Zuul源码分析

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zuul介绍：zuul是Netflix开原的一款服务网关。提供用户认证，复杂均衡，动态路由等功能。

准备工作：下载源码，安装JDK，Maven，以及IDE。

Zuul的核心代码是从ZuulServlet开始的。这个类继承自HttpServlet，然后重写了init()方法和service()方法。

init()方法如下

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@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);
}

在这个方法里面，首先调用父类的init方法初始化ServletConfig.同时也会对ZuulRunner进行初始化。

下面来看service方法。这个方法比较主要，下面会具体说。

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@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();
        }
    }

从这个方法里面可以看到先调用preRoute方法如果出现异常，就会调用postRoute方法。如果没有异常就会调用route方法。同样出现异常也会调用postRoute方法。这里面的三个方法，实际上就是你写配置文件的时候filter类型(一般zuul网关允许你定义三种类型的filter,pre,post,route)。我们跟进postRoute方法，看一下里面到底什么。进去之后会发现其实是委托给zuulRunner去工作。

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void postRoute() throws ZuulException {
       zuulRunner.postRoute();
   }

那就别废话了，来看下ZuulRunner的postRoute的方法。

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 /**
 * executes "post" filterType  ZuulFilters
 *
 * @throws ZuulException
 */
public void postRoute() throws ZuulException {
    FilterProcessor.getInstance().postRoute();
}

实际上ZuulRunner又交给FilterProcessor。那就走进FilterProcessor看个究竟，下面的代码是FilterProcessor里面的postRoute方法。发现也没干啥具体事儿，只是调了一个runFilters(“post”)方法。那我们就继续往前走吧。

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 /**
 * runs "post" filters which are called after "route" filters. ZuulExceptions from ZuulFilters are thrown.
 * Any other Throwables are caught and a ZuulException is thrown out with a 500 status code
 *
 * @throws ZuulException
 */
public void postRoute() throws ZuulException {
    try {
        runFilters("post");
    } catch (ZuulException e) {
        throw e;
    } catch (Throwable e) {
        throw new ZuulException(e, 500, "UNCAUGHT_EXCEPTION_IN_POST_FILTER_" + e.getClass().getName());
    }
}

下面是runFilters方法的具体实现。至此我们终于走到核心地带了。对于这段代码的解读大家可以看我在代码里面加的两行注释。已经说得很清楚了。

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 /**
 * runs all filters of the filterType sType/ Use this method within filters to run custom filters by type
 *
 * @param sType the filterType.
 * @return
 * @throws Throwable throws up an arbitrary exception
 */
public Object runFilters(String sType) throws Throwable {
    if (RequestContext.getCurrentContext().debugRouting()) {
        Debug.addRoutingDebug("Invoking {" + sType + "} type filters");
    }
    boolean bResult = false;
    //通过FilterLoader加载对应的Filter.
    List<ZuulFilter> list = FilterLoader.getInstance().getFiltersByType(sType);
    if (list != null) {
        for (int i = 0; i < list.size(); i++) {
            ZuulFilter zuulFilter = list.get(i);
            //分别执行每一个Filter
            Object result = processZuulFilter(zuulFilter);
            if (result != null && result instanceof Boolean) {
                bResult |= ((Boolean) result);
            }
        }
    }
    return bResult;
}

这段代码是上面提到的processZuulFilter方法的具体实现。同样，我也在这段代码的关键处加了一行注释。其实主要就是调用ZullFilter里面的runFilter()方法。

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/**
 * Processes an individual ZuulFilter. This method adds Debug information. Any uncaught Thowables are caught by this method and converted to a ZuulException with a 500 status code.
 *
 * @param filter
 * @return the return value for that filter
 * @throws ZuulException
 */
public Object processZuulFilter(ZuulFilter filter) throws ZuulException {

    RequestContext ctx = RequestContext.getCurrentContext();
    boolean bDebug = ctx.debugRouting();
    final String metricPrefix = "zuul.filter-";
    long execTime = 0;
    String filterName = "";
    try {
        long ltime = System.currentTimeMillis();
        filterName = filter.getClass().getSimpleName();

        RequestContext copy = null;
        Object o = null;
        Throwable t = null;

        if (bDebug) {
            Debug.addRoutingDebug("Filter " + filter.filterType() + " " + filter.filterOrder() + " " + filterName);
            copy = ctx.copy();
        }
        //这行是主要代码，调用ZuulFilter里面的runFilter()方法。
        ZuulFilterResult result = filter.runFilter();
        ExecutionStatus s = result.getStatus();
        execTime = System.currentTimeMillis() - ltime;

        switch (s) {
            case FAILED:
                t = result.getException();
                ctx.addFilterExecutionSummary(filterName, ExecutionStatus.FAILED.name(), execTime);
                break;
            case SUCCESS:
                o = result.getResult();
                ctx.addFilterExecutionSummary(filterName, ExecutionStatus.SUCCESS.name(), execTime);
                if (bDebug) {
                    Debug.addRoutingDebug("Filter {" + filterName + " TYPE:" + filter.filterType() + " ORDER:" + filter.filterOrder() + "} Execution time = " + execTime + "ms");
                    Debug.compareContextState(filterName, copy);
                }
                break;
            default:
                break;
        }

        if (t != null) throw t;

        usageNotifier.notify(filter, s);
        return o;

    } catch (Throwable e) {
        if (bDebug) {
            Debug.addRoutingDebug("Running Filter failed " + filterName + " type:" + filter.filterType() + " order:" + filter.filterOrder() + " " + e.getMessage());
        }
        usageNotifier.notify(filter, ExecutionStatus.FAILED);
        if (e instanceof ZuulException) {
            throw (ZuulException) e;
        } else {
            ZuulException ex = new ZuulException(e, "Filter threw Exception", 500, filter.filterType() + ":" + filterName);
            ctx.addFilterExecutionSummary(filterName, ExecutionStatus.FAILED.name(), execTime);
            throw ex;
        }
    }
}

来看下ZuulFilter里面的runFilter()方法。

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/**
 * runFilter checks !isFilterDisabled() and shouldFilter(). The run() method is invoked if both are true.
 *
 * @return the return from ZuulFilterResult
 */
public ZuulFilterResult runFilter() {
    ZuulFilterResult zr = new ZuulFilterResult();
    if (!isFilterDisabled()) {
        if (shouldFilter()) {
            Tracer t = TracerFactory.instance().startMicroTracer("ZUUL::" + this.getClass().getSimpleName());
            try {
                Object res = run();
                zr = new ZuulFilterResult(res, ExecutionStatus.SUCCESS);
            } catch (Throwable e) {
                t.setName("ZUUL::" + this.getClass().getSimpleName() + " failed");
                zr = new ZuulFilterResult(ExecutionStatus.FAILED);
                zr.setException(e);
            } finally {
                t.stopAndLog();
            }
        } else {
            zr = new ZuulFilterResult(ExecutionStatus.SKIPPED);
        }
    }
    return zr;
}

在这个方法里面首先检查以下该filter是否执行，然后就是调用run方法。其实就是调用你自己写的filter里面的run方法。

这就是zuul的整个工作流程了。

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