垃圾回收行为

Question

During start up of my application, database is queried, objects are created (from the result of the query) and are inserted in aa Arraylist. The arraylist is later looped and another data structure is created out of it. The arraylist (which is huge in size) is later garbage collected. My question is, is this a strain on a garbage collector to collect such a big object at once. What if I create a QUEUE data structure instead of arraylist. Reading the object from the queue would make them eligible for GC. Is that lesser strain on the GC? I am aware that GC could run anytime and there are no guarantees of it execution. More that the timing of execution, what I would like to understand is is it more work for the GC to collect from a contiguous location of memory (arraylist) as against a QUEUE , in which memory allocation is not contiguous?

Answer 1

is it more work for the GC to collect from a contiguous location of memory (arraylist) as against a QUEUE , in which memory allocation is not contiguous?

It is more work to clean up a linked list based queue than an ArrayList. This is becaume an ArrayList has two objects, the Queue has one object per element.

If you want to reduce the GC load, process the data as you read it. This way you won't need a queue or a list and you might find you have processed all the data by the time it has downloaded. ie it could be quite a bit faster too.

Answer 2

The biggest strain here comes from keeping objects, which are "huge in size" in memory. It can cause GC to work more frequently if other objects need to be created on a heap or even lead to "out of memory" exception when the size of your DB and ArrayList increase.

Any solution that would allow you to decrease the size of memory allocated to "huge" objects will help. If you can build your queue in such a way that queue elements are released fast without waiting for all other objects to be read from a DB, go for it.

As Peter mentioned in his answer, it would be even better to process an object as soon as it was read from a DB without queuing it or adding to a list.

One of the possible solutions would be to re-design your data access layer and use ResultSet ( http://docs.oracle.com/javase/7/docs/api/java/sql/ResultSet.html ), which is available in any Java platform that I can think of. Since ResultSet is kept on a DB side, you can read records one at a time and decrease the strain on your memory significantly.

Another approach would be to implement pagination, eg by changing your original query in such a way that only a portion of ListArray is read from a DB at a time.