Iteration that may Throw ArrayIndexOutOfBoundsException.

moonboat 2011-05-13

展开全文

Iteration that may Throw `ArrayIndexOutOfBoundsException`.

for (int i = 0; i < vector.size(); i++)
            doSomething(vector.get(i));

Even though the iteration in Listing 5.3 can throw an exception, this doesn't mean Vector isn't thread-safe. The state of the Vector is still valid and the exception is in fact in conformance with its specification. However, that something as mundane as fetching the last element or iteration throw an exception is clearly undesirable.

The problem of unreliable iteration can again be addressed by client-side locking, at some additional cost to scalability. By holding the Vector lock for the duration of iteration, as shown in Listing 5.4, we prevent other threads from modifying the Vector while we are iterating it. Unfortunately, we also prevent other threads from accessing it at all during this time, impairing concurrency.

Listing 5.4. Iteration with Client-side Locking.

synchronized (vector) {
            for (int i = 0; i < vector.size(); i++)
            doSomething(vector.get(i));
            }

Iterating a `List` with an `Iterator`.

List<Widget> widgetList
            = Collections.synchronizedList(new ArrayList<Widget>());
            ...
            // May throw ConcurrentModificationException
            for (Widget w : widgetList)
            doSomething(w);