SpringBoot接口如何对异常进行统一封装，并统一返回呢？以上文的参数校验为例，如何优雅的将参数校验的错误信息统一处理并封装返回呢？

为什么要优雅的处理异常

如果我们不统一的处理异常，经常会在controller层有大量的异常处理的代码， 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 add(@Valid @RequestBody UserParam userParam) {// 每个接口充斥着大量的异常处理try {// do something} catch(Exception e) {return ResponseEntity.fail("error");}return ResponseEntity.ok("success");}
}

那怎么实现统一的异常处理，特别是结合参数校验等封装？

实现案例

简单展示通过@ControllerAdvice进行统一异常处理。

@ControllerAdvice异常统一处理

对于400参数错误异常

@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 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");}}

对于自定义异常

/*** handle business exception.** @param businessException*            business exception* @return ResponseResult*/
@ResponseBody
@ExceptionHandler(BusinessException.class)
public ResponseResult processBusinessException(BusinessException businessException) {log.error(businessException.getLocalizedMessage(), businessException);// 这里可以屏蔽掉后台的异常栈信息，直接返回"business error"return ResponseResultEntity.fail(businessException, businessException.getLocalizedMessage());
}

对于其它异常

/*** handle other exception.** @param exception*            exception* @return ResponseResult*/
@ResponseBody
@ExceptionHandler(Exception.class)
public ResponseResult processException(Exception exception) {log.error(exception.getLocalizedMessage(), exception);// 这里可以屏蔽掉后台的异常栈信息，直接返回"server error"return ResponseResultEntity.fail(exception, exception.getLocalizedMessage());
}

Controller接口

（接口中无需处理异常）

@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 add(@Valid @RequestBody UserParam userParam) {return ResponseEntity.ok(userParam);}
}

运行测试

这里用postman测试下

进一步理解

我们再通过一些问题来帮助你更深入理解@ControllerAdvice。

@ControllerAdvice还可以怎么用？

除了通过@ExceptionHandler注解用于全局异常的处理之外，@ControllerAdvice还有两个用法：

• @InitBinder注解

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；

比如，在@ControllerAdvice注解的类中添加如下方法，来统一处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中传入参数（string类型）自动转化为Date类型

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

• @ModelAttribute注解

用来预设全局参数，比如最典型的使用Spring Security时将添加当前登录的用户信息（UserDetails)作为参数。

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

所有controller类中requestMapping方法都可以直接获取并使用currentUser

@PostMapping("saveSomething")
public ResponseEntity saveSomeObj(@ModelAttribute("currentUser") UserDetails operator) {// 保存操作，并设置当前操作人员的ID（从UserDetails中获得）return ResponseEntity.success("ok");
}

@ControllerAdvice是如何起作用的（原理）？

DispatcherServlet中onRefresh方法是初始化ApplicationContext后的回调方法，它会调用initStrategies方法，主要更新一些servlet需要使用的对象，包括国际化处理，requestMapping，视图解析等等。

/*** This implementation calls {@link #initStrategies}.*/
@Override
protected void onRefresh(ApplicationContext context) {initStrategies(context);
}/*** Initialize the strategy objects that this servlet uses.* 
May be overridden in subclasses in order to initialize further strategy objects.*/
protected void initStrategies(ApplicationContext context) {initMultipartResolver(context); // 文件上传initLocaleResolver(context); // i18n国际化initThemeResolver(context); // 主题initHandlerMappings(context); // requestMappinginitHandlerAdapters(context); // adaptersinitHandlerExceptionResolvers(context); // 异常处理initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);
}

从上述代码看，如果要提供@ControllerAdvice提供的三种注解功能，从设计和实现的角度肯定是实现的代码需要放在initStrategies方法中。

• @ModelAttribute和@InitBinder处理

具体来看，如果你是设计者，很显然容易想到：对于@ModelAttribute提供的参数预置和@InitBinder注解提供的预处理方法应该是放在一个方法中的，因为它们都是在进入requestMapping方法前做的操作。

如下方法是获取所有的HandlerAdapter，无非就是从BeanFactory中获取

private void initHandlerAdapters(ApplicationContext context) {this.handlerAdapters = null;if (this.detectAllHandlerAdapters) {// Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.Map 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");}}
}

我们要处理的是requestMapping的handlerResolver，作为设计者，就很容易出如下的结构

在RequestMappingHandlerAdapter中的afterPropertiesSet去处理advice

@Override
public void afterPropertiesSet() {// Do this first, it may add ResponseBody advice beansinitControllerAdviceCache();if (this.argumentResolvers == null) {List resolvers = getDefaultArgumentResolvers();this.argumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.initBinderArgumentResolvers == null) {List resolvers = getDefaultInitBinderArgumentResolvers();this.initBinderArgumentResolvers = new HandlerMethodArgumentResolverComposite().addResolvers(resolvers);}if (this.returnValueHandlers == null) {List handlers = getDefaultReturnValueHandlers();this.returnValueHandlers = new HandlerMethodReturnValueHandlerComposite().addHandlers(handlers);}
}private void initControllerAdviceCache() {if (getApplicationContext() == null) {return;}List adviceBeans = ControllerAdviceBean.findAnnotatedBeans(getApplicationContext());List