Synchronize on List-CodePudding

Suppose I have this code block:

List<Integer> lst = Collections.synchronizedCollection(new ArrayList<>());

And I have the following two methods:

public Integer returnFirst() {
  lst.get(0);
}

public void iterate() {
synchronized(lst) {
     Iterator i = lst.iterator();
     while (i.hasNext()) {
       System.out.println(i);
     }
   }
}

Supposed that a thread calls iterate(), then another thread calls returnFirst(). Would returnFirst() be blocked since you are synchronizing on the List object in iterate, and iterate is currently running?

CodePudding user response：

Yes, you're correct. returnFirst won't run until iterate is complete.

CodePudding user response：

Yes. Take a look at the SynchronizedCollection constructor amd SynchronizedList#get method:

        SynchronizedCollection(Collection<E> c) {
            this.c = Objects.requireNonNull(c);
            mutex = this;
        }

        public E get(int index) {
            synchronized (mutex) {return list.get(index);}
        }

You can see that the internal mutex used to control concurrent access is actually the object itself(mutex = this), this means that lst.get(0) and synchronized(LST) in your iterate() method compete for the same lock: the object itself.

In addition, your code will not compile, replace synchronizedCollection with synchronizedList.

Simple code test:

    public static void main(String[] args) throws InterruptedException {
        List<Integer> list = Collections.synchronizedList(new ArrayList<>());
        list.add(1);
        new Thread(() -> {
            try {
                synchronized(list) {
                    Thread.sleep(5000);
                    Iterator i = list.iterator();
                    while (i.hasNext()) {
                        System.out.println(i.next());
                    }
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }).start();
        Thread.sleep(100);
        new Thread(() -> {
            // It will block for 5 seconds and then output
            System.out.println(list.get(0));
        }).start();
    }