Override
public void afterPropertiesSet() {
    // Do this first, it may add ResponseBody advice beans
    initControllerAdviceCache();

    if (this.argumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
        this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.initBinderArgumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultInitBinderArgumentResolvers();
        this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.returnValueHandlers == null) {
        List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
        this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
    }
}

private void initControllerAdviceCache() {
    if (getApplicationContext() == null) {
        return;
    }

    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);
        }
        // 快取所有modelAttribute註解方法
        Set<Method> attrMethods = MethodIntrospector.selectMethods(beanType, MODEL_ATTRIBUTE_METHODS);
        if (!attrMethods.isEmpty()) {
            this.modelAttributeAdviceCache.put(adviceBean, attrMethods);
        }
        // 快取所有initBinder註解方法
        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);
    }
}

@ExceptionHandler處理

@ExceptionHandler顯然是在上述initHandlerExceptionResolvers(context)方法中。

同樣的，從BeanFactory中獲取HandlerExceptionResolver

/**
    * Initialize the HandlerExceptionResolver used by this class.
    * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
    * we default to no exception resolver.
    */
private void initHandlerExceptionResolvers(ApplicationContext context) {
    this.handlerExceptionResolvers = null;

    if (this.detectAllHandlerExceptionResolvers) {
        // Find all HandlerExceptionResolvers in the ApplicationContext, including ancestor contexts.
        Map<String, HandlerExceptionResolver> matchingBeans = BeanFactoryUtils
                .beansOfTypeIncludingAncestors(context, HandlerExceptionResolver.class, true, false);
        if (!matchingBeans.isEmpty()) {
            this.handlerExceptionResolvers = new ArrayList<>(matchingBeans.values());
            // We keep HandlerExceptionResolvers in sorted order.
            AnnotationAwareOrderComparator.sort(this.handlerExceptionResolvers);
        }
    }
    else {
        try {
            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.
        }
    }

    // Ensure we have at least some HandlerExceptionResolvers, by registering
    // default HandlerExceptionResolvers if no other resolvers are found.
    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");
        }
    }
}

我們很容易找到ExceptionHandlerExceptionResolver

同樣的在afterPropertiesSet去處理advice

@Override
public void afterPropertiesSet() {
    // Do this first, it may add ResponseBodyAdvice beans
    initExceptionHandlerAdviceCache();

    if (this.argumentResolvers == null) {
        List<HandlerMethodArgumentResolver> resolvers = getDefaultArgumentResolvers();
        this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);
    }
    if (this.returnValueHandlers == null) {
        List<HandlerMethodReturnValueHandler> handlers = getDefaultReturnValueHandlers();
        this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);
    }
}

private void initExceptionHandlerAdviceCache() {
    if (getApplicationContext() == null) {
        return;
    }

    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);
        }
    }
}

範例原始碼

https://github.com/realpdai/tech-pdai-spring-demos