嗨我正在尝试做一个递归的.pdf url收获,我得到一个ConcurrentModificationException ..我不明白这是怎么回事,我对并发性知之甚少;我将非常感谢对这种情况的发生以及如何解决这些问题的一些见解。
public class urlHarvester {
private URL rootURL;
private String fileExt;
private int depth;
private HashSet<String> targets;
private HashMap<Integer, LinkedList<String>> toVisit;
public urlHarvester(URL rootURL, String fileExt, int depth) {
this.rootURL = rootURL;
this.fileExt = fileExt;
this.depth = depth;
targets = new HashSet<String>();
toVisit = new HashMap<Integer, LinkedList<String>>();
for (int i = 1; i < depth + 1; i++) {
toVisit.put(i, new LinkedList<String>());
}
doHarvest();
}
private void doHarvest() {
try {
harvest(rootURL, depth);
while (depth > 0) {
for (String s : toVisit.get(depth)) {
toVisit.get(depth).remove(s);
harvest(new URL(s),depth-1);
}
depth--;
}
} catch (Exception e) {
System.err.println(e);
e.printStackTrace();
}
for (String s : targets) {
System.out.println(s);
}
}
private void harvest(URL url, int depth) {
try {
URLConnection urlConnection = url.openConnection();
InputStream inputStream = urlConnection.getInputStream();
Scanner scanner = new Scanner(new BufferedInputStream(inputStream));
java.lang.String source = "";
while (scanner.hasNext()) {
source = source + scanner.next();
}
inputStream.close();
scanner.close();
Matcher matcher = Pattern.compile("ahref=\"(.+?)\"").matcher(source);
while(matcher.find()) {
java.lang.String matched = matcher.group(1);
if (!matched.startsWith("http")) {
if (matched.startsWith("/") && url.toString().endsWith("/")) {
matched = url.toString() + matched.substring(1);
} else if ((matched.startsWith("/") && !url.toString().endsWith("/"))
|| (!matched.startsWith("/") && url.toString().endsWith("/"))) {
matched = url.toString() + matched;
} else if (!matched.startsWith("/") && !url.toString().endsWith("/")) {
matched = url.toString() + "/" + matched;
}
}
if (matched.endsWith(".pdf") && !targets.contains(matched)) {
targets.add(matched);System.out.println("ADDED");
}
if (!toVisit.get(depth).contains(matched)) {
toVisit.get(depth).add(matched);
}
}
} catch (Exception e) {
System.err.println(e);
}
}
带主调用的类:
urlHarvester harvester = new urlHarvester(new URL("http://anyasdf.com"), ".pdf", 5);
答案 0 :(得分:5)
该错误可能与并发无关,但是由此循环引起:
for (String s : toVisit.get(depth)) {
toVisit.get(depth).remove(s);
harvest(new URL(s),depth-1);
}
要在迭代时从集合中删除项目,您需要使用迭代器中的remove
方法:
List<String> list = toVisit.get(depth); //I assume list is not null
for (Iterator<String> it = list.iterator(); it.hasNext();) {
String s = it.next();
it.remove();
harvest(new URL(s),depth-1);
}
答案 1 :(得分:1)
尝试在迭代时直接从对象中删除对象时抛出ConcurrentModificationException
。
当您尝试从toVisit
HashMap
删除条目时会发生这种情况:
for (String s : toVisit.get(depth)) {
toVisit.get(depth).remove(s); <----
...
您可以使用迭代器而不是尝试直接从集合中删除:
Iterator<String> iterator = toVisit.get(depth).iterator();
while (iterator.hasNext()) {
String s = iterator.next();
iterator.remove();
harvest(new URL(s),depth-1);
}