ArrayList索引超出范围，尽管访问的索引在范围之内

Question

I am getting an Index Out of Bounds exception for an ArrayList . At the point of exception the size() and index seem to be fine when printed. Below is the relevant code. I have a custom table model which is refreshed when a button is clicked.

public class CallingClass
{
    public void buttonClicked()
    {
        new Thread(new Runnable()
        {
            public void run()
            {                   
                dataTableModel.refresh();
            }
        }).start();     
    }
}

public class DataTableModel extends AbstractTableModel
{
    protected ArrayList<Object> data;   

    public DataTableModel()
    {
        data = new ArrayList<Object>();
    }

    public Object getValueAt(int modelRow, int modelColumn)
    {
         // Throws Index Out of Bounds Exception though a println here shows modelRow value within 0 to data.size()-1
         // DataObject is just an interface to support getValue method
        return ((DataObject) data.get(modelRow)).getValue();
    }

    public void refresh()
    {
        reloadData();
        fireTableDataChanged();
    }

    protected void reloadData()
    {
        ArrayList<TextMessage> messageList = jmsConnection.getMessageList();

        data.clear();

        try
        {
            for(int i=0; i<messageList.size(); ++i)
            {
                data.add(new MyDataObject(messageList.get(i).getJMSMessageID()));
            }
        }
        catch(Exception e)
        {
        }
    }
}

As you can see the DataTableModel.refresh() runs in a new thread. The call to fireTableDataChanged probably causes the eventQueue thread to redraw the table, which in turn calls the getValueAt method. Here the exception is thrown, despite the modelRow value being less than data.size() . Not sure how there can be race condition either because the Runnable thread calls the fireTableDataChanged after calling reloadData (which repopulates the data object). Hence the data object should be stable when the eventQueue thread is calling getValueAt . Also note the the refresh button is disabled until the runnable thread comes out, to avoid overlapping calls to refresh.

Below is the exception message

Exception in thread "AWT-EventQueue-0" java.lang.IndexOutOfBoundsException: Index: 3, Size: 10
    at java.util.ArrayList.rangeCheck(Unknown Source)
    at java.util.ArrayList.get(Unknown Source)
    at datatablemodels.DataTableModel.getValueAt(DataTableModel.java:70)

If I change the reloadData code slightly, the exception goes away (at least not reproducible yet). My guess is that it is just reducing the odds of the exception.

protected void reloadData()
{
    ArrayList<TextMessage> messageList = jmsConnection.getMessageList();

    ArrayList<Object> tempdata = new ArrayList<Object>();

    try
    {
        for(int i=0; i<messageList.size(); ++i)
        {
            tempdata.add(new MyDataObject(messageList.get(i).getJMSMessageID()));
        }
    }
    catch(Exception e)
    {
    }

    data = tempdata ;
}

I am more interested in understanding what is going on here. I do have alternate solutions like synchronized , CopyOnWriteArrayList etc.

Answer 1

Since you have no synchronization, you have no guarantee about when data changes made in the refresh() method - which runs in its own thread - will be visible in other threads running the getValueAt() method, if ever. For example, the getValueAt() method, or part of it, may run "between" the data.clear() call in the reloadData() method and the repopulation of the data table.

And when you put the println() calls in, they may run before the data.clear() call or after the repopulation of data, even if the data.get() call does not.

The bottom line is, whenever data is touched by more than one thread, as it is here, you must synchronize access to the data, one way or another.