Springboot学习04-默认错误页面加载机制源码分析

 Springboot学习04-默认错误页面加载机制源码分析

前沿

 　　　　希望通过本文的学习，对错误页面的加载机制有这更神的理解

正文

 1-Springboot错误页面展示

2-Springboot默认错误处理逻辑

 1-将请求转发到BasicErrorController控制器来处理请求，

 2-浏览器请求响应BasicErrorController的errorHtml()方法，APP等客户端响应error()方法

 3-以浏览器的404错为例：最终返回一个modelAndView

  3-1-调用BasicErrorController的errorHtml(HttpServletRequest request, HttpServletResponse response)方法，其中status=404；//详见源码L-134

  3-2-调用AbstractErrorController的resolveErrorView方法，遍历ErrorMvcAutoConfiguration.errorViewResolvers，寻找需要的modelAndView；//详见源码L-142；162

  3-3-ErrorMvcAutoConfiguration.errorViewResolvers会有一个默认的DefaultErrorViewResolver，于是便执行DefaultErrorViewResolver.resolveErrorView()方法；//详见源码L-171；190

  3-4-DefaultErrorViewResolver.resolveErrorView()的具体实现：调用当前的this.resolve(status, model)，创建modelAndView；//即寻找error/404页面 //详见源码L-191；199

  3-5-如果创建error/404视图失败(即找不到error/404视图)，则创建error/4XX视图；否则，继续创建视图；//详见源码L-192；193

  3-6-如果创建error/4XX视图失败(即找不到error/4XX视图)，则创建默认名为error的视图，而error视图在静态累WhitelabelErrorViewConfiguration中进行配置和加载(即Springboot默认的Whitelabel Error Page页面)；//详见源码L-144

  3-7-根据实际获取到的视图，进行渲染

 3-源码分析 1//1-ErrorMvcAutoConfiguration配置类

//1-ErrorMvcAutoConfiguration配置类
package org.springframework.boot.autoconfigure.web.servlet.error;
@Configuration
@AutoConfigureBefore({WebMvcAutoConfiguration.class})//在WebMvcAutoConfiguration 配置之前完成peizhi 
public class ErrorMvcAutoConfiguration {
    
    //注册了一个 专门收集 error 发生时错误信息的bean 
    //DefaultErrorAttributes 实现了HandlerExceptionResolver， 通过对异常的处理， 填充 错误属性 ErrorAttributes 。 这个是boot 中的 controller 出现异常的时候会使用到的
    @Bean
    @ConditionalOnMissingBean(
        value = {ErrorAttributes.class},
        search = SearchStrategy.CURRENT
    )
    public DefaultErrorAttributes errorAttributes() {
        return new DefaultErrorAttributes(this.serverProperties.getError().isIncludeException());
    }
    
    //注册 BasicErrorController。 BasicErrorController 完成对所有 controller 发生异常情况的处理， 包括 异常和 4xx， 5xx 子类的；处理默认/error请求
    @Bean
    @ConditionalOnMissingBean(
        value = {ErrorController.class},
        search = SearchStrategy.CURRENT
    )
    public BasicErrorController basicErrorController(ErrorAttributes errorAttributes) {
        return new BasicErrorController(errorAttributes, this.serverProperties.getError(), this.errorViewResolvers);
    }
    //注册 错误页面的 定制器
    @Bean
    public ErrorMvcAutoConfiguration.ErrorPageCustomizer errorPageCustomizer() {
        return new ErrorMvcAutoConfiguration.ErrorPageCustomizer(this.serverProperties, this.dispatcherServletPath);
    }

}


//1-1-ErrorMvcAutoConfigurationde配置类的内部类:WhitelabelErrorViewConfiguration   
@Configuration
@ConditionalOnProperty(
    prefix = "server.error.whitelabel",
    name = {"enabled"},
    matchIfMissing = true
)
@Conditional({ErrorMvcAutoConfiguration.ErrorTemplateMissingCondition.class})
protected static class WhitelabelErrorViewConfiguration {
    //StaticView就是ErrorMvcAutoConfigurationde配置类的内部类:StaticView  
    private final ErrorMvcAutoConfiguration.StaticView defaultErrorView = new ErrorMvcAutoConfiguration.StaticView();

    protected WhitelabelErrorViewConfiguration() {
    }

    @Bean(name = {"error"})
    @ConditionalOnMissingBean(name = {"error"})
    public View defaultErrorView() {
        return this.defaultErrorView;
    }

    @Bean
    @ConditionalOnMissingBean
    public BeanNameViewResolver beanNameViewResolver() {
        BeanNameViewResolver resolver = new BeanNameViewResolver();
        resolver.setOrder(2147483637);
        return resolver;
    }
}


//1-2-ErrorMvcAutoConfigurationde配置类的内部类:StaticView  
//WhitelabelErrorViewConfiguration 逻辑
//1-WhitelabelErrorViewConfiguration 注册了 一个View， 同时 注册了BeanNameViewResolver，如果之前没有注册的话。
//2-BeanNameViewResolver 也是可以对View 进行处理的， 它的处理方式是根据 view 的name 查找对应的bean。 这里 defaultErrorView 也是一个bean， 其名字是 error。 即：如果发现请求是 /error， 那么如果其他 ViewResolver 处理不了， 就BeanNameViewResolver 来处理，BeanNameViewResolver 把defaultErrorView 渲染到浏览器
//3-可以看到， defaultErrorView 通常是异常处理的最后一个围墙， 因为 BeanNameViewResolver的优先级比较低defaultErrorView(实际就是StaticView)实现了 View ， 主要就是完成了对 页面的渲染， 提供了一个  render 方法。
private static class StaticView implements View {
    private static final Log logger = LogFactory.getLog(ErrorMvcAutoConfiguration.StaticView.class);

    private StaticView() {
    }

    public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
        if (response.isCommitted()) {
            String message = this.getMessage(model);
            logger.error(message);
        } else {
            StringBuilder builder = new StringBuilder();
            Date timestamp = (Date)model.get("timestamp");
            Object message = model.get("message");
            Object trace = model.get("trace");
            if (response.getContentType() == null) {
                response.setContentType(this.getContentType());
            }

            builder.append("<html><body><h1>Whitelabel Error Page</h1>").append("<p>This application has no explicit mapping for /error, so you are seeing this as a fallback.</p>").append("<div id='created'>").append(timestamp).append("</div>").append("<div>There was an unexpected error (type=").append(this.htmlEscape(model.get("error"))).append(", status=").append(this.htmlEscape(model.get("status"))).append(").</div>");
            if (message != null) {
                builder.append("<div>").append(this.htmlEscape(message)).append("</div>");
            }

            if (trace != null) {
                builder.append("<div style='white-space:pre-wrap;'>").append(this.htmlEscape(trace)).append("</div>");
            }

            builder.append("</body></html>");
            response.getWriter().append(builder.toString());
        }
    }
}

//1-3-ErrorMvcAutoConfigurationde配置类的内部类:ErrorPageCustomizer  
private static class ErrorPageCustomizer implements ErrorPageRegistrar, Ordered {
    private final ServerProperties properties;
    private final DispatcherServletPath dispatcherServletPath;

    protected ErrorPageCustomizer(ServerProperties properties, DispatcherServletPath dispatcherServletPath) {
        this.properties = properties;
        this.dispatcherServletPath = dispatcherServletPath;
    }
    //把 /error 这样的errorpage 注册到了servlet容器，使得它异常的时候,会转发到/error
    public void registerErrorPages(ErrorPageRegistry errorPageRegistry) {
        ErrorPage errorPage = new ErrorPage(this.dispatcherServletPath.getRelativePath(this.properties.getError().getPath()));
        errorPageRegistry.addErrorPages(new ErrorPage[]{errorPage});
    }

    public int getOrder() {
        return 0;
    }
}

//2-1-BasicErrorController类
package org.springframework.boot.autoconfigure.web.servlet.error;
@Controller
@RequestMapping({"${server.error.path:${error.path:/error}}"})
public class BasicErrorController extends AbstractErrorController {

    //当请求出现错误时，浏览器响应 ModelAndView errorHtml
    @RequestMapping(produces = {"text/html"})
    public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
        //示例：status = 404 NOT_FOUND
        HttpStatus status = this.getStatus(request);
        //这里的 model 是相关错误信息；示例：model={"timestamp":"Thu Dec 20 09:12:09 CST 2018","status" :"404","error": "Not Found","message": "No message available","path" :"/111"}
        Map<String, Object> model = Collections.unmodifiableMap(this.getErrorAttributes(request, this.isIncludeStackTrace(request, MediaType.TEXT_HTML)));
        response.setStatus(status.value());
        //这个完成了具体的处理过程；获取视图
        //这里的resolveErrorView 是AbstractErrorController.AbstractErrorController
        ModelAndView modelAndView = this.resolveErrorView(request, response, status, model);
        //如果modelAndView=null；则返回 new ModelAndView("error", model)；
        return modelAndView != null ? modelAndView : new ModelAndView("error", model);
    }
    
    //这里相对上面的方法，简单很多，它不会去使用 viewResolver 去处理， 因为它不需要任何的 view ，而是直接返回 text 格式数据，而不是 html 格式数据
    @RequestMapping
    public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {
        Map<String, Object> body = this.getErrorAttributes(request, this.isIncludeStackTrace(request, MediaType.ALL));
        HttpStatus status = this.getStatus(request);
        return new ResponseEntity(body, status);
    }

}


//2-2-AbstractErrorController抽象类
package org.springframework.boot.autoconfigure.web.servlet.error;
public abstract class AbstractErrorController implements ErrorController {
    
    protected ModelAndView resolveErrorView(HttpServletRequest request, HttpServletResponse response, HttpStatus status, Map<String, Object> model) {
        //这个errorViewResolvers 就是ErrorMvcAutoConfiguration.errorViewResolvers 成员变量，errorViewResolvers包含DefaultErrorViewResolver
        Iterator var5 = this.errorViewResolvers.iterator();
        ModelAndView modelAndView;
        do {
            if (!var5.hasNext()) {
                return null;
            }
            ErrorViewResolver resolver = (ErrorViewResolver)var5.next();
            modelAndView = resolver.resolveErrorView(request, status, model);
        } while(modelAndView == null);

        return modelAndView;
    }
}


//3-DefaultErrorViewResolver类
package org.springframework.boot.autoconfigure.web.servlet.error;
// DefaultErrorViewResolver 逻辑
//1-DefaultErrorViewResolver 作为 ErrorViewResolver 注入到 ErrorMvcAutoConfiguration 的构造中去；而 errorViewResolvers 其实就是直接 交给了 BasicErrorController。 也就是说， BasicErrorController 处理错误的时候，会使用 DefaultErrorViewResolver 提供的内容来进行页面渲染。
//2-DefaultErrorViewResolver是一个纯 boot 的内容，专门处理发生 error时候的 view
//3-当请求需要一个error view， 就先去 error/ 目录下面去找  error/  + viewName + .html 的文件（这里的viewName通常是 404 ，500 之类的错误的response status code）； 找到了 就直接展示（渲染）它。 否则就尝试去匹配4xx, 5xx，然后去找error/4xx.html或者 error/5xx.html两个页面，找到了就展示它。
public class DefaultErrorViewResolver implements ErrorViewResolver, Ordered {
    private static final Map<Series, String> SERIES_VIEWS;
    private ApplicationContext applicationContext;
    private final ResourceProperties resourceProperties;

    public ModelAndView resolveErrorView(HttpServletRequest request, HttpStatus status, Map<String, Object> model) {
        ModelAndView modelAndView = this.resolve(String.valueOf(status.value()), model);
        if (modelAndView == null && SERIES_VIEWS.containsKey(status.series())) {
            modelAndView = this.resolve((String)SERIES_VIEWS.get(status.series()), model);
        }

        return modelAndView;
    }

    private ModelAndView resolve(String viewName, Map<String, Object> model) {
        //示例：errorViewName = error/404
        String errorViewName = "error/" + viewName;
        //示例：从applicationContext中获取viewName="error/404"的可用模版
        TemplateAvailabilityProvider provider = this.templateAvailabilityProviders.getProvider(errorViewName, this.applicationContext);
        //如果provider=null，则返回this.resolveResource(errorViewName, model)
        //this.resolveResource<--见下面代码-->
        return provider != null ? new ModelAndView(errorViewName, model) : this.resolveResource(errorViewName, model);
    }

    //获取静态资源视图
    private ModelAndView resolveResource(String viewName, Map<String, Object> model) {
        // 静态的 location 包括 "classpath:/META-INF/resources/", "classpath:/resources/", "classpath:/static/", "classpath:/public/"
        String[] var3 = this.resourceProperties.getStaticLocations();
        int var4 = var3.length;

        for(int var5 = 0; var5 < var4; ++var5) {
            String location = var3[var5];

            try {
                Resource resource = this.applicationContext.getResource(location);
                resource = resource.createRelative(viewName + ".html");
                //资源必须要存在， 才会返回
                if (resource.exists()) {
                    return new ModelAndView(new DefaultErrorViewResolver.HtmlResourceView(resource), model);
                }
            } catch (Exception var8) {
                ;
            }
        }
        //如果各个静态目录下都没有找到那个html文件，那么就还是 返回null， 交给白标吧
        return null;
    }



    static {
        Map<Series, String> views = new EnumMap(Series.class);
        views.put(Series.CLIENT_ERROR, "4xx");
        views.put(Series.SERVER_ERROR, "5xx");
        SERIES_VIEWS = Collections.unmodifiableMap(views);
    }

    private static class HtmlResourceView implements View {
        private Resource resource;

        HtmlResourceView(Resource resource) {
            this.resource = resource;
        }

        public String getContentType() {
            return "text/html";
        }

        public void render(Map<String, ?> model, HttpServletRequest request, HttpServletResponse response) throws Exception {
            response.setContentType(this.getContentType());
            FileCopyUtils.copy(this.resource.getInputStream(), response.getOutputStream());
        }
    }
}

参考资料：

1-https://www.cnblogs.com/FlyAway2013/p/7944568.html