迭代器的并发修改异常

Question

I'm fairly new to java and I'm essentially writing a keylogger and having it write to file at regular intervals. Everytime a user presses a key it instantiates a NativeKeyEvent which invokes 'paramString()' and adds the information as a string to the arraylist below...

public static ArrayList<String> stringArray = new ArrayList<String>();

public synchronized String paramString() {
    StringBuilder param = new StringBuilder(255);
    // other code
    stringArray.add(param.toString());
}

Then at each interval the string array is passed across and is written to file in the TimerTask thread below...

public class SaveToArrayAndWriteTask extends TimerTask {

private ArrayList<String> anotherArray = NativeKeyEvent.stringArray;
private static String str;

   @Override
   public synchronized void run() {
     openFile();
     writeToFile(anotherArray);
     closeFile();
   }

   private static synchronized void writeToFile(ArrayList<String> localArray) {

    Iterator<String> iterator = localArray.iterator();
    try {
        while (iterator.hasNext()) {
            str = iterator.next().toString();
            output.format("%s\n", str);
        }
    } catch (Exception e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
}

The line 'str = iterator.next().toString();' then throws an exception if it attempts to iterate through the arraylist to write it to file while the program is recording / adding another key stroke. I thought by putting stringArray (which is being modified) into anotherArray and passing it as an argument would stop this from happening. As you can see I've tried using the synchronized keyword and I've also tried putting it into another thread and have read various other posts on stackoverflow but to no avail.

What would be the best way to solve this issue?

Answer 1

You aren't making a copy of the ArrayList, you are just making another reference to the same ArrayList.

private ArrayList<String> anotherArray = NativeKeyEvent.stringArray;

Should be something like:

private ArrayList<String> copy = new ArrayList<>(NativeKeyEvent.stringArray);

This will create a new ArrayList with the same elements as NativeKeyEvent.stringArray .

I would recommend using a getter instead of exposing stringArray as a public variable. The getter should make a copy of the stringArray and return that instead. That way any consumer of stringArray will operate on a copy and won't have the possibility of modifying the list.

Answer 2

I thought by putting stringArray (which is being modified) into anotherArray

private ArrayList<String> anotherArray = NativeKeyEvent.stringArray;

Is not creating a new ArrayList. anotherArray is still referencing the same object. You need to create a new ArrayList that contains the elements of stringArray.

private ArrayList<String> anotherArray = new ArrayList<>(NativeKeyEvent.stringArray);

Answer 3

Generally speaking, if you modify a collection while iterating over it, you'll get a ConcurrentModificationException .

You have two choices:

The hard way: synchronize access to the list so other threads block while you're iterating

The easy way: Use the CopyOnWriteArrayList list implementation from the java.util.concurrent package, which does all the hard work for you

public static ArrayList<String> stringArray = new CopyOnWriteArrayList<String>();

That's it. Nothing more needed, and you can remove all synchronization.