Zuul原理

@EnableZuulProxy和@EnableZuulServer

@EnableZuulProxy和@EnableZuulServer通过实例化不同的Marker，走不同的AutoConfiguration。

@EnableZuulProxy所走的ZuulProxyAutoConfiguration基础于@EnableZuulServer的ZuulServerAutoConfiguration。

所以两者的区别在于ZuulProxyAutoConfiguration在ZuulServerAutoConfiguration的基础上增加eureka、ribbon、hystrix等功能。

Zuul配置文件的读取

Zuul配置文件读取是通过在@Configuration注解的类上加@EnableConfigurationProperties注解引入ZuulProperties，而去自动读取的。

ZuulProperties通过routes存放每个路由信息。

Zuul的核心——ZuulServlet

ZuulServlet的实例化位于ZuulServerAutoConfiguration中，其通过ServletRegistrationBean注册了监听/zuul路径的ZuulServlet。

	@Bean
	@ConditionalOnMissingBean(name = "zuulServlet")
	@ConditionalOnProperty(name = "zuul.use-filter", havingValue = "false", matchIfMissing = true)
	public ServletRegistrationBean zuulServlet() {
		ServletRegistrationBean<ZuulServlet> servlet = new ServletRegistrationBean<>(
				new ZuulServlet(), this.zuulProperties.getServletPattern());
		servlet.addInitParameter("buffer-requests", "false");
		return servlet;
	}

但我们使用过程中访问的路径是不带/zuul的，其实现是通过ZuulServerAutoConfiguration实例化的ZuulController和ZuulHandlerMapping来实现的。

ZuulController继承了ServletWrappingController，将请求委托给ZuulServlet处理。

public class ZuulController extends ServletWrappingController {

	public ZuulController() {
		setServletClass(ZuulServlet.class);
		setServletName("zuul");
		setSupportedMethods((String[]) null); // Allow all
	}

	@Override
	public ModelAndView handleRequest(HttpServletRequest request,
			HttpServletResponse response) throws Exception {
		try {
			return super.handleRequestInternal(request, response);
		}
		finally {
		RequestContext.getCurrentContext().unset();
		}
	}

}

而ZuulHandlerMapping继承了AbstractUrlHandlerMapping，通过重写lookupHandler，将配置文件所配置的路径交给ZuulController处理。

public class ZuulHandlerMapping extends AbstractUrlHandlerMapping {

	private final RouteLocator routeLocator;

	private final ZuulController zuul;

	private volatile boolean dirty = true;

	@Override
	protected Object lookupHandler(String urlPath, HttpServletRequest request)
			throws Exception {
		RequestContext ctx = RequestContext.getCurrentContext();
		if (ctx.containsKey("forward.to")) {
			return null;
		}
        //刷新新Route配置
		if (this.dirty) {
			synchronized (this) {
				if (this.dirty) {
					registerHandlers();
					this.dirty = false;
				}
			}
		}
		return super.lookupHandler(urlPath, request);
	}

    //Route配置的路径，注册处理器为ZuulController
	private void registerHandlers() {
		Collection<Route> routes = this.routeLocator.getRoutes();
		if (routes.isEmpty()) {
			this.logger.warn("No routes found from RouteLocator");
		}
		else {
			for (Route route : routes) {
				registerHandler(route.getFullPath(), this.zuul);
			}
		}
	}

}

于是当我们配置一个route，hello->localhost:8080，我们可以通过/zuul/hello去访问，也可以通过/hello去访问。

下面我们看下ZuulServlet的逻辑，可以发现ZuulServlet的逻辑很简单，都是交给ZuulFilter去实现。

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);

            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();
        }
    }

}