Building Blocks - Java Concurrency in Practice - page 93

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 con-
formance with its specification. However, that something as mundane as fetching the last ele-
ment 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.