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摘要: 接着上一篇《Spring Cloud Zuul遗失的世界(一)》，ZuulController继承了ServletWrappingController，将当前应用中的ZuulServlet直接包装为一个Controller，暴露为入口访问，在本篇文章中介绍etflix-zuul-core的代码Zuul的执行的生命周期等。

一.Netflix zuul core源码分析

com.netflix.zuul.http.ZuulServlet是ZuulFilter链执行的入口，通过service方法，提取请求到RequestContext，然后通过调用ZuulRunner，依次按顺序执行每种类型的Filter，完成整个Filter的生命周期，架构图如下所示。

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1.1 ZuulServlet代码分析

在ZuulConfiguration中点击com.netflix.zuul.http.ZuulServlet打开代码如下所示：

public class ZuulServlet extends HttpServlet {

    private static final long serialVersionUID = -3374242278843351500L;
    private ZuulRunner zuulRunner;


    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);

        String bufferReqsStr = config.getInitParameter("buffer-requests");
        boolean bufferReqs = bufferReqsStr != null && bufferReqsStr.equals("true") ? true : false;

        zuulRunner = new ZuulRunner(bufferReqs);
    }

    @Override
    public void service(javax.servlet.ServletRequest servletRequest, javax.servlet.ServletResponse servletResponse) throws ServletException, IOException {
        try {
            //调用zuulRunner.init(servletRequest, servletResponse)进行请求上下文的传递
            init((HttpServletRequest) servletRequest, (HttpServletResponse) servletResponse);

            // Marks this request as having passed through the "Zuul engine", as opposed to servlets
            // explicitly bound in web.xml, for which requests will not have the same data attached
            RequestContext context = RequestContext.getCurrentContext();
            context.setZuulEngineRan();

            try {
                preRoute();
            } catch (ZuulException e) {
                error(e);
                postRoute();
                return;
            }
            try {
                route();
            } catch (ZuulException e) {
                error(e);
                postRoute();
                return;
            }
            try {
                postRoute();
            } catch (ZuulException e) {
                error(e);
                return;
            }

        } catch (Throwable e) {
            error(new ZuulException(e, 500, "UNHANDLED_EXCEPTION_" + e.getClass().getName()));
        } finally {
            RequestContext.getCurrentContext().unset();
        }
    }

    /**
     * 执行 "post" ZuulFilters
     *
     * @throws ZuulException
     */
    void postRoute() throws ZuulException {
        zuulRunner.postRoute();
    }

    /**
     * 执行 "route" filters
     *
     * @throws ZuulException
     */
    void route() throws ZuulException {
        zuulRunner.route();
    }

    /**
     * 执行 "pre" filters
     *
     * @throws ZuulException
     */
    void preRoute() throws ZuulException {
        zuulRunner.preRoute();
    }

    /**
     * initializes request
     *
     * @param servletRequest
     * @param servletResponse
     */
    void init(HttpServletRequest servletRequest, HttpServletResponse servletResponse) {
        zuulRunner.init(servletRequest, servletResponse);
    }

    /**
     * sets error context info and executes "error" filters
     *
     * @param e
     */
    void error(ZuulException e) {
        RequestContext.getCurrentContext().setThrowable(e);
        zuulRunner.error();
    }


}

如上所示，ZuulServlet其实就是一个Servlet，service方法包含了Zuul的整个生命周期。

1.1.1 ZuulServlet init代码拆解分析

//init() - 注册一个ZuulRunner用于调用整个filter 链
void init(HttpServletRequest servletRequest, HttpServletResponse servletResponse) {
        zuulRunner.init(servletRequest, servletResponse);
  }

1.1.2 ZuulServlet service代码分析

   // 调用init将req,res置入上下文.获取并标记上下文此session已经通过进入
   @Override
    public void service(javax.servlet.ServletRequest servletRequest, javax.servlet.ServletResponse servletResponse) throws ServletException, IOException {
        try {
            init((HttpServletRequest) servletRequest, (HttpServletResponse) servletResponse);

            // Marks this request as having passed through the "Zuul engine", as opposed to servlets
            // explicitly bound in web.xml, for which requests will not have the same data attached
            RequestContext context = RequestContext.getCurrentContext();
            context.setZuulEngineRan();

            try {
                preRoute();
            } catch (ZuulException e) {
                error(e);
                postRoute();
                return;
            }
            try {
                route();
            } catch (ZuulException e) {
                error(e);
                postRoute();
                return;
            }
            try {
                postRoute();
            } catch (ZuulException e) {
                error(e);
                return;
            }

        } catch (Throwable e) {
            error(new ZuulException(e, 500, "UNHANDLED_EXCEPTION_" + e.getClass().getName()));
        } finally {
            RequestContext.getCurrentContext().unset();
        }
    }

正常情况下，请求只经过pre -> route -> post。
两层try…catch，内层只捕获ZuulException，而其他异常由外层捕获。
内层3个try…catch语句，只有pre,route抛出ZuulException时，才会执行errror，再执行post。而当post(88行)抛出ZuulException后，只会执行error。
外层捕获其他异常(内层try语句块中抛出的非ZuulException异常以及内层catch语句中抛出的所有异常)后，将HTTP状态码设置为500，同时交给error处理。
整个流程的终点有两个：post及error；而非只有post一个。

1.2 Zuul的请求上下文 RequestContext

com.netflix.zuul.context.RequestContext继承了ConcurrentHashMap<String, Object>，是Zuul Filter生命周期中处理http请求上下文,是一个threadlocal的Map.

public class RequestContext extends ConcurrentHashMap<String, Object> {

    private static final Logger LOG = LoggerFactory.getLogger(RequestContext.class);

    protected static Class<? extends RequestContext> contextClass = RequestContext.class;

    private static RequestContext testContext = null;

    protected static final ThreadLocal<? extends RequestContext> threadLocal = new ThreadLocal<RequestContext>() {
        @Override
        protected RequestContext initialValue() {
            try {
                return contextClass.newInstance();
            } catch (Throwable e) {
                throw new RuntimeException(e);
            }
        }
    };
    //其余省略
}

1.3 ContextLifecycleFilter

com.netflix.zuul.context.ContextLifecycleFilter是调用链外围finally中remove上文中threadlocal.

 public class ContextLifecycleFilter implements Filter {

    public void destroy() {}

    public void init(FilterConfig filterConfig) throws ServletException {}

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
        throws IOException, ServletException {
        try {
            chain.doFilter(req, res);
        } finally {
            RequestContext.getCurrentContext().unset();
        }
    }

}

1.4 Zuul的运行执行器-ZuulRunner

com.netflix.zuul.ZuulRunner，

public class ZuulRunner {

    private boolean bufferRequests;

    /**
     * Creates a new <code>ZuulRunner</code> instance.
     */
    public ZuulRunner() {
        this.bufferRequests = true;
    }

    /**
     *
     * @param bufferRequests - whether to wrap the ServletRequest in HttpServletRequestWrapper and buffer the body.
     */
    public ZuulRunner(boolean bufferRequests) {
        this.bufferRequests = bufferRequests;
    }

    /**
     * sets HttpServlet request and HttpResponse
     *
     * @param servletRequest
     * @param servletResponse
     */
    // ZuulRunner内传入的req/res就会被替换为wrapper类增强功能:
    public void init(HttpServletRequest servletRequest, HttpServletResponse servletResponse) {

        RequestContext ctx = RequestContext.getCurrentContext();
        if (bufferRequests) {
            ctx.setRequest(new HttpServletRequestWrapper(servletRequest));
        } else {
            ctx.setRequest(servletRequest);
        }

        ctx.setResponse(new HttpServletResponseWrapper(servletResponse));
    }
}

com.netflix.zuul.filters.ZuulServletFilter跟跟ZuulServlet是同一份代码.
com.netflix.zuul.monitoring，预留了CounterFactory与TracerFactory的接口，用来扩展实现counter与timer.