CODEWITHSUNDEEP
javamultithreadinglistsynchronization
Сергей Никитин
Сергей Никитин

Reputation: 635

Is it necessary to use synchronizedList instead List if iteration synchronized already?

Task is a simple. I have a dozen threads and one "global" list (of some objects). Each thread (periodically) iterate through all list to find the desired object and change it (or add it if not present). And then iterate again and save all object to the file. The JavaDoc says: "It is imperative that the user manually synchronize on the returned list when iterating over it" And now - all that I do with my list I do inside synchronized block. Inside implementation of synchronizedList, as I understand, also present some synchronization, and they (I suppose) add undesirable delay.

What if I will use simple List? I think - if all my doing with the list already in critical section - what I will lose if I change 

private static final List<JobSummary> SyncJS = Collections.synchronizedList(new ArrayList<JobSummary>());

to 

private static final List<JobSummary> SyncJS = new ArrayList<>();

Or I miss something?

Upvotes: 0

Views: 100

Answers (2)

Naresh Vavilala
Naresh Vavilala

Reputation: 608

The operation "find the desired object and change it" is not atomic, hence you need to synchronize the list before performing any non atomic actions.

If you synchronize the list for all of your operations, then you do not need Collections.synchronizedList(new ArrayList<JobSummary>()). new ArrayList<>() would work well.

Upvotes: 0

Motomotes
Motomotes

Reputation: 4237

The point of the synchronizedObjects is to avoid manually synchronizing. If you are inside a synchronized block then you don't need them. It will only be extra overhead.

Upvotes: 1

Related Questions