SpringCloud学习笔记(三)-网关Zuul

Zuul在SpringCloud中起到网关的作用，可用于请求路由转发、过滤、安全检查等作用，通过@EnableZuulProxy来开启网关的配置。

一、Zuul初始化

/**
 * 路由网关
 */
@EnableZuulProxy
@EnableEurekaClient
@SpringBootApplication(exclude={DataSourceAutoConfiguration.class})
@EnableSwagger2
public class RsmsZuulApplication {

    public static void main(String[] args) {
        SpringApplication.run(RsmsZuulApplication.class, args);
    }
}

该注解用于设置Zuul服务器断点，及配置一系列的前置或后置过滤器。可以看到在该注解中引入了ZuulProxyMarkerConfiguration类。

/**
 * Sets up a Zuul server endpoint and installs some reverse proxy filters in it, so it can
 * forward requests to backend servers. The backends can be registered manually through
 * configuration or via DiscoveryClient.
 *
 * @see EnableZuulServer for how to get a Zuul server without any proxying
 *
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Biju Kunjummen
 */
@EnableCircuitBreaker
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(ZuulProxyMarkerConfiguration.class)
public @interface EnableZuulProxy {
}

继续跟踪ZuulProxyMarkerConfiguration类，通过注释可以看到，这个类的作用主要是通过配置的Marker(Bean)来激活ZuulProxyAutoConfiguration类，ZuulProxyAutoConfiguration是用于zuul自动装配的类，springcloud中大量使用这种装配方式，在系统启动时完成自动装配。

/**
 * Responsible for adding in a marker bean to trigger activation of 
 * {@link ZuulProxyAutoConfiguration}
 *
 * @author Biju Kunjummen
 */

@Configuration
public class ZuulProxyMarkerConfiguration {
    @Bean
    public Marker zuulProxyMarkerBean() {
        return new Marker();
    }
    class Marker {
    }
}

ZuulProxyAutoConfiguration配置如下
RibbonCommandFactoryConfiguration：用户处理负载均衡相关

@Configuration
@Import({ RibbonCommandFactoryConfiguration.RestClientRibbonConfiguration.class,
        RibbonCommandFactoryConfiguration.OkHttpRibbonConfiguration.class,
        RibbonCommandFactoryConfiguration.HttpClientRibbonConfiguration.class,
        HttpClientConfiguration.class })
@ConditionalOnBean(ZuulProxyMarkerConfiguration.Marker.class)
public class ZuulProxyAutoConfiguration extends ZuulServerAutoConfiguration {
...
}

ZuulProxyAutoConfiguration继承ZuulServerAutoConfiguration，ZuulServerAutoConfiguration定义了一系列的过滤器、控制器、监听器等。
其中比较重要的有zuulServlet。
1：初始化ZuulServlet
ZuulServlet类似用SpringMVC的请求转发器DispatcherServlet，当有Http请求到来时，ZuulController会交给ZuulServlet处理。ZuulServlet在根据其所管理的Filter，按执行顺序执行Filter。zuulservlet的具体执行过程后续会单独讲解。

@Bean
    @ConditionalOnMissingBean(name = "zuulServlet")
    public ServletRegistrationBean zuulServlet() {
        ServletRegistrationBean servlet = new ServletRegistrationBean<>(new ZuulServlet(),
                this.zuulProperties.getServletPattern());
        // The whole point of exposing this servlet is to provide a route that doesn't
        // buffer requests.
        servlet.addInitParameter("buffer-requests", "false");
        return servlet;
    }

2：路由定位器
定义了多路定位器和简单定位器。

@Bean
@Primary
public CompositeRouteLocator primaryRouteLocator(
            Collection routeLocators) {
        return new CompositeRouteLocator(routeLocators);
}

@Bean
@ConditionalOnMissingBean(SimpleRouteLocator.class)
public SimpleRouteLocator simpleRouteLocator() {
        return new SimpleRouteLocator(this.server.getServletPrefix(),
                this.zuulProperties);
}

SimpleRouteLocator定义路由获取的具体过程，如对前缀StripPrefix的处理、重试的配置、路由的刷新、路由规则的匹配等都是在这个类中完成。

protected Route getSimpleMatchingRoute(final String path) {
        if (log.isDebugEnabled()) {
            log.debug("Finding route for path: " + path);
        }

        // This is called for the initialization done in getRoutesMap()
        getRoutesMap();

        if (log.isDebugEnabled()) {
            log.debug("servletPath=" + this.dispatcherServletPath);
            log.debug("zuulServletPath=" + this.zuulServletPath);
            log.debug("RequestUtils.isDispatcherServletRequest()="
                    + RequestUtils.isDispatcherServletRequest());
            log.debug("RequestUtils.isZuulServletRequest()="
                    + RequestUtils.isZuulServletRequest());
        }

                //预处理URL路径
        String adjustedPath = adjustPath(path);
                //根据路径获取匹配的路由
        ZuulRoute route = getZuulRoute(adjustedPath);

        return getRoute(route, adjustedPath);
}

protected Route getRoute(ZuulRoute route, String path) {
        if (route == null) {
            return null;
        }
        if (log.isDebugEnabled()) {
            log.debug("route matched=" + route);
        }
        String targetPath = path;
        String prefix = this.properties.getPrefix();
                //处理前缀
        if (path.startsWith(prefix) && this.properties.isStripPrefix()) {
            targetPath = path.substring(prefix.length());
        }
        if (route.isStripPrefix()) {
            int index = route.getPath().indexOf("*") - 1;
            if (index > 0) {
                String routePrefix = route.getPath().substring(0, index);
                targetPath = targetPath.replaceFirst(routePrefix, "");
                prefix = prefix + routePrefix;
            }
        }
                //是否重试
        Boolean retryable = this.properties.getRetryable();
        if (route.getRetryable() != null) {
            retryable = route.getRetryable();
        }
        return new Route(route.getId(), targetPath, route.getLocation(), prefix,
                retryable,
                route.isCustomSensitiveHeaders() ? route.getSensitiveHeaders() : null, 
                route.isStripPrefix());
    }

3：控制器
Http请求被控制器拦截后交给ZuulServlet处理。

@Bean
public ZuulController zuulController() {
        return new ZuulController();
}

执行顺序越小会越先执行，所有pre过滤器执行完成，才会执行route过滤器，同样所有的post过滤器都会再route过滤器之后执行。（zuulFileter的compareTo方法会比较Order大小，执行过程中，从注册的过滤器列表中获取到相关类型的过滤器，根据优先级从小到大排序。）

大概整理了下zuul自带的一些过滤器

过滤器类型	名称	执行顺序	功能
pre	ServletDetectionFilter	-3	判断请求头来源是zuulServlet还是DispatcherServlet，决定后续的处理机制
pre	FormBodyWrapperFilter	-1	解析表单数据重新包装，提供给下游服务使用
pre	DebugFilter	1	是否开启调试标志
pre	Servlet30WrapperFilter	-2	包装为符合servlet3.0规范的请求体
pre	PreDecorationFilter	5	路由转发前基本信息配置
post	SendResponseFilter	1000	处理正常的请求响应
error	SendErrorFilter	0	错误请求响应，早起版本是post的一种，新版本单独列为一种
post	LocationRewriteFilter	900	重写位置头信息为zuul url
route	SendForwardFilter	500	处理内部转发
route	RibbonRoutingFilter	10	使用Ribbon, Hystrix转发请求
route	SimpleHostRoutingFilter	100	普通请求转发

二、ZuulServlet执行过程

前面说过ZuulServlet是Http请求的处理入口，类似用SpringMVC的请求转发器DispatcherServlet，当有Http请求到来时，会先交给ZuulServlet。
ZuulServlet继承HttpServlet，所以大概过程也是初始化-调用service()-销毁。
从service方法可以看出，按pre过滤器-route过滤器-postRoute顺序执行，如果出现错误会调用filterType为error的过滤器处理。
前置过滤器出现错误，会调用error过滤器处理后再交给post过滤器处理。
route过滤器出现错误，也是会先调用error过滤器处理再交给post过滤器处理。
而当post过滤器出现错误，直接调用error过滤器处理，结束流程。

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 {
            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();
        }
    }
...
}

所有过滤器的执行都是通过FilterProcessor来完成，这是过滤器执行的核心类，FilterProcessor是一个单例对象，会根据传入的过滤器类型，获取相关过滤器，根据order排序后依次执行。