同步块和方法未按预期工作答案

【问题标题】：synchronized block and method not working as intended同步块和方法未按预期工作
【发布时间】：2014-03-10 05:35:37
【问题描述】：

状态：在方法中添加静态，变量队列和同步（crawler.class）解决了这个问题。谢谢大家！！

http://pastie.org/8724549#41-42,46,49,100-101,188-189,191

突出显示的方法/块是synchronized。

那个块/方法应该be accessed by one method at a particular time.。

应该是这样的 = 第一个线程进入方法，更新大小，所有其他人看到那个大小。更新了一个。更新应该只由第一个线程进行。不是别人

为什么它甚至被运行。它由所有 11 个线程运行。
它正在运行而不等待前一个线程完成。 “queue loaded, new size ------------”它的创建/添加元素

package crawler;

import crawler.Main;
import static crawler.Main.basicDAO;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.URL;
import java.util.*;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.ConcurrentMap;
import java.util.logging.Level;
import java.util.logging.Logger;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;


/**
 *
 * @author syncsys
 */
public class Crawler implements Runnable, InterfaceForConstants {
public static final String patternString = "[_A-Za-z0-9-]+(\\.[_A-Za-z0-9-]+)*@[A-Za-z0-9]+(\\.[A-Za-z0-9]+)*(\\.[A-Za-z]{2,})";
public ConcurrentLinkedQueue<Link> queue =  new ConcurrentLinkedQueue<Link>();
private volatile String url;





    private void crawl(String url) {


        synchronized (Crawler.class){
            System.out.println("queue size "+queue.size());
            if(queue.size() < (totalSizeOfEmailBucket / 3)){
                updateQueue();
            }
        System.out.println("This is inside of sync block.   -----------    queue size "+queue.size());
        }
        System.out.println("This is at the end of sync block.   -----------    queue size "+queue.size());





        BufferedReader bf = null;
        try {
            url = queue.poll().getLink();
            URL target = new URL(url);
            bf = new BufferedReader(
                    new InputStreamReader(target.openStream())
                 );
            StringBuilder html = new StringBuilder();
            String inputLine;
            while ((inputLine = bf.readLine()) != null) {
                html.append(inputLine);
            }
            List emailList = new ArrayList( getEmailList(html.toString()) );
           // List linkList = new ArrayList( getLinkList(html.toString(), url) );
            System.out.println("Just worked on --------- "+ url);
            Main.processedLinksCount++;
            if(emailList.size()>0){
                putEmailsInDB(emailList);
            }

            // putLinksInDB(linkList);
        } catch (IOException ex) {
            Logging.logError(ex.toString());
            basicDAO.deleteLink(url);
        } catch (Exception ex) {
            Logging.logError(ex.toString()); 
            basicDAO.deleteLink(url);
        }finally{
            if(bf !=null){
                try {
                bf.close();
                } catch (IOException ex) {
                    Logging.logError(ex.toString());
                }

            }
            crawl(null);
        }
    }

    public synchronized void  updateQueue() {



            Queue<Link> tempQueue =  new PriorityQueue<Link>();
            tempQueue = getNonProcessedLinkFromDB() ;

            queue.addAll(tempQueue);
            BasicDAO.markLinkAsProcesed(tempQueue);
            System.out.println("queue loaded, new  size ------------------------------------ "+queue.size());

    }

    private  List getLinkList(String html, String url) {
        Document doc = Jsoup.parse(html);
        Elements bodies = doc.select("body");
        List linkList =  new ArrayList();
        for(Element body : bodies ){
            Elements aTags = body.getElementsByTag("a");
            for (Element a: aTags){
               String link =  a.attr("href");
               if ( !(link.startsWith("#")) 
                     && 
                    !(link.contains("()"))
                     && 
                    !(link.endsWith(".jpg")) 
                     && 
                    !(link.endsWith(".jpeg"))  
                     && 
                    !(link.endsWith(".png"))  
                     && 
                    !(link.endsWith(".gif"))     ){

                    if( link.startsWith("/") ){
                        link = url+link;
                    }
                 linkList.add(link);
                 //put link in db
               }    
            }
        }

        return linkList;
    }

    private  List getEmailList(String html) {
        Pattern p = Pattern.compile(patternString);
        Matcher m = p.matcher(html);
        List emailList = new ArrayList();
        while(m.find()){
            emailList.add(m.group());
            Main.nonUniqueEmailsCount++;
        }

        return emailList;    
    }



    private Queue<Link> getNonProcessedLinkFromDB() {
        return ( basicDAO.getNonProcessedLink() );
    }

    private  void putEmailsInDB(List emailList) {
        basicDAO.insertEmail(emailList);
    }

    private  void putLinksInDB(List linkList) {
       basicDAO.insertLinks(linkList);
    }

    @Override
    public void run() {
        if(url != null){
            crawl(url);
        }else{
 //          crawl();
        }

    }
    public Crawler(String url){
        this.url = url;
    }

    public Crawler(){
        this.url =  null;
    }
}

启动线程的方式：非乐观。我知道。未使用执行器服务或池，但以下是有效代码：

for (int i = 0; i < 11; i++) {


                    try {



                             Thread thread = new Thread(new Crawler("https://www.google.com.pk/?gws_rd=cr&ei=-q8vUqqNDIny4QTLlYCwAQ#q=pakistan"/*new BasicDAO().getNonProcessedLink()*/)); 
                             System.out.println("resume with saved link true");


                        thread.start();

                        System.out.println("thread stared");
                        threadList.add(thread);
                        System.out.println("thread added to arraylist");

                    } catch (Exception ex) {
                        new Logging().logError(ex.toString());
                    }

               }

调试：

for 11 threads , its says in logs:

queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
queue size 0
This is at the end of sync block.   -----------    queue size 1000
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
queue size 0
queue loaded, new  size ------------------------------------ 1000
This is inside of sync block.   -----------    queue size 1000
This is at the end of sync block.   -----------    queue size 1000
Just worked on --------- http://ao.com/Advice/Washing-Machines/Top-Tens/Top-Five-Washing-Machines/Advice/Freezers/Top-Tens/Top-Five-Freezers/flavel
queue size 999
Just worked on --------- http://ao.com/Advice/Washing-Machines/Top-Tens/Top-Five-Washing-Machines/l/fridges-width_less_than_50_cm/1-26/29-30//zanussi
queue loaded, new  size ------------------------------------ 1999
This is inside of sync block.   -----------    queue size 1999
This is at the end of sync block.   -----------    queue size 1999
queue size 999
queue loaded, new  size ------------------------------------ 1999
This is inside of sync block.   -----------    queue size 1999
This is at the end of sync block.   -----------    queue size 1999
Just worked on --------- http://ao.com/Advice/Washing-Machines/Top-Tens/Top-Five-Washing-Machines/Advice/Refrigerators/Top-Tens/Top-Five-Fridges/l/small_appliances-bodum/1-6/55/
queue size 999
queue loaded, new  size ------------------------------------ 1999
This is inside of sync block.   -----------    queue size 1999
This is at the end of sync block.   -----------    queue size 1999
8692 characters / 254 lines
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【问题讨论】：

让我们也看看你在哪里创建和启动你的线程。
@SotiriosDelimanolis 您刚刚回答并删除了您的回答？为什么？
因为我不确定你不理解的行为是什么。你能澄清一下你的期望吗？

标签： java multithreading thread-safety synchronized java.util.concurrent

【解决方案1】：

由于这一行，代码将无法按预期运行：同步（Crawler.class）您已锁定 Crawler 的“类”对象，每个类加载器一个，但您的同步方法是非静态的。用静态同步方法替换非静态同步方法，这应该可以解决问题。

【讨论】：

关于静态/非静态方法的要点。我应该为同步块做什么/替换？
没什么。我只看到一种同步方法，因此只需将 public synchronized void updateQueue() 替换为 public static synchronized void updateQueue() ，编译器可能还会期望您将“队列”作为静态变量，因此您可能还必须更改 public ConcurrentLinkedQueue 队列到公共静态 ConcurrentLinkedQueue 队列。
我会尽快更改 synchronized(Crawles.class) 块以同步其他内容，而不是将方法更改为 static 方法。
静态同步 methodName(..)
与我的确切代码一样，synchronized (Crawler.class) 应该更改为什么？我也试过synchronized(this)

【解决方案2】：

假设这个

第一个线程进入方法，更新大小，所有其他线程都看到该大小。

是你的期望，这是根本原因。

您的 queue 字段是一个实例字段

public ConcurrentLinkedQueue<Link> queue =  new ConcurrentLinkedQueue<Link>();

你有 11 个实例

Thread thread = new Thread(new Crawler("https://www.google.com.pk/?gws_rd=cr&ei=-q8vUqqNDIny4QTLlYCwAQ#q=pakistan"/*new BasicDAO().getNonProcessedLink()*/));

所以每个Crawler 都有自己的队列，因此每个线程都进入这个块

if(queue.size() < (totalSizeOfEmailBucket / 3)){
    updateQueue();
}

因为每个线程都在更新不同的ConcurrentLinkedQueue 对象。

它正在运行而不等待前一个线程完成。

这不是真的。所有线程都会阻塞在

synchronized (Crawler.class){

但除此之外的任何代码都是公平的游戏。

【讨论】：

我应该改为 synchronized(this) 吗？或者除了将静态单词添加到队列之外，我还需要更改什么
@MasoodAhmad 您的代码有很多问题。 synchronized 的重点是在代码关键部分周围的共享对象监视器上阻塞。 synchronized(this) 不会改变任何东西，因为你有 11 个不同的对象，即。 11 个不同的this 引用。考虑只使用一个Crawler 对象或让它们都共享同一个ConcurrentLinkedQueue 对象。
@MasoodAhmad 不要制作 static 来分享它们。将它们作为参数传递给构造函数。
在方法中添加静态变量队列和同步（crawler.class）解决了这个问题。谢谢大家！！
@MasoodAhmad 请阅读this whole tutorial。这里的问题是共享与非共享状态。使方法static 没有任何改变。您需要做的是了解synchronized 的工作原理以及线程如何交互。