008-Sentinel清洗RESTful的@PathVariable

這是堅持技術寫做計劃（含翻譯）的第8篇，定個小目標999，每週最少2篇。

前段時間的文章可能是運維方面的，最近放出一波後端相關的。

背景

最近開始使用Sentinel進行流量保護，可是默認的web servlet filter是攔截所有http請求。在傳統的項目中問題不大。可是若是項目中用了Spring MVC，而且用了@PathVariable就尷尬了。
好比 uri pattern是  /foo/{id} ,而從Sentinel監控看 /foo/1 和 /foo/2 就是兩個資源了，而且Sentinel最大支持6000個資源，再多就不生效了。

解決辦法

官方給的方案是:UrlCleaner

WebCallbackManager.setUrlCleaner(new UrlCleaner() {
            @Override
            public String clean(String originUrl) {
                if (originUrl.startsWith(fooPrefix)) {
                    return "/foo/*";
                }
                return originUrl;
            }
        });
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可是想一想就吐， /v1/{foo}/{bar}/qux/{baz} 這種的來個20來個，截一個我看看。

AOP

換種思路，uri pattern難搞，用笨辦法 aop總行吧？答案是能夠的。

@Aspect
public class SentinelResourceAspect {
    @Pointcut("within(com.anjia.*.web.rest..*)")
    public void sentinelResourcePackagePointcut() {
        // Method is empty as this is just a Pointcut, the implementations are
        // in the advices.
    }
    @Around("sentinelResourcePackagePointcut()")
    public Object sentinelResourceAround(ProceedingJoinPoint joinPoint) throws Throwable {
        Entry entry = null;
        // 務必保證finally會被執行
        try {
          // 資源名可以使用任意有業務語義的字符串
          // 注意此處只是類名#方法名，方法重載是合併的，若是須要進行區分，
          // 能夠獲取參數類型加入到資源名稱上
          entry = SphU.entry(joinPoint.getSignature().getDeclaringTypeName()+
                             "#"+joinPoint.getSignature().getName());
          // 被保護的業務邏輯
          // do something...
        } catch (BlockException ex) {
          // 資源訪問阻止，被限流或被降級
          // 進行相應的處理操做
        } finally {
          if (entry != null) {
            entry.exit();
          }
        }
        return result;
    }
}
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攔截器

溫習一下 Spring mvc的執行流程 doFilter -> doService -> dispatcher -> preHandle -> controller -> postHandle -> afterCompletion -> filterAfter

核心的是 String pattern = (String) request.getAttribute(HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE); 可是是在dispatcher階段才賦值的，因此在CommFilter是取不到的，因此致使使用官方的Filter是不行的。只能用攔截器

import com.alibaba.csp.sentinel.EntryType;
import com.alibaba.csp.sentinel.SphU;
import com.alibaba.csp.sentinel.adapter.servlet.callback.RequestOriginParser;
import com.alibaba.csp.sentinel.adapter.servlet.callback.UrlCleaner;
import com.alibaba.csp.sentinel.adapter.servlet.callback.WebCallbackManager;
import com.alibaba.csp.sentinel.adapter.servlet.util.FilterUtil;
import com.alibaba.csp.sentinel.context.ContextUtil;
import com.alibaba.csp.sentinel.log.RecordLog;
import com.alibaba.csp.sentinel.slots.block.BlockException;
import com.alibaba.csp.sentinel.util.StringUtil;
import org.apache.commons.lang3.StringUtils;
import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.HandlerMapping;
import org.springframework.web.servlet.ModelAndView;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@Component
public class SentinelHandlerInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        String origin = parseOrigin(request);
        String pattern = (String) request.getAttribute(HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE);
        String uriTarget = StringUtils.defaultString(pattern,FilterUtil.filterTarget(request));
        try {
            // Clean and unify the URL.
            // For REST APIs, you have to clean the URL (e.g. `/foo/1` and `/foo/2` -> `/foo/:id`), or
            // the amount of context and resources will exceed the threshold.
            UrlCleaner urlCleaner = WebCallbackManager.getUrlCleaner();
            if (urlCleaner != null) {
                uriTarget = urlCleaner.clean(uriTarget);
            }
            RecordLog.info(String.format("[Sentinel Pre Filter] Origin: %s enter Uri Path: %s", origin, uriTarget));
            SphU.entry(uriTarget, EntryType.IN);
            return true;
        } catch (BlockException ex) {
            RecordLog.warn(String.format("[Sentinel Pre Filter] Block Exception when Origin: %s enter fall back uri: %s", origin, uriTarget), ex);
            WebCallbackManager.getUrlBlockHandler().blocked(request, response, ex);
            return false;
        }
    }

    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        while (ContextUtil.getContext() != null && ContextUtil.getContext().getCurEntry() != null) {
            ContextUtil.getContext().getCurEntry().exit();
        }
        ContextUtil.exit();
    }

    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {

    }

    private String parseOrigin(HttpServletRequest request) {
        RequestOriginParser originParser = WebCallbackManager.getRequestOriginParser();
        String origin = EMPTY_ORIGIN;
        if (originParser != null) {
            origin = originParser.parseOrigin(request);
            if (StringUtil.isEmpty(origin)) {
                return EMPTY_ORIGIN;
            }
        }
        return origin;
    }


    private static final String EMPTY_ORIGIN = "";
}

複製代碼

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport;

@Configuration
public class WebConfig extends WebMvcConfigurerAdapter {
    @Inject
    SentinelHandlerInterceptor sentinelHandlerInterceptor;

    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(sentinelHandlerInterceptor);
    }
}

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UrlBlockHandler和UrlCleaner和WebServletConfig.setBlockPage(blockPage)

上面說過，UrlCleaner是爲了歸併請求，清洗url用的。而UrlBlockHandler是在被攔截後的默認處理器。可是clean和handler都不是鏈式的，因此若是有多種處理，須要本身在一個方法裏，進行邏輯判斷。

UrlCleaner

WebCallbackManager.setUrlCleaner(new UrlCleaner() {
            @Override
            public String clean(String originUrl) {
                if (originUrl.startsWith(fooPrefix)) {
                    return "/foo/*";
                }
                return originUrl;
            }
        });
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UrlBlockHandler
若是通用一點的，能夠本身根據request的 content-type進行自適應返回內容(PLAN_TEXT和JSON)

WebCallbackManager.setUrlBlockHandler((request, response, ex) -> {
    response.addHeader("Content-Type","application/json;charset=UTF-8");
    PrintWriter out = response.getWriter();
    out.print("{\"code\"":429,\"msg\":\"系統繁忙，請稍後重試\""}");
    out.flush();
    out.close();
});
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WebServletConfig.setBlockPage(blockPage)

WebServletConfig.setBlockPage("http://www.baidu.com")
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注意，三個方法都不是不支持調用鏈，好比我寫兩個UrlBlockHandler,只認最後一個。

參考資料

• wiki/主流框架的適配#web-servlet
• issues#REST API pattern UrlCleaner同一處理

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