1、 为什么需要异常处理？

　　在软件开发过程中，往往会出现一些不可预知的错误，这些错误有可能花费你数小时甚至数天的时间。异常处理通常是防止这种未知错误的常用措施，它的好处是你不用再绞尽脑汁去考虑各种错误，这为处理某一类错误提供了一个很有效的方法，使编程效率大大提高。 

 

2、springmvc的异常处理

　　

package com.roton.app.controller.resolver;import com.roton.app.bean.ResponseVo;import com.roton.app.bean.StatusCode;import com.roton.app.exception.RotonException;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.core.annotation.AnnotationUtils;import org.springframework.http.HttpOutputMessage;import org.springframework.http.MediaType;import org.springframework.http.converter.HttpMessageConverter;import org.springframework.http.converter.HttpMessageNotWritableException;import org.springframework.http.server.ServletServerHttpResponse;import org.springframework.stereotype.Component;import org.springframework.web.bind.annotation.ResponseBody;import org.springframework.web.bind.annotation.RestController;import org.springframework.web.method.HandlerMethod;import org.springframework.web.servlet.HandlerExceptionResolver;import org.springframework.web.servlet.ModelAndView;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import java.io.IOException;import java.lang.reflect.Method;import java.lang.reflect.Type;public class ExceptionResolverCustom implements HandlerExceptionResolver {    // json转换器    // 将异常信息转json       private HttpMessageConverter jsonMessageConverter;    private static final Logger logger = LoggerFactory.getLogger(ExceptionResolverCustom.class);    // 前端控制器调用此方法执行异常处理    // handler，执行的action类就包装了一个方法（对应url的方法）    @Override    public ModelAndView resolveException(HttpServletRequest request,                                         HttpServletResponse response, Object handler, Exception ex) {        logger.error(   ex.getMessage()   );        // 输出 异常信息        // 转成springmvc底层对象（就是对action方法的封装对象，只有一个方法）        HandlerMethod handlerMethod = (HandlerMethod) handler;        // 取出方法        Object obj = handlerMethod.getBean();        Method method = handlerMethod.getMethod();        //TODO   更具方法的返回值去定义返回        Type[] paramTypeList = method.getGenericParameterTypes();// 方法的参数列表        // 判断方法是否返回json        // 只要方法上有responsebody注解表示返回json        // 查询method是否有responsebody注解        RestController rest = AnnotationUtils.findAnnotation(obj.getClass(), RestController.class);        ResponseBody responseBody = AnnotationUtils.findAnnotation(method,                ResponseBody.class);        if (responseBody != null || rest != null) {            // 将异常信息转json输出            return this.resolveJsonException(request, response, handlerMethod, ex);        }        // 这里说明action返回的是jsp页面        // 解析异常        RotonException appException = resolveExceptionCustom(ex);        // 将异常信息在异常页面显示        // 转向错误页面        ModelAndView modelAndView = new ModelAndView();        modelAndView.addObject("exception",                appException.getMessage());        modelAndView.addObject("url",                request.getRequestURI());        if (appException.getMessage().contains("权限")){            modelAndView.setViewName("/401");// 逻辑视图名        }else {            modelAndView.setViewName("/error");// 逻辑视图名        }        return modelAndView;    }    // 异常信息解析方法    private RotonException resolveExceptionCustom(Exception ex) {        RotonException resultInfo = null;        if (ex instanceof RotonException) {            // 抛出的是系统自定义异常            resultInfo = (RotonException) ex;        } else {            //  AppException(String statusCode, String message) {            // 重新构造“未知错误”异常            resultInfo = new RotonException(StatusCode.FAILED, "未知错误！");        }        return resultInfo;    }    // 将异常信息转json输出    private ModelAndView resolveJsonException(HttpServletRequest request,                                              HttpServletResponse response, Object handler, Exception ex) {        // 解析异常        RotonException appException = resolveExceptionCustom(ex);        HttpOutputMessage outputMessage = new ServletServerHttpResponse(response);        try {            //将AppException对象转成json输出            //boolean status, String statusCode, String message            ResponseVo responseVo = new ResponseVo(false, StatusCode.FAILED, appException.getMessage());            jsonMessageConverter.write(responseVo, MediaType.APPLICATION_JSON, outputMessage);        } catch (HttpMessageNotWritableException e) {            e.printStackTrace();        } catch (IOException e) {            e.printStackTrace();        }        return new ModelAndView();    }    public HttpMessageConverter
     
       getJsonMessageConverter() {        return jsonMessageConverter;    }    public void setJsonMessageConverter(            HttpMessageConverter
      
        jsonMessageConverter) {        this.jsonMessageConverter = jsonMessageConverter;    }}