public final class ApplicationFilterChain implements FilterChain {
  private int pos = 0; //当前执行filter的offset
  private int n; //当前filter的数量
  private ApplicationFilterConfig[] filters; //filter配置类，通过getFilter()方法获取Filter
  private Servlet servlet
  @Override
  public void doFilter(ServletRequest request, ServletResponse response) {
    if (pos < n) {
      ApplicationFilterConfig filterConfig = filters[pos++];
      Filter filter = filterConfig.getFilter();
      filter.doFilter(request, response, this);
    } else {
      // filter都处理完毕后，执行servlet
      servlet.service(request, response);
    }
  }
}

private static <T> Invoker<T> buildInvokerChain(final Invoker<T> invoker, String key, String group) {
  Invoker<T> last = invoker;
  //只获取满足条件的Filter
  List<Filter> filters = ExtensionLoader.getExtensionLoader(Filter.class).getActivateExtension(invoker.getUrl(), key, group);
  if (filters.size() > 0) {
    for (int i = filters.size() - 1; i >= 0; i --) {
      final Filter filter = filters.get(i);
      final Invoker<T> next = last;
      last = new Invoker<T>() {
        ...
        public Result invoke(Invocation invocation) throws RpcException {
          return filter.invoke(next, invocation);
        }
        ...
      };
    }
  }
  return last;
}

public class Plugin implements InvocationHandler{
  private Object target;
  private Interceptor interceptor;
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {   
    if (满足代理条件) {
      return interceptor.intercept(new Invocation(target, method, args));
    }
    return method.invoke(target, args);   
  }
  //对传入的对象进行代理，可能是实际的Executor类，也可能是Plugin代理类
  public static Object wrap(Object target, Interceptor interceptor) {
    Class<?> type = target.getClass();
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }
}