Motivation
说一下，为什么要阅读Netflix Zuul，最近在看alibaba/Sentinel 在网关方面的应用。发现网关的设计模式有很多通用的地方。Netflix Zuul 对比Zuul2 更能直接的看出网关的主要功能和核心设计，易于上手，所以选择这个。同时也再次实践一下怎么高效阅读源代码。
Design and Function
Zuul 的主要功能，包括，鉴权，路由，流量监控，负载，实时响应（动态配置和开关），错误处理。
![zuul]( "zuul")
这张图就是根据这些功能提出的设计，通过Filter 将整个过程连接起来，
可扩展性：
- 在不同阶段做不同的处理，做隔离。
- 通过groovy 脚本加载来实现动态加载新的filter。
ZuulFilter
下面我们就来看总重要的 ZuulFilter 的设计吧，首先是类结构图 ![]()
先看最基本的IZuulFilter 接口, 定义了两个方法，这个filter 要不要执行，怎么执行。
public interface IZuulFilter {
/**
* a "true" return from this method means that the run() method should be invoked
*
* @return true if the run() method should be invoked. false will not invoke the run() method
*/
boolean shouldFilter();
/**
* if shouldFilter() is true, this method will be invoked. this method is the core method of a ZuulFilter
*
* @return Some arbitrary artifact may be returned. Current implementation ignores it.
* @throws ZuulException if an error occurs during execution.
*/
Object run() throws ZuulException;
}
这里暂时提出疑问，throw ZuulException 会怎样？，返回任意result 怎么处理。
下面来看 Abstratct ZuulFilter类:
/**
* Base abstract class for ZuulFilters. The base class defines abstract methods to define:
* filterType() - to classify a filter by type. Standard types in Zuul are "pre" for pre-routing filtering,
* "route" for routing to an origin, "post" for post-routing filters, "error" for error handling.
* We also support a "static" type for static responses see StaticResponseFilter.
* Any filterType made be created or added and run by calling FilterProcessor.runFilters(type)
* <p/>
* filterOrder() must also be defined for a filter. Filters may have the same filterOrder if precedence is not
* important for a filter. filterOrders do not need to be sequential.
* <p/>
* ZuulFilters may be disabled using Archius Properties.
* <p/>
* By default ZuulFilters are static; they don't carry state. This may be overridden by overriding the isStaticFilter() property to false
*
* @author Mikey Cohen
* Date: 10/26/11
* Time: 4:29 PM
*/
public abstract class ZuulFilter implements IZuulFilter, Comparable<ZuulFilter> {
private final AtomicReference<DynamicBooleanProperty> filterDisabledRef = new AtomicReference<>();
/**
* to classify a filter by type. Standard types in Zuul are "pre" for pre-routing filtering,
* "route" for routing to an origin, "post" for post-routing filters, "error" for error handling.
* We also support a "static" type for static responses see StaticResponseFilter.
* Any filterType made be created or added and run by calling FilterProcessor.runFilters(type)
*
* @return A String representing that type
*/
abstract public String filterType();
/**
* filterOrder() must also be defined for a filter. Filters may have the same filterOrder if precedence is not
* important for a filter. filterOrders do not need to be sequential.
*
* @return the int order of a filter
*/
abstract public int filterOrder();
public int compareTo(ZuulFilter filter) {
return Integer.compare(this.filterOrder(), filter.filterOrder());
}
}
根据设计图，我们知道Filter有三个阶段 分别是 pre, route,post, 这里的filterType 就是指定filter type（不同type的执行顺序不一样）。filterOrder 方法就是指定相同type 下filter的执行顺序了。同时也是实现 Comparable接口的原因了。
关于自定义 type，我们稍后再看。
那我们来看看 这些type的顺序是怎么指定的，
/**
* Zuul Servlet filter to run Zuul within a Servlet Filter. The filter invokes pre-routing filters first,
* then routing filters, then post routing filters. Handled exceptions in pre-routing and routing
* call the error filters, then call post-routing filters. Errors in post-routing only invoke the error filters.
* Unhandled exceptions only invoke the error filters
*
* @author Mikey Cohen
* Date: 10/12/11
* Time: 2:54 PM
*/
public class ZuulServletFilter implements Filter {
private ZuulRunner zuulRunner;
@Override
public void init(FilterConfig filterConfig) throws ServletException {
String bufferReqsStr = filterConfig.getInitParameter("buffer-requests");
boolean bufferReqs = bufferReqsStr != null && bufferReqsStr.equals("true") ? true : false;
zuulRunner = new ZuulRunner(bufferReqs);
}
@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
try {
init((HttpServletRequest) servletRequest, (HttpServletResponse) servletResponse);
try {
preRouting();
} catch (ZuulException e) {
error(e);
postRouting();
return;
}
// Only forward onto to the chain if a zuul response is not being sent
if (!RequestContext.getCurrentContext().sendZuulResponse()) {
filterChain.doFilter(servletRequest, servletResponse);
return;
}
try {
routing();
} catch (ZuulException e) {
error(e);
postRouting();
return;
}
try {
postRouting();
} catch (ZuulException e) {
error(e);
return;
}
} catch (Throwable e) {
error(new ZuulException(e, 500, "UNCAUGHT_EXCEPTION_FROM_FILTER_" + e.getClass().getName()));
} finally {
RequestContext.getCurrentContext().unset();
}
}
}
现在 看到了，ZuulFilter是通过实现在Servlet Filter的生命周期中实现的。那现在我们可以猜想 preRouting() 里会把所有的pre Type 按顺序执行：
/**
* runs all "pre" filters. These filters are run before routing to the orgin.
*
* @throws ZuulException
*/
public void preRoute() throws ZuulException {
try {
runFilters("pre");
} catch (ZuulException e) {
throw e;
} catch (Throwable e) {
throw new ZuulException(e, 500, "UNCAUGHT_EXCEPTION_IN_PRE_FILTER_" + e.getClass().getName());
}
}
/**
* 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;
List<ZuulFilter> list = FilterLoader.getInstance().getFiltersByType(sType);
if (list != null) {
for (int i = 0; i < list.size(); i++) {
ZuulFilter zuulFilter = list.get(i);
Object result = processZuulFilter(zuulFilter);
if (result != null && result instanceof Boolean) {
bResult |= ((Boolean) result);
}
}
}
return bResult;
}
从代码可以看出process根据列表里的filter顺序执行，顺序在返回前就按order 排序了，这样，当一个 preFilter 执行抛出 ZuulException 之后，pre阶段就结束了，进到最初ServletFilter的后续阶段。
![]()
所以到这里为止，我们就知道Zuul是怎么工作和扩展的了，大家就可以根据自己的需要进行扩展了。
