CODEWITHSUNDEEP
androidmultithreading
llappall
llappall

Reputation: 2952

Android queries modifying a variable in the UI Thread

I have a simple query returning a Cursor, and then I walk the cursor and create objects that I throw in an ArrayList, like this:

List<Element> myElements = new ArrayList<Element>();  
Cursor c = db.query(...);  
c.moveToFirst();  
while (c != null && !c.isAfterLast()) {  
    myElements.add(new Element(cursor.getString(0).........)); <-- CREATING THE ELEMENT  
    c.moveToNext();  
}

...

You get the idea.

The problem is that I need to run 4 queries like this hitting different tables, etc, but they all return the same Element object in the end (after walking the cursor).
Being a good Android citizen I created a class extending AsyncTask to avoid hogging the UI Thread. Also, I want to run the 4 queries in 4 threads to speed things up.

The question:
in my onPostExecute(Cursor c), I'm running the logic marked as "CREATING THE ELEMENT" above. If I run 4 threads with 4 queries and all modifying the List, will I have thread conflicts touching the same variable from them? How do I prevent that? Do I gain anything by threading this if the list I need to modify is synchronized? I mean, the threads will have to wait in line anyway, I might as well write the 4 queries and run them sequentially... or not?
I understand I want to keep this out of the UI Thread. The question is if I want to create 4 threads (each running in an AsyncTask) or just ONE AsyncTask that runs the 4 queries sequentially.

Thanks!

Llappall

Upvotes: 1

Views: 235

Answers (2)

Aleadam
Aleadam

Reputation: 40401

Vector, as opposed to ArrayList, is synchronized and thread safe, so I would suggest to use it instead. http://download.oracle.com/javase/6/docs/api/java/util/Vector.html

Another alternative would be to create a new List per thread and then use Collections.addAll() to incorporate the elements to the original list.

To answer the question whether you would gain anything by starting multiple threads, probably the answer will depend on how expensive are the queries you are doing. Starting a new thread has an intrinsic overhead, so you want to make sure that the query you are starting is worth the cost.

Upvotes: 0

advantej
advantej

Reputation: 20325

will I have thread conflicts touching the same variable from them?

You will certainly have race conditions - if you are fine with it then no issues.

How do I prevent that? Do I gain anything by threading this if the list I need to modify is synchronized?

I don't think so.

I mean, the threads will have to wait in line anyway, I might as well write the 4 queries and run them sequentially... or not?
The question is if I want to create 4 threads (each running in an AsyncTask) or just ONE AsyncTask that runs the 4 queries sequentially.

I would run all the 4 queries in one AsyncTask, creating 4 AsyncTasks would be a lot to do and maintain.

Upvotes: 1

Related Questions