So if a program does this:
static ArrayList<X> a = null;
static{
for(;;){X x = new X(); a.add(x)}
}
and the only operations called after the static initializing, on the shared list, are get() and x.t()
X x = a.get(i); x.t();
and X doesn't have access to container & is thread safe, that should mean that using Arraylist like this, without synchronization is thread safe, correct?
If nothing is modifying the ArrayList than there is no reason that you should be worried about it's thread-safety.
That static block is thread-safe by default, because it's ran only once when the class is loaded (for initialization).
t does
ArrayList, then I would argue that you should be worried about its thread safety.
If the intention is for the list to not be modified once it is created, then enforce that: wrap the ArrayList in a Collections.unmodifiableList().
Better yet, if you have Guava available, use an ImmutableList.
Vector is the synchronized collection you want.
Just because the elements of the Collection are thread safe does not guarantee that the container itself is thread safe. Based on my understanding of concurrency.
If t modifies the backing Collection it is not thread safe. For instance t is defined as follows:
this.list.remove(this.x);
It would not be safe.
Vector is not the answer. Please read here.Collections.synchronizedList(new ArrayList<T>())) over Vector in modern Java because it makes the intent clearer. Better yet, use a lock-free implementation like CopyOnWriteArrayList.Vector is not an option: Why is Java Vector class considered obsolete or deprecated?.Thread-safety is only an issue on writable objects. Once initialised, your ArrayList is immutable - thus thread safe. Consider using unmodifiable wrappers around your arraylist - it will
Just for sake of completeness, please be aware of the fact that, while operations on the ArrayList are threadsafe, operation on the elements are not.
staticblock code provides in your example, it will throw aExceptionInInitializerError.tdo? Or istany arbitrary function we can think of?