限流用法，时机：请求被转发之前调用。

• 令牌桶限流
• 一段时间里面往令牌桶里面放令牌，需求从令牌里拿令牌，拿不到（说明名额满了）就等着不给访问

package com.imooc.apigateway.filter;

import com.google.common.util.concurrent.RateLimiter;
import com.imooc.apigateway.exception.RateLimitException;
import com.netflix.zuul.ZuulFilter;
import org.springframework.stereotype.Component;
import static org.springframework.cloud.netflix.zuul.filters.support.FilterConstants.PRE_TYPE;
import static org.springframework.cloud.netflix.zuul.filters.support.FilterConstants.SERVLET_DETECTION_FILTER_ORDER;

/**
 * 限流
 */
@Component
public class RateLimitFilter extends ZuulFilter{

  // 谷歌提供的guava组件实现令牌桶的场景，每秒钟往里面放100个令牌。
  private static final RateLimiter RATE_LIMITER = RateLimiter.create(100);

  /**
   * 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
   */
  @Override
  public String filterType() {
    return PRE_TYPE;
  }

  /**
   * 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
   */
  @Override
  public int filterOrder() {
    return SERVLET_DETECTION_FILTER_ORDER - 1;
  }

  /**
   * 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
   */
  @Override
  public boolean shouldFilter() {
    return true;
  }

  /**
   * 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.
   */
  @Override
  public Object run() {
    if (!RATE_LIMITER.tryAcquire()) {
      throw new RateLimitException();
    }

    return null;
  }
}