Java Vector Thread safety

Question

Is there any danger, if im using one Vector(java.util.Vector) on my server program when im accessing it from multiple threads only for reading? (myvector .size() .get() ...) For writing im using synchronized methods. Thank you.

Answer 1

Vector is a thread-safe collection - all its methods are synchronized by default. This is why it's recommended to use ArrayList instead - it's not thread-safe which results in a better performance for single-thread applications.

Answer 2

As stated above, every single method of Vector is thread-safe by its own because of synchronized modifiers. But, if you need some complex operations, such as get() or add() based on condition which is related to the same vector, this is not thread-safe. See example below:

if (vector.size() > 0) {
    System.out.println(vector.get(0));
}

This code has a race condition between size() and get() - the size of vector might be changed by other thread after our thread verified the vector is not empty, and thus get() call may return unexpected results. To avoid this, the sample above should be changed like this:

synchronized (vector) {
    if (vector.size() > 0) {
        System.out.println(vector.get(0));
    }
}

Now this "get-if-not-empty" operation is atomic and race condition-free.

Answer 3

The java.util.Vector methods are all synchronized. So using it from multiple threads is "safe". You only need to synchronize if you need a read-evaluate-write process to be atomic. Synchronizing your own methods does not necessarily make your code thread-safe for those scenarios. If the shared state is the Vector object, then you need to synchronize on the instance of your Vector, not on an instance of your own classes.

Answer 4

Vector class is thread safe (size, get, set methods are synchronized), but if you want to achieve better performance you should try to use CopyOnWriteArrayList or Collections.synchronizedList instead.