Question

I have one thread populating a vector object with values, and one other thread that periodically fetches the values from it and clears it periodically.

I want that either thread accessing the vector pauses while the other one is accessing it. Do I need to use the wait/notify/notifyAll keywords ?

private Vector<Long> _recordIdsSent;

# Called by main thread (before thread A and thread B are started)
public ConstructorOfThisClass() {
    _recordIdsSent = new Vector<Long>();
    // Starts thread A & B
}

# Called by thread A
public void addSentRecordIds( List<Long> ids ) {
    synchronized (_recordIdsSent) {
        _recordIdsSent.addAll( ids );
    }
}

# Called by thread B
public void deleteRecords()
{
    List<Long> ids;
    synchronized (_recordIdsSent) {
        ids = (List<Long>) _recordIdsSent.clone();
        _recordIdsSent.clear();
    }

    // Delete the records matching ids....
}

Note: I clone the _recordIdsSent vector because the delete operation can take some time.

[EDIT]
Moved the synchronized keyword from the method signature to the variable _recordIdsSent

Answer 1

As already noted, in provided code access to vector from addSentRecordIds is not synchronized – that makes your code unsafe

Also, this code snippet does not guarantee that object used for locking (_recordIdsSent) doesn’t change – this makes it unsafe too. Myself I typically prefer dedicated objects for locking because it makes my intent cleaner and is less error prone. Like this:

private final Object lock = new Object(); // dedicated lock object
//... _recordIdsSent, addSentRecordIds etc...
public void deleteRecords()
{
    List<Long> ids;
    synchronized (lock) {
        ids = (List<Long>) _recordIdsSent.clone();
        _recordIdsSent.clear();
   }

   // Delete the records matching ids....
}