Reading multiple files - java.util.ConcurrentModificationException

Question

Error "java.util.ConcurrentModificationException" sometimes appear, when I test more than three times the error will show up, I do not understand why it is always pointing to the "Word w = iter.next ();" and why it does not always show error.

//LoadFile.java

@Override
    public void run() {
        BufferedReader reader = null;
        try {
            reader = new BufferedReader(new InputStreamReader(
                    new FileInputStream(this.fileName), "UTF-8"));
            String line = reader.readLine();
            while (line != null) {
                w = null;
                String[] result1 = line.split(":");
                w = new Word();
                w.setId(result1[0]);
                w.setWord(result1[1]);
                w.setMean(result1[2]);
                lw.add(w);
                line = reader.readLine();
            }
            reader.close();
        } catch (FileNotFoundException e) {
            System.err.println(e.getMessage());
        } catch (IOException e) {
            e.printStackTrace();
        }
        System.out.println("Thread " + fileName + " exiting.");
        Gui.showWord(lw);
    }

    public void start() {
        System.out.println("Starting " + fileName);
        if (t == null) {
            t = new Thread(this, fileName);
            t.start();
        }
    }

//Gui.java

private void chooseFile() {

        fileChooser = new JFileChooser();
        fileChooser.setMultiSelectionEnabled(true);
        int returnVal = fileChooser.showOpenDialog(Gui.this);
        if (returnVal == JFileChooser.APPROVE_OPTION) {
            file = fileChooser.getSelectedFiles();
            for (int i = 0; i <= file.length - 1; i++) {
                System.out.println(file[i].getName()
                        + ".........................");
                lf = new LoadFile(file[i].getName());
                lf.start();
            }

        } else {
            textAreaWord.append("Operation Canceled \n" + "new Line\n");
        }
    }

    public static void showWord(List<Word> ls) {
        Iterator<Word> iter = ls.iterator();
        while (iter.hasNext()) {
            Word w = iter.next();
            textAreaWord.append(w.getWord() + "\n");
        }

    }

// error

Exception in thread "hello.txt" Thread hello.txt exiting.
Thread hi.txt exiting.
java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at dictionary.Gui.showWord(Gui.java:138)
    at dictionary.LoadFile.run(LoadFile.java:46)
    at java.lang.Thread.run(Unknown Source)

Thanks!

score 2 · Answer 1 · answered Mar 23 '15 at 02:17

Java Collection classes are fail-fast which means that if the Collection will be changed while some thread is traversing over it using iterator, the iterator.next() will throw a ConcurrentModificationException.

While one thread is iterating over lw (in showWord method), another thread is trying to add a Word in lw.

score 2 · Answer 2 · edited May 23 '17 at 12:12

2

In multi Threaded environment while multiple threads are accessing same List it is safe to use CopyOnWriteArrayList.

Java 5 improves on the synchronized collections by providing several concurrent collection classes in java.util.concurrent package.

Also here is stackAnswer.

edited May 23 '17 at 12:12

Community

1
1

answered Mar 23 '15 at 04:25

rns

1,047
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25

score 1 · Answer 3 · answered Mar 23 '15 at 02:10

1

Is lw an ArrayList? if so when multiple threads access it at the same time it will throw the java.util.ConcurrentModificationException since it is not thread safe. You should use java.util.Vector instead as it is synchronized.

answered Mar 23 '15 at 02:10

Abe

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74

Reading multiple files - java.util.ConcurrentModificationException

3 Answers3