线程"main"java.util.ConcurrentModificationException中的异常

Question

当我运行以下代码时,我得到一个例外.我搜索但找不到任何解决方案.

Exception in thread "main" java.util.ConcurrentModificationException
     at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
     at java.util.HashMap$KeyIterator.next(Unknown Source)
     at com.aybits.software.linkgrabber.Grabber.main(Grabber.java:45)

第45行用于(String linkFromCollection:linksList){

public class Grabber {

static String url;
Document doc;
static Set<String> linksList = new HashSet<String>();
String matchingString ="java2s.com/Code";
static boolean isCrawling = true;
static int STOP_WATCH = 0;

public Grabber(String url){
    Grabber.url = url;
}

public void grabLinks(String urlToCrawl) throws IOException{
    doc = Jsoup.connect(urlToCrawl).timeout(20 * 1000).get();
    Elements links = doc.select("a[href]");

    for (Element link : links) {
        //print(" * a: <%s>  (%s)", link.attr("abs:href"), trim(link.text(), 35));
        if(link.attr("abs:href").toString().contains(matchingString)){
            if(!linksList.contains(link.attr("abs:href").toString())){
                System.out.println("Added - " + link.attr("abs:href"));
                linksList.add(link.attr("abs:href").toString());
            }
        }
    }
}

public static void main(String[] args) throws IOException {
    Grabber app = new Grabber("http://java2s.com");
    app.grabLinks(url);

    while(isCrawling){
        for(String linkFromCollection : linksList){
            app.grabLinks(linkFromCollection);

            if(linksList.contains(linkFromCollection)){
                STOP_WATCH += 5;
                System.out.println("STOP_WATCH IS " + STOP_WATCH);
            }else{
                STOP_WATCH -= 1;
                System.out.println("STOP_WATCH IS " + STOP_WATCH);
            }

            if(STOP_WATCH >= 100){
                isCrawling = false;
                System.out.println("STOP_WATCH IS " + STOP_WATCH);
            }
        }


    }
    ICVSWrite writer = new ICVSWrite();

    String[] strArray = (String[]) linksList.toArray();
    writer.write(strArray);

}

}

Answer 1

这条线

linksList.add(link.attr("abs:href").toString());

在linksList迭代时修改集合.下一次for循环进入main,Java调用next集合,看到集合已被修改,并抛出异常.

在进行增强for循环时,无法添加或删除集合.

Answer 2

您不能在其上循环调用addwhile Collection。这里：

for (Element link : links) {        
    if(...){
        if(...){
            ...
            linksList.add(link.attr("abs:href").toString());
                      ^^^ <- here
        }
    }
}

您可以在循环中从方法中调用grabLinks方法：mainlinksList

for(String linkFromCollection : linksList) {
   app.grabLinks(linkFromCollection);       

您必须将您的项目添加到另一个项目中Collection，然后将它们复制过来。

让我困惑了一段时间的是为什么异常来自 aHashMap因为我假设那linksList是 a List-显然它是 a Set。不是世界上最好的名字。

这应该有帮助。