Java开发工程师必备技能：了解搜索引擎如何展示结果

一、搜索引擎结果展示的基本原理

搜索引擎的主要功能是根据用户输入的关键词，从众多网页中筛选出最相关的内容，并将这些内容显示在搜索结果页面上。这个过程可以分为三个基本步骤：

1、爬虫抓取：搜索引擎利用爬虫程序（也称为蜘蛛）从互联网上抓取网页。

public class Spider {

    private static final int MAX_PAGES_TO_SEARCH = 10;

    private Set pagesVisited = new HashSet
   ();
    private List
     pagesToVisit = new LinkedList
     ();

    /**
     * Our main launching point for the Spider's functionality. Internally it creates spider legs
     * that make an HTTP request and parse the response (the web page).
     * 
     * @param url
     *            - The starting point of the spider
     * @param searchWord
     *            - The word or string that you are searching for
     */
    public void search(String url, String searchWord)
    {
        while(this.pagesVisited.size() < MAX_PAGES_TO_SEARCH)
        {
            String currentUrl;
            SpiderLeg leg = new SpiderLeg();
            if(this.pagesToVisit.isEmpty())
            {
                currentUrl = url;
                this.pagesVisited.add(url);
            }
            else
            {
                currentUrl = this.nextUrl();
            }
            leg.crawl(currentUrl); // Lots of stuff happening here. Look at the crawl method in
                                   // SpiderLeg
            boolean success = leg.searchForWord(searchWord);
            if(success)
            {
                System.out.println(String.format("**Success** Word %s found at %s", searchWord, currentUrl));
                break;
            }
            this.pagesToVisit.addAll(leg.getLinks());
        }
        System.out.println(String.format("**Done** Visited %s web page(s)", this.pagesVisited.size()));
    }

    /**
     * Returns the next URL to visit (in the order that they were found). We also do a check to
     * make sure this method doesn't return a URL that has already been visited.
     * 
     * @return
     */
    private String nextUrl()
    {
        String nextUrl;
        do
        {
            nextUrl = this.pagesToVisit.remove(0);
        } while(this.pagesVisited.contains(nextUrl));
        this.pagesVisited.add(nextUrl);
        return nextUrl;
    }
}

     
    
   
  

2、索引处理：搜索引擎将爬虫抓取到的网页存储进索引库中，同时对网页的主要内容进行索引处理。索引处理的目的是能够快速准确地找到包含关键字的网页，这个过程主要是通过计算TF-IDF等算法来实现。

public class Indexer {

    private WebCrawler spider;

    private Map frequencyToUrlMap;
    private Map
   > wordUrlsMap;

    public Indexer(WebCrawler spider) {
        this.spider = spider;
        frequencyToUrlMap = new HashMap
    ();
        wordUrlsMap = new HashMap
     >();
    }

    /**
     * Index a page by its URL
     * 
     * @param url
     *            - The URL of the page to be indexed
     */
    public void indexPage(String url) {
        System.out.println("Indexing " + url);
        Document document = spider.getDocument(url);
        String text = document.text();
        List
       words = spider.getWordsFromDocument(text);

        // Count frequency of web pages
        for (String word : words) {
            if (!wordUrlsMap.containsKey(word)) {
                wordUrlsMap.put(word, new HashMap
       
        ()); } Map
        
         urlToCountMap = wordUrlsMap.get(word); if (!urlToCountMap.containsKey(url)) { urlToCountMap.put(url, 0); } urlToCountMap.put(url, urlToCountMap.get(url) + 1); } // Map frequency to URL int frequency = 0; for (Map.Entry
         
          > entry : wordUrlsMap.entrySet()) { String word = entry.getKey(); Map
          
           urlToCountMap = entry.getValue(); frequency = 0; for (Map.Entry
           
            urlEntry : urlToCountMap.entrySet()) { frequency += urlEntry.getValue(); } // TODO: implement sorting by frequency frequencyToUrlMap.put(word, url); System.out.println("indexing " + word + ", " + url); } } }
           
          
         
        
       
      
     
    
   
  

3、结果展示：当用户输入关键字之后，搜索引擎会将包含关键字的网页内容展示在搜索结果页面上，同时还会对这些网页进行排名，将最相关的网页排名靠前。

二、搜索引擎结果展示的主要形式

搜索引擎结果通常有以下几种主要形式：

1、蓝色链接+标题+描述：这是搜索结果最常见的展示形式，用户在搜索后会看到一系列链接，每个链接后面跟着网页的标题和描述信息，让用户可以对结果进行初步筛选。

Example Domain
This domain is established to be used for illustrative examples in documents. You may use this domain in examples without prior coordination or asking for permission.

2、图文结果：对于一些特定的搜索，搜索引擎也会展示图文结果，包括图片、视频、新闻等信息。这种展示形式更加直观且易于用户理解。

  

    
    Example Link
    
   
This is an example description of the linked page.

  

3、补全输入：搜索引擎在用户输入关键字的时候，可能会显示一些类似的查询建议，这些建议通常是搜索目前所拥有的数据中出现过的最近最频繁的关键字组合。

三、优化 Java 程序，提高搜索效率

搜索引擎的搜索效率直接影响用户的使用感受，因此优化程序是非常重要的。以下是一些可以提高搜索效率的方法：

1、多线程并发处理：多线程并发处理可以让爬虫同时抓取多个网页，减少等待时间，提高效率。

public void search(List urls, String searchWord) {
    ExecutorService executor = Executors.newFixedThreadPool(10); // use a Thread pool
    final Spider spider = new Spider();

    for (final String url : urls) {
        executor.execute(new Runnable() {
            @Override
            public void run() {
                spider.search(url, searchWord);
            }
        });
    }

    executor.shutdown();
    try {
        executor.awaitTermination(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

  

2、数据缓存：由于爬虫会重复抓取相同的网页，因此可以将已经抓取到的网页存储到缓存中，下次再抓取相同网页时，可以直接从缓存中获取，提高效率。

public class WebCache {

    private Map cache = new HashMap
   ();

    public void put(String url, Document document) {
        cache.put(url, document);
    }

    public Document get(String url) {
        return cache.get(url);
    }
}

   
  

3、避免重复索引：为了避免重复索引已经被索引过的网页，可以记录下已经索引过的网页URL，下次进行索引时，进行判断，如果该URL已经被索引过，直接跳过即可。

public class DeduplicationIndexer extends Indexer {

    private Set visitedUrls;

    public DeduplicationIndexer(WebCrawler spider) {
        super(spider);
        visitedUrls = new HashSet
   ();
    }

    /**
     * Index a page by its URL
     * 
     * @param url
     *            - The URL of the page to be indexed
     */
    public void indexPage(String url) {
        if (!visitedUrls.contains(url)) {
            visitedUrls.add(url);
            super.indexPage(url);
        }
    }
}