Correct way to synchronize ArrayList in java

Question

I m not sure if this is the correct way to synchronize my ArrayList   I have an ArrayList in queue which is passed in from the registerInQueue function   ArrayList lt Record gt  in queue   null   public void registerInQueue ArrayList in queue        this in queue   in queue      Now I m trying to synchronize it  Is this sychronizing my in queue object correctly   List lt Record gt  in queue list   Collections synchronizedList in queue    synchronized  in queue list        while  in queue list size    gt  0            in queue list remove 0

User · Answer

That s correct  and documented   http   java sun com javase 6 docs api java util Collections html synchronizedList java util List   However  to clear the list  just call List clear

User · Answer

Yes it is the correct way  but the synchronised block is required if you want all the removals together to be safe - unless the queue is empty no removals allowed  My guess is that you just want safe queue and dequeue operations  so you can remove the synchronised block   However  there are far advanced concurrent queues in Java such as ConcurrentLinkedQueue

User · Answer

You re synchronizing twice  which is pointless and possibly slows down the code  changes while iterating over the list need a synchronnization over the entire operation  which you are doing with synchronized  in queue list  Using Collections synchronizedList   is superfluous in that case  it creates a wrapper that synchronizes individual operations    However  since you are emptying the list completely  the iterated removal of the first element is the worst possible way to do it  sice for each element all following elements have to be copied  making this an O n 2  operation - horribly slow for larger lists   Instead  simply call clear   - no iteration needed   Edit  If you need the single-method synchronization of Collections synchronizedList   later on  then this is the correct way   List lt Record gt  in queue list   Collections synchronizedList in queue   in queue list clear       synchronized implicitly     But in many cases  the single-method synchronization is insufficient  e g  for all iteration  or when you get a value  do computations based on it  and replace it with the result   In that case  you have to use manual synchronization anyway  so Collections synchronizedList   is just useless additional overhead

User · Answer

Looking at your example  I think ArrayBlockingQueue  or its siblings  may be of use  They look after the synchronisation for you  so threads can write to the queue or peek take without additional synchronisation work on your part

User · Answer

Let s take a normal list  implemented by the ArrayList class  and make it synchronized  This is shown in the SynchronizedListExample class       We pass the Collections synchronizedList method a new ArrayList of Strings  The method returns a synchronized List of Strings         Here is SynchronizedArrayList class   package com mnas technology automation utility  import java util ArrayList  import java util Collections  import java util Iterator  import java util List  import org apache log4j Logger            author manoj kumar    email kumarmanoj mtech gmail com       public class SynchronizedArrayList       static Logger log   Logger getLogger SynchronizedArrayList class getName         public static void main String   args                List lt String gt  synchronizedList   Collections synchronizedList new ArrayList lt String gt              synchronizedList add  Aditya            synchronizedList add  Siddharth            synchronizedList add  Manoj               when iterating over a synchronized list  we need to synchronize access to the synchronized list         synchronized  synchronizedList                Iterator lt String gt  iterator   synchronizedList iterator                while  iterator hasNext                      log info  Synchronized Array List Items      iterator next                                           Notice that when iterating over the list  this access is still done using a synchronized block that locks on the synchronizedList object   In general  iterating over a synchronized collection should be done in a synchronized block

[java] Correct way to synchronize ArrayList in java

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