Spring MVC异常处理详解 ExceptionHandler good

@ControllerAdvice(basePackageClasses = AcmeController.class)
public class AcmeControllerAdvice extends ResponseEntityExceptionHandler {

    @ExceptionHandler(YourException.class)
    @ResponseBody
    ResponseEntity<?> handleControllerException(HttpServletRequest request, Throwable ex) {
        HttpStatus status = getStatus(request);
        return new ResponseEntity<>(new CustomErrorType(status.value(), ex.getMessage()), status);
    }

    private HttpStatus getStatus(HttpServletRequest request) {
        Integer statusCode = (Integer) request.getAttribute("javax.servlet.error.status_code");
        if (statusCode == null) {
            return HttpStatus.INTERNAL_SERVER_ERROR;
        }
        return HttpStatus.valueOf(statusCode);
    }

}

​​https://docs.spring.io/spring-boot/docs/2.0.0.RELEASE/reference/htmlsingle/​​​

下图中，我画出了Spring MVC中，跟异常处理相关的主要类和接口。

1.包含文件

• spring.xml
• messages_zh_CN.properties
• messages_en_US.properties
• ExceptionHandle.java
• XXController.java

2.文件内容

• spring.xml

<mvc:annotation-driven validator="validator" >  
     <mvc:message-converters>  
         <ref bean="stringHttpMessageConverter" />  
     </mvc:message-converters>  
  </mvc:annotation-driven>

    <!--避免错误信息是乱码-->
<util:list id="messageConverters">  
      <ref bean="stringHttpMessageConverter" />  
  </util:list>  

<bean id="stringHttpMessageConverter"  
    class="org.springframework.http.converter.StringHttpMessageConverter">  
      <constructor-arg value="UTF-8" index="0"/>
      <property name="supportedMediaTypes">  
          <list>  
            <!--项目中返回都是转成json格式的string，所以是text类型-->
              <value>text/plain;charset=UTF-8</value>  
               <value>text/html;charset=UTF-8</value>  
          </list>  
      </property> 
  </bean> 

<bean id="validator" class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"> 
   <property name="providerClass" value="org.hibernate.validator.HibernateValidator"/>  
       <property name="validationMessageSource" ref="messageSource"/>  
  </bean>  

  <bean id="messageSource" class="org.springframework.context.support.ReloadableResourceBundleMessageSource">  
      <property name="basenames">  
          <list>  
       <value>classpath:messages</value>  <value>classpath:org/hibernate/validator/ValidationMessages</value>  
          </list>  
      </property>  
      <property name="useCodeAsDefaultMessage" value="false"/>  
      <property name="defaultEncoding" value="UTF-8"/>  
      <property name="cacheSeconds" value="60"/>  
  </bean>

• messages_zh_CN.properties

#注意了，如果你的启动jvm是local是US他会去读取messages_en_US.properties文件
#如果不存在，就不会解析{uname.null}
uname.null=用户名不能为空

• ExceptionHandle.java

import java.util.stream.Collectors;
import lombok.extern.slf4j.Slf4j;
import org.springframework.validation.BindException;
import org.springframework.validation.ObjectError;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.ResponseBody;
import 
org.springframework.web.method.annotation.MethodArgumentTypeMismatchException;
import com.xx.customquery.exception.BusinessException;
import com.xx.customquery.exception.CommonMessageCode.SystemMessageCode;
import com.xx.customquery.util.Result;
@ControllerAdvice
@Slf4j
public class ExceptionHandle {
  @ExceptionHandler(BindException.class)
  @ResponseBody
  public String processValidationError(BindException ex) {
      log.error(ex.getMessage(), ex);
      String result = ex
              .getBindingResult()
              .getAllErrors()
              .stream()
              .map(ObjectError::getDefaultMessage)
              .collect(Collectors.joining(","));

      return Result.returnParamFailResult(result);
  }

  @ExceptionHandler(MethodArgumentTypeMismatchException.class)
  @ResponseBody
  public String processArgumentTypeMismatchException(MethodArgumentTypeMismatchException ex) {
      log.error(ex.getMessage(), ex);
      return Result.returnParamFailResult(ex.getMessage());
  }

  @ExceptionHandler(BusinessException.class)
  @ResponseBody
  public String processBusinessException(BusinessException ex) {
      log.error(ex.getMessage(), ex);
      return Result.returnFailResult(ex);
  }

  @ExceptionHandler(Throwable.class)
  @ResponseBody
  public String processException(Throwable ex) {
      log.error(ex.getMessage(), ex);
      return Result.returnFailResult(new BusinessException(SystemMessageCode.ERROR_SYSTEM));
  }
}

• Result.java

import com.google.gson.Gson;
import com.xx.customquery.exception.BusinessException;
public class Result<E> {
  public final static int SUCCESS = 0;
  public final static int FAIL = 999;
  public final static int PARAM_FAIL = -1;
  private final static Gson GSON = new GsonBuilder().disableHtmlEscaping().create();

  public static String returnSuccResult(){
      return GSON.toJson(new Result<String>(SUCCESS, "", ""));
  }
  public static String returnFailResult(String message){
      return GSON.toJson(new Result<String>(FAIL, message, ""));
  }
  public static String returnParamFailResult(String message){
      return GSON.toJson(new Result<String>(PARAM_FAIL, message, ""));
  }
  public static String returnFailResult(BusinessException exception){
      return GSON.toJson(new Result<String>(exception.getCode(), exception.getMessage(), ""));
  }
  public static <T> String returnDataResult(T data){
      return GSON.toJson(new Result<T>(SUCCESS, "", data));
  }

  private int code;
  private String message;
  private E data;
  public Result(){}
  public Result(int code,String message, E data){
      this.code = code;
      this.message = message;
      this.data = data;
  }
  public int getCode() {
      return code;
  }
  public void setCode(int code) {
      this.code = code;
  }
  public String getMessage() {
      return message;
  }
  public void setMessage(String message) {
      this.message = message;
  }
  public E getData() {
      return data;
  }
  public void setData(E data) {
      this.data = data;
  }
}

• XXController.java

@ResponseBody
@RequestMapping(value = "save", method = RequestMethod.POST)
public String saveQuery(@Valid Query querys) {    
}

• Query

import lombok.Data;
import org.hibernate.validator.constraints.NotBlank;
@Data
public class Queries {
  private long id;
  @NotBlank(message="{uname.null}")
  private String user;
}

​​http://www.jianshu.com/p/1390dc477d92​​

@ControllerAdvice源码

package org.springframework.web.bind.annotation;

import java.lang.annotation.Annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.core.annotation.AliasFor;
import org.springframework.stereotype.Component;

@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 {};

}

源码分析

@ ControllerAdvice是一个@ Component，
用于定义@ ExceptionHandler的，@InitBinder和@ModelAttribute方法，适用于所有使用@ RequestMapping方法，并处理所有@ RequestMapping标注方法出现异常的统一处理。

项目图片

这里写图片描述

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.jege.spring.boot</groupId>
    <artifactId>spring-boot-controller-advice</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>spring-boot-controller-advice</name>
    <url>http://maven.apache.org</url>

    <!-- 公共spring-boot配置，下面依赖jar文件不用在写版本号 -->
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.4.1.RELEASE</version>
        <relativePath />
    </parent>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>

        <!-- web -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>

        <!-- 持久层 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-jpa</artifactId>
        </dependency>

        <!-- h2内存数据库 -->
        <dependency>
            <groupId>com.h2database</groupId>
            <artifactId>h2</artifactId>
            <scope>runtime</scope>
        </dependency>

        <!-- 测试 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <!-- 只在test测试里面运行 -->
            <scope>test</scope>
        </dependency>

    </dependencies>

    <build>
        <finalName>spring-boot-controller-advice</finalName>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>${java.version}</source>
                    <target>${java.version}</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

全局异常处理类CommonExceptionAdvice

package com.jege.spring.boot.exception;

import java.util.Set;

import javax.validation.ConstraintViolation;
import javax.validation.ConstraintViolationException;
import javax.validation.ValidationException;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.dao.DataIntegrityViolationException;
import org.springframework.http.HttpStatus;
import org.springframework.http.converter.HttpMessageNotReadableException;
import org.springframework.validation.BindException;
import org.springframework.validation.BindingResult;
import org.springframework.validation.FieldError;
import org.springframework.web.HttpMediaTypeNotSupportedException;
import org.springframework.web.HttpRequestMethodNotSupportedException;
import org.springframework.web.bind.MethodArgumentNotValidException;
import org.springframework.web.bind.MissingServletRequestParameterException;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.annotation.ResponseBody;
import org.springframework.web.bind.annotation.ResponseStatus;

import com.jege.spring.boot.json.AjaxResult;

/**
 * @author JE哥
 * @email [email protected]
 * @description:全局异常处理
 */
@ControllerAdvice
@ResponseBody
public class CommonExceptionAdvice {

  private static Logger logger = LoggerFactory.getLogger(CommonExceptionAdvice.class);

  /**
   * 400 - Bad Request
   */
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  @ExceptionHandler(MissingServletRequestParameterException.class)
  public AjaxResult handleMissingServletRequestParameterException(MissingServletRequestParameterException e) {
    logger.error("缺少请求参数", e);
    return new AjaxResult().failure("required_parameter_is_not_present");
  }

  /**
   * 400 - Bad Request
   */
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  @ExceptionHandler(HttpMessageNotReadableException.class)
  public AjaxResult handleHttpMessageNotReadableException(HttpMessageNotReadableException e) {
    logger.error("参数解析失败", e);
    return new AjaxResult().failure("could_not_read_json");
  }

  /**
   * 400 - Bad Request
   */
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  @ExceptionHandler(MethodArgumentNotValidException.class)
  public AjaxResult handleMethodArgumentNotValidException(MethodArgumentNotValidException e) {
    logger.error("参数验证失败", e);
    BindingResult result = e.getBindingResult();
    FieldError error = result.getFieldError();
    String field = error.getField();
    String code = error.getDefaultMessage();
    String message = String.format("%s:%s", field, code);
    return new AjaxResult().failure(message);
  }

  /**
   * 400 - Bad Request
   */
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  @ExceptionHandler(BindException.class)
  public AjaxResult handleBindException(BindException e) {
    logger.error("参数绑定失败", e);
    BindingResult result = e.getBindingResult();
    FieldError error = result.getFieldError();
    String field = error.getField();
    String code = error.getDefaultMessage();
    String message = String.format("%s:%s", field, code);
    return new AjaxResult().failure(message);
  }

  /**
   * 400 - Bad Request
   */
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  @ExceptionHandler(ConstraintViolationException.class)
  public AjaxResult handleServiceException(ConstraintViolationException e) {
    logger.error("参数验证失败", e);
    Set<ConstraintViolation<?>> violations = e.getConstraintViolations();
    ConstraintViolation<?> violation = violations.iterator().next();
    String message = violation.getMessage();
    return new AjaxResult().failure("parameter:" + message);
  }

  /**
   * 400 - Bad Request
   */
  @ResponseStatus(HttpStatus.BAD_REQUEST)
  @ExceptionHandler(ValidationException.class)
  public AjaxResult handleValidationException(ValidationException e) {
    logger.error("参数验证失败", e);
    return new AjaxResult().failure("validation_exception");
  }

  /**
   * 405 - Method Not Allowed
   */
  @ResponseStatus(HttpStatus.METHOD_NOT_ALLOWED)
  @ExceptionHandler(HttpRequestMethodNotSupportedException.class)
  public AjaxResult handleHttpRequestMethodNotSupportedException(HttpRequestMethodNotSupportedException e) {
    logger.error("不支持当前请求方法", e);
    return new AjaxResult().failure("request_method_not_supported");
  }

  /**
   * 415 - Unsupported Media Type
   */
  @ResponseStatus(HttpStatus.UNSUPPORTED_MEDIA_TYPE)
  @ExceptionHandler(HttpMediaTypeNotSupportedException.class)
  public AjaxResult handleHttpMediaTypeNotSupportedException(Exception e) {
    logger.error("不支持当前媒体类型", e);
    return new AjaxResult().failure("content_type_not_supported");
  }

  /**
   * 500 - Internal Server Error
   */
  @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
  @ExceptionHandler(ServiceException.class)
  public AjaxResult handleServiceException(ServiceException e) {
    logger.error("业务逻辑异常", e);
    return new AjaxResult().failure("业务逻辑异常：" + e.getMessage());
  }

  /**
   * 500 - Internal Server Error
   */
  @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
  @ExceptionHandler(Exception.class)
  public AjaxResult handleException(Exception e) {
    logger.error("通用异常", e);
    return new AjaxResult().failure("通用异常：" + e.getMessage());
  }

  /**
   * 操作数据库出现异常:名称重复，外键关联
   */
  @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
  @ExceptionHandler(DataIntegrityViolationException.class)
  public AjaxResult handleException(DataIntegrityViolationException e) {
    logger.error("操作数据库出现异常:", e);
    return new AjaxResult().failure("操作数据库出现异常：字段重复、有外键关联等");
  }
}

自定义异常ServiceException

package com.jege.spring.boot.exception;

/**
 * @author JE哥
 * @email [email protected]
 * @description:自定义异常类
 */
public class ServiceException extends RuntimeException {
  public ServiceException(String msg) {
    super(msg);
  }
}

不需要application.properties

控制器AdviceController

package com.jege.spring.boot.controller;

import java.util.List;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import com.jege.spring.boot.exception.ServiceException;

/**
 * @author JE哥
 * @email [email protected]
 * @description:全局异常处理演示入口
 */
@RestController
public class AdviceController {

  @RequestMapping("/hello1")
  public String hello1() {
    int i = 1 / 0;
    return "hello";
  }

  @RequestMapping("/hello2")
  public String hello2(Long id) {
    String string = null;
    string.length();
    return "hello";
  }

  @RequestMapping("/hello3")
  public List<String> hello3() {
    throw new ServiceException("test");
  }
}

访问

• ​​http://localhost:8080/hello1​​{"meta":{"success":false,"message":"通用异常：/ by zero"},"data":null}
• ​​http://localhost:8080/hello2​​{"meta":{"success":false,"message":"通用异常：null"},"data":null}
• ​​http://localhost:8080/hello3​​{"meta":{"success":false,"message":"业务逻辑异常：test"},"data":null}

源码地址

​​https://github.com/je-ge/spring-boot​​

​​http://www.jianshu.com/p/5c5601789626​​

在Spring MVC中，所有用于处理在请求映射和请求处理过程中抛出的异常的类，都要实现HandlerExceptionResolver接口。AbstractHandlerExceptionResolver实现该接口和Orderd接口，是HandlerExceptionResolver类的实现的基类。ResponseStatusExceptionResolver等具体的异常处理类均在AbstractHandlerExceptionResolver之上，实现了具体的异常处理方式。一个基于Spring MVC的Web应用程序中，可以存在多个实现了HandlerExceptionResolver的异常处理类，他们的执行顺序，由其order属性决定, order值越小，越是优先执行, 在执行到第一个返回不是null的ModelAndView的Resolver时，不再执行后续的尚未执行的Resolver的异常处理方法。。

下面我逐个介绍一下SpringMVC提供的这些异常处理类的功能。

DefaultHandlerExceptionResolver

HandlerExceptionResolver接口的默认实现，基本上是Spring MVC内部使用，用来处理Spring定义的各种标准异常，将其转化为相对应的HTTP Status Code。其处理的异常类型有：

handleNoSuchRequestHandlingMethod
handleHttpRequestMethodNotSupported
handleHttpMediaTypeNotSupported
handleMissingServletRequestParameter
handleServletRequestBindingException
handleTypeMismatch
handleHttpMessageNotReadable
handleHttpMessageNotWritable
handleMethodArgumentNotValidException
handleMissingServletRequestParameter
handleMissingServletRequestPartException
handleBindException

ResponseStatusExceptionResolver

用来支持ResponseStatus的使用，处理使用了ResponseStatus注解的异常，根据注解的内容，返回相应的HTTP Status Code和内容给客户端。如果Web应用程序中配置了ResponseStatusExceptionResolver，那么我们就可以使用ResponseStatus注解来注解我们自己编写的异常类，并在Controller中抛出该异常类，之后ResponseStatusExceptionResolver就会自动帮我们处理剩下的工作。

这是一个自己编写的异常，用来表示订单不存在：

@ResponseStatus(value=HttpStatus.NOT_FOUND, reason="No such Order")  // 404
    public class OrderNotFoundException extends RuntimeException {
        // ...
    }

这是一个使用该异常的Controller方法：

@RequestMapping(value="/orders/{id}", method=GET)
    public String showOrder(@PathVariable("id") long id, Model model) {
        Order order = orderRepository.findOrderById(id);
        if (order == null) throw new OrderNotFoundException(id);
        model.addAttribute(order);
        return "orderDetail";
    }

这样，当OrderNotFoundException被抛出时，ResponseStatusExceptionResolver会返回给客户端一个HTTP Status Code为404的响应。

AnnotationMethodHandlerExceptionResolver和ExceptionHandlerExceptionResolver

用来支持ExceptionHandler注解，使用被ExceptionHandler注解所标记的方法来处理异常。其中AnnotationMethodHandlerExceptionResolver在3.0版本中开始提供，ExceptionHandlerExceptionResolver在3.1版本中开始提供，从3.2版本开始，Spring推荐使用ExceptionHandlerExceptionResolver。
如果配置了AnnotationMethodHandlerExceptionResolver和ExceptionHandlerExceptionResolver这两个异常处理bean之一，那么我们就可以使用ExceptionHandler注解来处理异常。

下面是几个ExceptionHandler注解的使用例子：

@Controller
public class ExceptionHandlingController {

  // @RequestHandler methods
  ...
  
  // 以下是异常处理方法
  
  // 将DataIntegrityViolationException转化为Http Status Code为409的响应
  @ResponseStatus(value=HttpStatus.CONFLICT, reason="Data integrity violation")  // 409
  @ExceptionHandler(DataIntegrityViolationException.class)
  public void conflict() {
    // Nothing to do
  }
  
  // 针对SQLException和DataAccessException返回视图databaseError
  @ExceptionHandler({SQLException.class,DataAccessException.class})
  public String databaseError() {
    // Nothing to do.  Returns the logical view name of an error page, passed to
    // the view-resolver(s) in usual way.
    // Note that the exception is _not_ available to this view (it is not added to
    // the model) but see "Extending ExceptionHandlerExceptionResolver" below.
    return "databaseError";
  }

  // 创建ModleAndView，将异常和请求的信息放入到Model中，指定视图名字，并返回该ModleAndView
  @ExceptionHandler(Exception.class)
  public ModelAndView handleError(HttpServletRequest req, Exception exception) {
    logger.error("Request: " + req.getRequestURL() + " raised " + exception);

    ModelAndView mav = new ModelAndView();
    mav.addObject("exception", exception);
    mav.addObject("url", req.getRequestURL());
    mav.setViewName("error");
    return mav;
  }
}

需要注意的是，上面例子中的ExceptionHandler方法的作用域，只是在本Controller类中。如果需要使用ExceptionHandler来处理全局的Exception，则需要使用ControllerAdvice注解。

@ControllerAdvice
class GlobalDefaultExceptionHandler {
    public static final String DEFAULT_ERROR_VIEW = "error";

    @ExceptionHandler(value = Exception.class)
    public ModelAndView defaultErrorHandler(HttpServletRequest req, Exception e) throws Exception {
        // 如果异常使用了ResponseStatus注解，那么重新抛出该异常，Spring框架会处理该异常。 
        if (AnnotationUtils.findAnnotation(e.getClass(), ResponseStatus.class) != null)
            throw e;

        // 否则创建ModleAndView，处理该异常。
        ModelAndView mav = new ModelAndView();
        mav.addObject("exception", e);
        mav.addObject("url", req.getRequestURL());
        mav.setViewName(DEFAULT_ERROR_VIEW);
        return mav;
    }
}

SimpleMappingExceptionResolver

提供了将异常映射为视图的能力，高度可定制化。其提供的能力有：

1. 根据异常的类型，将异常映射到视图；
2. 可以为不符合处理条件没有被处理的异常，指定一个默认的错误返回；
3. 处理异常时，记录log信息；
4. 指定需要添加到Modle中的Exception属性，从而在视图中展示该属性。

@Configuration
@EnableWebMvc 
public class MvcConfiguration extends WebMvcConfigurerAdapter {
    @Bean(name="simpleMappingExceptionResolver")
    public SimpleMappingExceptionResolver createSimpleMappingExceptionResolver() {
        SimpleMappingExceptionResolver r = new SimpleMappingExceptionResolver();

        Properties mappings = new Properties();
        mappings.setProperty("DatabaseException", "databaseError");
        mappings.setProperty("InvalidCreditCardException", "creditCardError");

        r.setExceptionMappings(mappings);  // 默认为空
        r.setDefaultErrorView("error");    // 默认没有
        r.setExceptionAttribute("ex"); 
        r.setWarnLogCategory("example.MvcLogger"); 
        return r;
    }
    ...
}

自定义ExceptionResolver

Spring MVC的异常处理非常的灵活，如果提供的ExceptionResolver类不能满足使用，我们可以实现自己的异常处理类。可以通过继承SimpleMappingExceptionResolver来定制Mapping的方式和能力，也可以直接继承AbstractHandlerExceptionResolver来实现其它类型的异常处理类。

Spring MVC是如何创建和使用这些Resolver的？

首先看Spring MVC是怎么加载异常处理bean的。

1. Spring MVC有两种加载异常处理类的方式，一种是根据类型，这种情况下，会加载ApplicationContext下所有实现了ExceptionResolver接口的bean，并根据其order属性排序，依次调用；一种是根据名字，这种情况下会加载ApplicationContext下，名字为handlerExceptionResolver的bean。
2. 不管使用那种加载方式，如果在ApplicationContext中没有找到异常处理bean，那么Spring MVC会加载默认的异常处理bean。
3. 默认的异常处理bean定义在DispatcherServlet.properties中。

org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
  org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
  org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver

以下代码摘自ispatcherServlet，描述了异常处理类的加载过程：

/**
 * Initialize the HandlerMappings used by this class.
 * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
 * we default to BeanNameUrlHandlerMapping.
 */
private void initHandlerMappings(ApplicationContext context) {
  this.handlerMappings = null;

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

  // Ensure we have at least one HandlerMapping, by registering
  // a default HandlerMapping if no other mappings are found.
  if (this.handlerMappings == null) {
    this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
    if (logger.isDebugEnabled()) {
      logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
    }
  }
}

然后看Spring MVC是怎么使用异常处理bean的。

1. Spring MVC把请求映射和处理过程放到try catch中，捕获到异常后，使用异常处理bean进行处理。
2. 所有异常处理bean按照order属性排序，在处理过程中，遇到第一个成功处理异常的异常处理bean之后，不再调用后续的异常处理bean。

以下代码摘自DispatcherServlet，描述了处理异常的过程。

/**
 * Process the actual dispatching to the handler.
 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
 * to find the first that supports the handler class.
 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
 * themselves to decide which methods are acceptable.
 * @param request current HTTP request
 * @param response current HTTP response
 * @throws Exception in case of any kind of processing failure
 */
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
  HttpServletRequest processedRequest = request;
  HandlerExecutionChain mappedHandler = null;
  boolean multipartRequestParsed = false;

  WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

  try {
    ModelAndView mv = null;
    Exception dispatchException = null;

    try {
      processedRequest = checkMultipart(request);
      multipartRequestParsed = (processedRequest != request);

      // Determine handler for the current request.
      mappedHandler = getHandler(processedRequest);
      if (mappedHandler == null || mappedHandler.getHandler() == null) {
        noHandlerFound(processedRequest, response);
        return;
      }

      // Determine handler adapter for the current request.
      HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

      // Process last-modified header, if supported by the handler.
      String method = request.getMethod();
      boolean isGet = "GET".equals(method);
      if (isGet || "HEAD".equals(method)) {
        long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
        if (logger.isDebugEnabled()) {
          logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
        }
        if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
          return;
        }
      }

      if (!mappedHandler.applyPreHandle(processedRequest, response)) {
        return;
      }

      // Actually invoke the handler.
      mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

      if (asyncManager.isConcurrentHandlingStarted()) {
        return;
      }

      applyDefaultViewName(request, mv);
      mappedHandler.applyPostHandle(processedRequest, response, mv);
    }
    catch (Exception ex) {
      dispatchException = ex;
    }
    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
  }
  catch (Exception ex) {
    triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
  }
  catch (Error err) {
    triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
  }
  finally {
    if (asyncManager.isConcurrentHandlingStarted()) {
      // Instead of postHandle and afterCompletion
      if (mappedHandler != null) {
        mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
      }
    }
    else {
      // Clean up any resources used by a multipart request.
      if (multipartRequestParsed) {
        cleanupMultipart(processedRequest);
      }
    }
  }
}


/**
 * Determine an error ModelAndView via the registered HandlerExceptionResolvers.
 * @param request current HTTP request
 * @param response current HTTP response
 * @param handler the executed handler, or {@code null} if none chosen at the time of the exception
 * (for example, if multipart resolution failed)
 * @param ex the exception that got thrown during handler execution
 * @return a corresponding ModelAndView to forward to
 * @throws Exception if no error ModelAndView found
 */
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
    Object handler, Exception ex) throws Exception {

  // Check registered HandlerExceptionResolvers...
  ModelAndView exMv = null;
  for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {
    exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);
    if (exMv != null) {
      break;
    }
  }
  if (exMv != null) {
    if (exMv.isEmpty()) {
      request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
      return null;
    }
    // We might still need view name translation for a plain error model...
    if (!exMv.hasView()) {
      exMv.setViewName(getDefaultViewName(request));
    }
    if (logger.isDebugEnabled()) {
      logger.debug("Handler execution resulted in exception - forwarding to resolved error view: " + exMv, ex);
    }
    WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
    return exMv;
  }

  throw ex;
}

何时该使用何种ExceptionResolver？

Spring提供了很多选择和非常灵活的使用方式，下面是一些使用建议：

1. 如果自定义异常类，考虑加上ResponseStatus注解；
2. 对于没有ResponseStatus注解的异常，可以通过使用ExceptionHandler+ControllerAdvice注解，或者通过配置SimpleMappingExceptionResolver，来为整个Web应用提供统一的异常处理。
3. 如果应用中有些异常处理方式，只针对特定的Controller使用，那么在这个Controller中使用ExceptionHandler注解。
4. 不要使用过多的异常处理方式，不然的话，维护起来会很苦恼，因为异常的处理分散在很多不同的地方。