从头学习爬虫（二十四）重构篇----WebMagic框架分析之scheduler

这系列文章主要分析分析webmagic框架，没有实战内容，如有实战问题可以讨论，也可以提供技术支持。

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Scheduler

我们先来看看接口

package us.codecraft.webmagic.scheduler;

import us.codecraft.webmagic.Request;
import us.codecraft.webmagic.Task;

/**
 * Scheduler is the part of url management.<br>
 * You can implement interface Scheduler to do:
 * manage urls to fetch
 * remove duplicate urls
 *
 * @author [email protected] <br>
 * @since 0.1.0
 */
public interface Scheduler {

    /**
     * add a url to fetch
     *
     * @param request request
     * @param task task
     */
    public void push(Request request, Task task);

    /**
     * get an url to crawl
     *
     * @param task the task of spider
     * @return the url to crawl
     */
    public Request poll(Task task);

}

也很简单，放，取 两个方法

我们再来看看默认调用实现scheduler的那个类QueueScheduler

package us.codecraft.webmagic.scheduler;

import us.codecraft.webmagic.Request;
import us.codecraft.webmagic.Task;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;


/**
 * Basic Scheduler implementation.<br>
 * Store urls to fetch in LinkedBlockingQueue and remove duplicate urls by HashMap.
 *
 * @author [email protected] <br>
 * @since 0.1.0
 */
public class QueueScheduler extends DuplicateRemovedScheduler implements MonitorableScheduler {

    private BlockingQueue<Request> queue = new LinkedBlockingQueue<Request>();

    @Override
    public void pushWhenNoDuplicate(Request request, Task task) {
        queue.add(request);
    }

    @Override
    public Request poll(Task task) {
        return queue.poll();
    }

    @Override
    public int getLeftRequestsCount(Task task) {
        return queue.size();
    }

    @Override
    public int getTotalRequestsCount(Task task) {
        return getDuplicateRemover().getTotalRequestsCount(task);
    }
}

没啥好看的。。我们主要看下实现那个接口和继承的类

DuplicateRemovedScheduler

package us.codecraft.webmagic.scheduler;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import us.codecraft.webmagic.Request;
import us.codecraft.webmagic.Task;
import us.codecraft.webmagic.scheduler.component.DuplicateRemover;
import us.codecraft.webmagic.scheduler.component.HashSetDuplicateRemover;
import us.codecraft.webmagic.utils.HttpConstant;

/**
 * Remove duplicate urls and only push urls which are not duplicate.<br><br>
 *
 * @author [email protected]
 * @since 0.5.0
 */
public abstract class DuplicateRemovedScheduler implements Scheduler {

    protected Logger logger = LoggerFactory.getLogger(getClass());

    private DuplicateRemover duplicatedRemover = new HashSetDuplicateRemover();

    public DuplicateRemover getDuplicateRemover() {
        return duplicatedRemover;
    }

    public DuplicateRemovedScheduler setDuplicateRemover(DuplicateRemover duplicatedRemover) {
        this.duplicatedRemover = duplicatedRemover;
        return this;
    }

    @Override
    public void push(Request request, Task task) {
        logger.trace("get a candidate url {}", request.getUrl());
        if (shouldReserved(request) || noNeedToRemoveDuplicate(request) || !duplicatedRemover.isDuplicate(request, task)) {
            logger.debug("push to queue {}", request.getUrl());
            pushWhenNoDuplicate(request, task);
        }
    }

    protected boolean shouldReserved(Request request) {
        return request.getExtra(Request.CYCLE_TRIED_TIMES) != null;
    }

    protected boolean noNeedToRemoveDuplicate(Request request) {
        return HttpConstant.Method.POST.equalsIgnoreCase(request.getMethod());
    }

    protected void pushWhenNoDuplicate(Request request, Task task) {

    }
}

简单理解下就是request get 重复请求去除，post 重复不去除，没有用布隆过滤，还有个接口MonitorableScheduler接口是提供监控功能，也就是查看还剩下多少待爬取的URL，和总共有多少URL

还有带优先级PriorityScheduler 

扩展

BloomFilterDuplicateRemover 用了布隆过滤器 重复post 也支持过滤了，没有测试过

FileCacheQueueScheduler 文件方式，主要是用于增量爬取记录url,这个指的是比如今天共100个页面我爬了20个下班了我关闭了爬虫，第二天他先把20个去重了。

RedisScheduler 加入了redis

RedisScheduler 加入了redis和优先级

总结

我们基本上把所有模块都分析完了，知道了工作原理，分析源码，知道了如何正确使用，接下来带来最后一篇。