Synchronized List/Map in Java if only one thread i

Question

The first thread is filling a collection continuously with objects. A second thread needs to iterate over these objects, but it will not change the collection.

Currently I use Collection.synchronized for making it thread-safe, but is there a fast way to doing it?

Update

It's simple: The first thread (ui) continuously writes the mouse position to the ArrayList, as long as the mousebutton is pressed down. The second thread (render) draws a line based on the list.

Answer 1

As mentioned in comments, you need explicit synchronization on this list, because iteration is not atomic:

List<?> list = // ...

Thread 1:

synchronized(list) {
    list.add(o);   
}

Thread 2:

synchronized(list) {
    for (Object o : list) {
        // do actions on object
    }
}

Answer 2

Even if you synchronize the list, it's not necessarily thread-safe while iterating over it, so make sure you synchronize on it:

synchronized(synchronizedList) {
    for (Object o : synchronizedList) {
        doSomething()
    }
}

Edit:

Here's a very clearly written article on the matter: http://java67.blogspot.com/2014/12/how-to-synchronize-arraylist-in-java.html

Answer 3

Use java.util.concurrent.ArrayBlockingQueue.ArrayBlockingQueue implementation of BlockingQueue. It perfectly suits your needs.

1. It is perfectly suited for producer-consumer cases as that is one in yours.

2. You can also configure access policy. Javadoc explains access policy like this:

   Fair if true then queue accesses for threads blocked on insertion or removal, are processed in FIFO order; if false the access order is unspecified.

Answer 4

There are 3 options which I can currently think of to handle concurrency in ArrayList:-

1. Using Collections.synchronizedList(list) - currently you are using it.

2. CopyOnWriteArrayList - behaves much like ArrayList class, except that when the list is modified, instead of modifying the underlying array, a new array in created and the old array is discarded. It will be slower than 1.

3. Creating custom ArrayList class using ReentrantReadWriteLock. You can create a wrapper around ArrayList class. Use read lock when reading/iterating/looping and use write lock when adding elements in array. For e.g:-

   import java.util.List;
   import java.util.concurrent.locks.Lock;
   import java.util.concurrent.locks.ReadWriteLock;
   import java.util.concurrent.locks.ReentrantReadWriteLock;
   
   public class ReadWriteList<E> {
   
       private final List<E> list;
       private ReadWriteLock lock = new ReentrantReadWriteLock();
       private final Lock  r =lock.readLock();
       private final Lock  w =lock.writeLock();
   
       public ReadWriteList(List<E> list){
           this.list=list;
       }
   
       public boolean add(E e){
   
           w.lock();
           try{
               return list.add(e);
           }
           finally{
           w.unlock();
           }
       }
   
       //Do the same for other modification methods
   
       public E getElement(int index){
           r.lock();
           try{
   
               return list.get(index);
           }
           finally{
               r.unlock();
           }   
       }
   
       public List<E> getList(){
           r.lock();
           try{
           return list;
           }
           finally{
           r.unlock();
           }   
       }
       //Do the same for other read methods
   }
   

Answer 5

Rather than a List will a Set suit your needs?

If so, you can use Collections.newSetFromMap(new ConcurrentHashMap<>())

Answer 6

If you're reading far more often than writing, you can use CopyOnWriteArrayList