A fixed rate limiter for HTTP requests. Can be used to avoid being blocked by throttle-limited API's like Facebook.

FixedRateInterceptor.java

package net.webapp.util;

import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.util.ArrayList;

import org.apache.http.HttpException;
import org.apache.http.HttpRequest;
import org.apache.http.HttpRequestInterceptor;
import org.apache.http.protocol.HttpContext;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.google.api.client.util.Lists;

public class FixedRateInterceptor implements HttpRequestInterceptor {
  private static final Logger logger = LoggerFactory
      .getLogger(FixedRateInterceptor.class);
  private ArrayList<Long> samples = Lists.newArrayList();
  private String host;
  private double maxRate;

  public FixedRateInterceptor(double rate, String host) {
    this.host = host;
    this.maxRate = rate;
  }

  public synchronized void process(HttpRequest request, HttpContext context)
      throws HttpException, IOException {
    String requestHost;
    try {
      requestHost = new URI(context.getAttribute("http.target_host")
          .toString()).getHost();
    } catch (URISyntaxException e1) {
      throw new IOException(e1);
    }
    if (requestHost.equalsIgnoreCase(host)) {
      while (rate() > maxRate) {
        if (logger.isDebugEnabled()) {
          logger.debug("rate {} maxRate {}", rate(), maxRate);
        }
        try {
          Thread.sleep(500);
        } catch (InterruptedException e) {
        }
      }
      sample();
    }
  }

  private synchronized double rate() {
    Long now = System.currentTimeMillis();
    if (samples.size() == 0 || now.equals(samples.get(0))) {
      return 0;
    }
    if (logger.isDebugEnabled()) {
      logger.debug("samples {} {} now {}", samples.size(),
          samples.get(0), now);
    }
    return (samples.size() * 1000D) / (now - samples.get(0));
  }

  private synchronized void sample() {
    samples.add(System.currentTimeMillis());
    if (samples.size() > 20) {
      samples.remove(0);
    }
  }
}