java - 致命异常 : AsyncTask caused by ConcurrentModificationException

Question

我是 Android 新手，一般都在开发，因此非常感谢您的帮助。关于“AsyncTask 中的致命异常”的大多数问题(和答案)都是 NullpointerException，所以我认为我可以提问。

我使用这个 Midi 驱动程序 https://github.com/kshoji/USB-MIDI-Driver从外部键盘获取笔记。我将收到的音符添加到数组中，以便获得播放音符的序列。然后，我使用 AsyncTask 将部分输入与给定的笔记列表进行匹配。 “计算矩阵”和“同步”这两种方法基本上是从一篇关于同步音乐数据的论文中复制(简化)的。

所以我的 Activity 是这样做的:

private void syncInput() {
    if(syncTask.getStatus() != AsyncTask.Status.RUNNING){
        syncTask = new SynchronisationTask(this, synchronizer);
        syncTask.execute();
    }
}

我的 AsyncTask 执行此操作:

public SynchronisationTask(ShowNotesActivity sna, Synchronizer s){
    this.showNotesActivity = sna;
    this.synchronizer = s;
}
@Override
protected Integer doInBackground(Mat... params) {
    return synchronizer.getPosition();
}

我的同步器是这样的:

public int getPosition() {
...
    if(input.size() < 10){
        length = input.size();
    }
    currentPart = partNotes.subList(0, 11);
    currentInput = input.subList(input.size() - length, input.size());

    if(currentPart.size() > 0 && currentInput.size() > 0){
        cost = new int[currentPart.size()][currentInput.size()];
        calculateMatrix();
        sync();
    }
    //not yet sure what I want to return so its 0
    return 0;
}

private void sync() {
    int i = currentPart.size()-1;
    int j = currentInput.size()-1;
    List<Pair<Integer,Integer>> pmMatch = new ArrayList<>();
    while(i > 0 && j > 0){
        if(cost[i][j] == cost[i][j-1]){
            j--;
        }else{
            if(cost[i][j] == cost[i-1][j]){
                i--;
            } else{
                pmMatch.add(Pair.create(i,j));
                i--;
                j--;
            }
        }
    }
    int max = 1;
    for(Pair p : pmMatch){
        if((int)p.first > max)
            max = (int)p.first;
    }
    lastNote = max;
    lastMatch = pmMatch.size();
    match++;
}

private void calculateMatrix(){
    int p = currentPart.size();
    int m = currentInput.size();

    int c = fivePointTwo();
    for(int i = 0; i < p; i++){
        cost[i][0] = c;
    }
    for(int j = 1; j < m; j++){
        cost[0][j] = c;
    }
    for(int i = 1; i < p; i++){
        for (int j = 1; j < m; j++) {
            int d = fivePointFour(currentPart.get(i), currentInput.get(j));
            if(cost[i-1][j-1] + d < Math.min(cost[i-1][j], cost[i][j-1])){
                cost[i][j] = cost[i-1][j-1] + d;
            }
            else if(cost[i-1][j] < cost[i][j-1]){
                cost[i][j] = cost[i-1][j];
            } else{
                cost[i][j] = cost[i][j-1];
            }
        }
    }
}

我的应用程序或多或少地随机崩溃，现在我终于得到了一些信息......我不明白。我希望你能帮助我解决这个错误:

06-08 01:30:18.149  26521-27123/ba.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #3
Process: ba.myapplication, PID: 26521
java.lang.RuntimeException: An error occured while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:300)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
        at java.util.concurrent.FutureTask.run(FutureTask.java:242)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
        at java.lang.Thread.run(Thread.java:818)
 Caused by: java.util.ConcurrentModificationException
        at java.util.AbstractList$SubAbstractList.size(AbstractList.java:360)
        at ba.myapplication.Synchronizer.sync(Synchronizer.java:89)

sync(Synchronizer.java:89)是“sync”方法中的第二行:int j = currentInput.size()-1;

Answer 1

在 getPosition()代码 currentInput = input.subList(input.size() - length, input.size());导致了这个问题。看起来您正在修改 input列出同时 getPosition()工作中。

来自 subList javadoc:

The semantics of the list returned by this method become undefined if the backing list (i.e., this list) is structurally modified in any way other than via the returned list. (Structural modifications are those that change the size of this list, or otherwise perturb it in such a fashion that iterations in progress may yield incorrect results.)

您可以尝试创建输入子列表的副本:currentInput = new ArrayList<>(input.subList(input.size() - length, input.size())); ，或将同步添加到您的代码中。