springboot情操陶冶-web配置(五)-白红宇

springboot情操陶冶-web配置(五)

阅读量：4699 次

发布时间：2019-06-09

本文共 10759 字，大约阅读时间需要 35 分钟。

本文讲讲mvc的异常处理机制，方便查阅以及编写合理的异常响应方式

入口例子

很简单，根据之前的文章，我们只需要复写WebMvcConfigurer接口的异常添加方法即可，如下

1.创建简单的异常处理类，本例针对绑定异常

package com.example.demo.web.validation;import com.example.demo.web.model.ResEntity;import com.google.gson.Gson;import org.slf4j.Logger;import org.slf4j.LoggerFactory;import org.springframework.validation.BindException;import org.springframework.validation.ObjectError;import org.springframework.web.servlet.ModelAndView;import org.springframework.web.servlet.handler.AbstractHandlerExceptionResolver;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import java.io.IOException;import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;/** * @author nanco * ------------- * resolve bindexception * ------------- * @create 18/9/9 */public class SimpleExceptionResolver extends AbstractHandlerExceptionResolver {    private static final Logger EXCEPTION_LOG = LoggerFactory.getLogger(SimpleExceptionResolver.class);    private final Map
    
     > errorResultMap = new HashMap<>(2);    private final String ERROR_KEY = "error_result";    private Gson gson = new Gson();    @Override    protected ModelAndView doResolveException(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) {        // only process BindException,unless return null to allow the next handler understanding the exception        if (BindException.class.isInstance(ex)) {            ResEntity resEntity = new ResEntity();            try {                BindException bindException = BindException.class.cast(ex);                List
     
       allErrors = bindException.getAllErrors();                List
      
        resMessages = new ArrayList<>(allErrors.size());                allErrors.stream().forEach(error -> {                    resMessages.add(error.getDefaultMessage());                });                errorResultMap.put(ERROR_KEY, resMessages);                resEntity.setData(errorResultMap);                response.getOutputStream().write(gson.toJson(resEntity).getBytes());            } catch (IOException e) {                EXCEPTION_LOG.error("process BindException fail.", e);            }            return new ModelAndView();        }        return null;    }}

2.实现WebMvcConfigurer接口后复写其中的extendHandlerExceptionResolvers()方法

package com.example.demo.web.config;import com.example.demo.web.validation.SimpleExceptionResolver;import org.springframework.context.annotation.Configuration;import org.springframework.http.converter.HttpMessageConverter;import org.springframework.web.servlet.HandlerExceptionResolver;import org.springframework.web.servlet.config.annotation.InterceptorRegistry;import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;import java.util.List;/** * @author nanco * ------------- * color the mvc config * ------------- * @create 2018/9/5 **/@Configurationpublic class BootWebMvcConfigurer implements WebMvcConfigurer {    @Override    public void addInterceptors(InterceptorRegistry registry) {    }    @Override    public void configureMessageConverters(List
    
     > converters) {    }    @Override    public void extendHandlerExceptionResolvers(List
     
       resolvers) {        // response first        resolvers.add(0, new SimpleExceptionResolver());    }}

上述简单的代码便会对系统抛出的BindException异常进行针对性的处理，从而返回合乎格式的响应体。当然这只是一小部分，笔者可以稍微从源码的角度来分析下spring的异常机制

源码层

查阅过DispatcherServlet源码的都知道，当出现异常的时候，则会尝试调用HandlerExceptionResolver解析器去根据异常进行视图渲染或者直接返回对应的错误信息。笔者按步骤来进行简单分析，从WebMvcConfigurationSupport入手

1.异常解析器注册

@Bean    public HandlerExceptionResolver handlerExceptionResolver() {        List
    
      exceptionResolvers = new ArrayList<>();        // 优先加载用户自定义的异常解析器，也可通过WebMvcConfigurer来复写        configureHandlerExceptionResolvers(exceptionResolvers);        // 当用户没有复写上述方法后，采取默认的异常解析器        if (exceptionResolvers.isEmpty()) {            addDefaultHandlerExceptionResolvers(exceptionResolvers);        }        // 扩增异常解析器，可见上文中的例子        extendHandlerExceptionResolvers(exceptionResolvers);        HandlerExceptionResolverComposite composite = new HandlerExceptionResolverComposite();        composite.setOrder(0);        composite.setExceptionResolvers(exceptionResolvers);        return composite;    }

2.直接看下spring内置的默认异常解析器吧，参考addDefaultHandlerExceptionResolvers()方法

protected final void addDefaultHandlerExceptionResolvers(List
    
      exceptionResolvers) {        // 1.异常的方法处理，跟@RequestMapping注解的方法调用类似        ExceptionHandlerExceptionResolver exceptionHandlerResolver = createExceptionHandlerExceptionResolver();        exceptionHandlerResolver.setContentNegotiationManager(mvcContentNegotiationManager());        exceptionHandlerResolver.setMessageConverters(getMessageConverters());        exceptionHandlerResolver.setCustomArgumentResolvers(getArgumentResolvers());        exceptionHandlerResolver.setCustomReturnValueHandlers(getReturnValueHandlers());        if (jackson2Present) {            exceptionHandlerResolver.setResponseBodyAdvice(                    Collections.singletonList(new JsonViewResponseBodyAdvice()));        }        if (this.applicationContext != null) {            exceptionHandlerResolver.setApplicationContext(this.applicationContext);        }        exceptionHandlerResolver.afterPropertiesSet();        exceptionResolvers.add(exceptionHandlerResolver);        // 2.携带@ResponseStatus注解的解析器        ResponseStatusExceptionResolver responseStatusResolver = new ResponseStatusExceptionResolver();        responseStatusResolver.setMessageSource(this.applicationContext);        exceptionResolvers.add(responseStatusResolver);        // 3.默认的异常解析器，针对spring的内置异常作下简单的response        exceptionResolvers.add(new DefaultHandlerExceptionResolver());    }

笔者主要关注ExceptionHandlerExceptionResolver和ResponseStatusExceptionResolver解析器，那就分块来简单的讲解把

ExceptionHandlerExceptionResolver

初始化状态的代码就不罗列了，读者直接阅读源码就知道，笔者此处作下初始化的总结

寻找所有的携带@ControllerAdvice注解的bean，包装成ExceptionHandlerMethodResolver方法解析器，由此来从中挑选出携带@ExceptionHandler注解的方法集合

对第一条中所得的方法集合，读取其中@ExceptionHandler注解的值(Throwable实现类)；无则读取对应方法实现了Throwable异常接口的参数集合。即得出exceptionTypes集合

对上述的exceptionTypes集合依次与对应的method形成映射，即方便针对指定的异常可以调用相应的方法来返回结果

对上述满足条件的ControllerAdvice ，结合ExceptionHandlerMethodResolver装入exceptionHandlerAdviceCache属性map中

封装参数解析器集合与返回值解析器集合，和处理@RequestMapping的操作一样

具体的解析过程，笔者此处点一下，方便与上文对照着看，直接看关键的getExceptionHandlerMethod()方法

protected ServletInvocableHandlerMethod getExceptionHandlerMethod(            @Nullable HandlerMethod handlerMethod, Exception exception) {        Class
     handlerType = null;        if (handlerMethod != null) {            // 获取出现异常类方法的所在类            handlerType = handlerMethod.getBeanType();            // 优先判断如果此类直接返回的是异常类，则尝试寻找解析器            ExceptionHandlerMethodResolver resolver = this.exceptionHandlerCache.get(handlerType);            if (resolver == null) {                                // 查找异常所在类是否有符合的@ExceptionHandler语法方法                resolver = new ExceptionHandlerMethodResolver(handlerType);                this.exceptionHandlerCache.put(handlerType, resolver);            }            // 得到映射的方法            Method method = resolver.resolveMethod(exception);            if (method != null) {                return new ServletInvocableHandlerMethod(handlerMethod.getBean(), method);            }            // For advice applicability check below (involving base packages, assignable types            // and annotation presence), use target class instead of interface-based proxy.            if (Proxy.isProxyClass(handlerType)) {                handlerType = AopUtils.getTargetClass(handlerMethod.getBean());            }        }        // 进入@ControlleAdvice的语法环境了，判断抛异常的所在类，ControllerAdvice是否支持        for (Map.Entry
    
      entry : this.exceptionHandlerAdviceCache.entrySet()) {            ControllerAdviceBean advice = entry.getKey();                        // 如果@ControllerAdvice注解无任何的属性配置，则默认是支持的            if (advice.isApplicableToBeanType(handlerType)) {                ExceptionHandlerMethodResolver resolver = entry.getValue();                Method method = resolver.resolveMethod(exception);                if (method != null) {                    return new ServletInvocableHandlerMethod(advice.resolveBean(), method);                }            }        }        return null;    }

最终就是根据Exception的类型找寻符合条件的method，然后按照@RequestMapping注解的处理方式得到相应的视图对象供视图解析器去渲染

ResponseStatusExceptionResolver

针对携带@ResponseStatus注解的异常类来返回响应体的，简单的看下代码吧

@Override    @Nullable    protected ModelAndView doResolveException(            HttpServletRequest request, HttpServletResponse response, @Nullable Object handler, Exception ex) {        try {            // 直接返回的是ResponseStatusException类型的异常则直接处理            if (ex instanceof ResponseStatusException) {                return resolveResponseStatusException((ResponseStatusException) ex, request, response, handler);            }            // 读取异常类上携带的@ResponseStatus注解，有则返回结果            ResponseStatus status = AnnotatedElementUtils.findMergedAnnotation(ex.getClass(), ResponseStatus.class);            if (status != null) {                return resolveResponseStatus(status, request, response, handler, ex);            }            // 递归调用下            if (ex.getCause() instanceof Exception) {                ex = (Exception) ex.getCause();                return doResolveException(request, response, handler, ex);            }        }        catch (Exception resolveEx) {            logger.warn("ResponseStatus handling resulted in exception", resolveEx);        }        // 无符合条件的，直接返回null，调用下一个异常解析器        return null;    }

最终调用的也就是HttpServletResponse#sendError(int statusCode,String reason)方法直接返回结果

DispatcherServlet异常处理逻辑

此处还是贴下重要的代码片段，加深印象，直接查阅processHandlerException()方法

protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,            @Nullable Object handler, Exception ex) throws Exception {....if (this.handlerExceptionResolvers != null) {            // 对异常解析器集合进行遍历            for (HandlerExceptionResolver handlerExceptionResolver : this.handlerExceptionResolvers) {                exMv = handlerExceptionResolver.resolveException(request, response, handler, ex);                // ModelAndView对象不为null则直接跳出，否则采取下一个异常解析器                if (exMv != null) {                    break;                }            }        }}....

温馨提示:

根据上述代码的逻辑可见，用户在自定义相应的异常解析器时，需要注意如果满足解析指定的异常，则最后返回不为null的视图对象(return new ModelAndView())，以免其跑至下一个异常解析器，影响服务执行结果。

遍历的异常解析器顺序此处提一下，其采取的是简单的ArrayList集合来保持顺序，所以用户如果想自己的异常解析器保持较高的优先级，则可以采取List接口的add(int index, T value)方法添加或者直接实现HandlerExceptionResolver并设置order属性来保持即可

结语

了解异常解析器的加载机制以及运行逻辑，方便我们写出合乎spring逻辑的代码，以此保证代码的整洁性。

转载于:https://www.cnblogs.com/question-sky/p/9729958.html

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