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前言

在4. 分布式链路追踪客户端工具包Starter设计一文中，我们实现了基础的Starter包，里面提供了我们自己定义的Servlet过滤器和RestTemplate拦截器，其中Servlet过滤器叫做HoneyTracingFilter，仅提供了提取SpanContext，创建Span和开启Span的基础功能，所以本文将围绕如何增强Servlet过滤器展开讨论。

正文

一. Opentracing提供的TracingFilter

其实最简单的增强方式，就是使用TracingFilter来替换我们自己实现的HoneyTracingFilter，下面给出TracingFilter的源码实现。

@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain chain)throws IOException, ServletException {HttpServletRequest httpRequest = (HttpServletRequest) servletRequest;HttpServletResponse httpResponse = (HttpServletResponse) servletResponse;if (!isTraced(httpRequest, httpResponse)) {chain.doFilter(httpRequest, httpResponse);return;}if (servletRequest.getAttribute(SERVER_SPAN_CONTEXT) != null) {chain.doFilter(servletRequest, servletResponse);} else {SpanContext extractedContext = tracer.extract(Format.Builtin.HTTP_HEADERS,new HttpServletRequestExtractAdapter(httpRequest));final Span span = tracer.buildSpan(httpRequest.getMethod()).asChildOf(extractedContext).withTag(Tags.SPAN_KIND.getKey(), Tags.SPAN_KIND_SERVER).start();httpRequest.setAttribute(SERVER_SPAN_CONTEXT, span.context());for (ServletFilterSpanDecorator spanDecorator: spanDecorators) {spanDecorator.onRequest(httpRequest, span);}try (Scope scope = tracer.activateSpan(span)) {chain.doFilter(servletRequest, servletResponse);if (!httpRequest.isAsyncStarted()) {for (ServletFilterSpanDecorator spanDecorator : spanDecorators) {spanDecorator.onResponse(httpRequest, httpResponse, span);}}} catch (Throwable ex) {for (ServletFilterSpanDecorator spanDecorator : spanDecorators) {spanDecorator.onError(httpRequest, httpResponse, ex, span);}throw ex;} finally {if (httpRequest.isAsyncStarted()) {httpRequest.getAsyncContext().addListener(new AsyncListener() {@Overridepublic void onComplete(AsyncEvent event) throws IOException {HttpServletRequest httpRequest = (HttpServletRequest) event.getSuppliedRequest();HttpServletResponse httpResponse = (HttpServletResponse) event.getSuppliedResponse();for (ServletFilterSpanDecorator spanDecorator: spanDecorators) {spanDecorator.onResponse(httpRequest,httpResponse,span);}span.finish();}@Overridepublic void onTimeout(AsyncEvent event) throws IOException {HttpServletRequest httpRequest = (HttpServletRequest) event.getSuppliedRequest();HttpServletResponse httpResponse = (HttpServletResponse) event.getSuppliedResponse();for (ServletFilterSpanDecorator spanDecorator : spanDecorators) {spanDecorator.onTimeout(httpRequest,httpResponse,event.getAsyncContext().getTimeout(),span);}}@Overridepublic void onError(AsyncEvent event) throws IOException {HttpServletRequest httpRequest = (HttpServletRequest) event.getSuppliedRequest();HttpServletResponse httpResponse = (HttpServletResponse) event.getSuppliedResponse();for (ServletFilterSpanDecorator spanDecorator: spanDecorators) {spanDecorator.onError(httpRequest,httpResponse,event.getThrowable(),span);}}@Overridepublic void onStartAsync(AsyncEvent event) throws IOException {}});} else {span.finish();}}}
}

通过阅读TracingFilter源码，我们可以得到如下几种扩展增强。

Servlet自身的urlPatterns机制。可以通过配置urlPatterns，决定哪些请求需要打印链路信息；
TracingFilter的skipPattern机制。可以通过配置skipPattern，决定哪些请求不需要打印链路信息；
装饰器ServletFilterSpanDecorator。可以提供ServletFilterSpanDecorator给到TracingFilter，这样在收到请求，返回响应和处理异常时均可以做一些扩展操作；

二. urlPatterns和skipPattern设计

在第一节中得到的TracingFilter的几种增强，其中第1和第2点的urlPatterns和skipPattern，可以提供出来供用户配置，本节对这部分进行实现。

首先是配置属性类里面需要加入urlPatterns和skipPattern的配置属性，如下所示。

* 分布式链路追踪配置属性类。*/
@ConfigurationProperties("honey.tracing")
public class HoneyTracingProperties {private boolean enabled;private HttpUrlProperties httpUrl = new HttpUrlProperties();public boolean isEnabled() {return enabled;}public void setEnabled(boolean enabled) {this.enabled = enabled;}public HttpUrlProperties getHttpUrl() {return httpUrl;}public void setHttpUrl(HttpUrlProperties httpUrl) {this.httpUrl = httpUrl;}public static class HttpUrlProperties {* 按照/url1,/url2这样配置。*/private String urlPattern = "/*";* 按照/url1|/honey.*这样配置。*/private String skipPattern = "";public String getUrlPattern() {return urlPattern;}public void setUrlPattern(String urlPattern) {this.urlPattern = urlPattern;}public String getSkipPattern() {return skipPattern;}public void setSkipPattern(String skipPattern) {this.skipPattern = skipPattern;}}}

然后注册过滤器的配置类HoneyTracingFilterConfig需要做如下修改。

* Servlet过滤器配置类。*/
@Configuration
@AutoConfigureAfter(HoneyTracingConfig.class)
public class HoneyTracingFilterConfig {@Autowiredprivate HoneyTracingProperties honeyTracingProperties;@Beanpublic FilterRegistrationBean<TracingFilter> honeyTracingFilter(Tracer tracer) {String urlPattern = honeyTracingProperties.getHttpUrl().getUrlPattern();String skipPatternStr = honeyTracingProperties.getHttpUrl().getSkipPattern();Pattern skipPattern = Pattern.compile(skipPatternStr);FilterRegistrationBean<TracingFilter> registrationBean = new FilterRegistrationBean<>();registrationBean.addUrlPatterns(urlPattern);registrationBean.setOrder(Integer.MIN_VALUE);registrationBean.setFilter(new TracingFilter(tracer, new ArrayList<>(), skipPattern));return registrationBean;}}

三. TracingFilter的装饰器设计

通过为TracingFilter注册ServletFilterSpanDecorator装饰器，可以让我们在收到请求，返回响应和处理异常时做一些扩展操作，例如记录请求url，api和返回码等，下面实现一个装饰器HoneyServletFilterSpanDecorator，其提供如下几个功能。

收到请求时记录：

请求的host；
请求的api。

返回响应时记录：

响应码。

处理异常时记录：

响应码。

实现如下。

* {@link TracingFilter}的装饰器。*/
public class HoneyServletFilterSpanDecorator implements ServletFilterSpanDecorator {@Overridepublic void onRequest(HttpServletRequest httpServletRequest, Span span) {span.setTag(FIELD_HOST, getHostFromRequest(httpServletRequest));span.setTag(FIELD_API, httpServletRequest.getRequestURI());}@Overridepublic void onResponse(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Span span) {span.setTag(FIELD_HTTP_CODE, httpServletResponse.getStatus());}@Overridepublic void onError(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, Throwable exception, Span span) {span.setTag(FIELD_HTTP_CODE, httpServletResponse.getStatus());}@Overridepublic void onTimeout(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, long timeout, Span span) {}private String getHostFromRequest(HttpServletRequest httpServletRequest) {return httpServletRequest.getScheme()+ "://"+ httpServletRequest.getServerName()+ ":"+ httpServletRequest.getServerPort();}}

相关的常量字段记录在CommonConstants中，如下所示。

public class CommonConstants {public static final double DEFAULT_SAMPLE_RATE = 1.0;public static final String HONEY_TRACER_NAME = "HoneyTracer";public static final String HONEY_REST_TEMPLATE_NAME = "HoneyRestTemplate";public static final String FIELD_HOST = "host";public static final String FIELD_API = "api";public static final String FIELD_HTTP_CODE = "httpCode";}

在注册TracingFilter时需要将HoneyServletFilterSpanDecorator设置给TracingFilter，对应的配置类HoneyTracingFilterConfig修改如下。

* Servlet过滤器配置类。*/
@Configuration
@AutoConfigureAfter(HoneyTracingConfig.class)
public class HoneyTracingFilterConfig {@Autowiredprivate HoneyTracingProperties honeyTracingProperties;@Beanpublic FilterRegistrationBean<TracingFilter> honeyTracingFilter(Tracer tracer,List<ServletFilterSpanDecorator> decorators) {String urlPattern = honeyTracingProperties.getHttpUrl().getUrlPattern();String skipPatternStr = honeyTracingProperties.getHttpUrl().getSkipPattern();Pattern skipPattern = Pattern.compile(skipPatternStr);FilterRegistrationBean<TracingFilter> registrationBean = new FilterRegistrationBean<>();registrationBean.addUrlPatterns(urlPattern);registrationBean.setOrder(Integer.MIN_VALUE);registrationBean.setFilter(new TracingFilter(tracer, decorators, skipPattern));return registrationBean;}@Beanpublic ServletFilterSpanDecorator honeyServletFilterSpanDecorator() {return new HoneyServletFilterSpanDecorator();}}

至此，我们就使用装饰器装饰了TracingFilter，效果就是最终在TracingFilter调用到Span的finish() 方法时，我们可以从Span的tags中拿到本次请求的host，api和httpCode，这些数据可以最终在打印链路日志时使用。

最后给出工程目录结构图。