springboot情操陶冶-web配置(五)

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<String, List<String>> 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<ObjectError> allErrors = bindException.getAllErrors();

                List<String> 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
 **/
@Configuration
public class BootWebMvcConfigurer implements WebMvcConfigurer {

    @Override
    public void addInterceptors(InterceptorRegistry registry) {

    }

    @Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {

    }

    @Override
    public void extendHandlerExceptionResolvers(List<HandlerExceptionResolver> 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<HandlerExceptionResolver> 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<HandlerExceptionResolver> 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

1. 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

2. 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

3. 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

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

5. 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<ControllerAdviceBean, ExceptionHandlerMethodResolver> 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:

1. 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。
2. 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。