how to create Synchronized arraylist

Question

i have created synchronized arrayList like this

import java.text.SimpleDateFormat;
import java.util.*;


class HelloThread  
{

 int i=1;
 List arrayList;
  public  void go()
  {
 arrayList=Collections.synchronizedList(new ArrayList());
 Thread thread1=new Thread(new Runnable() {

  public void run() {
  while(i<=10)
  {
   arrayList.add(i);
   i++;
  }
  }
 });
 thread1.start();
 Thread thred2=new Thread(new Runnable() {
  public void run() {
     while(true)
     {
   Iterator it=arrayList.iterator();
      while(it.hasNext())
      {
       System.out.println(it.next());
      }
     }
  }
 });
 thred2.start();
  }
 }

public class test
{
  public static void main(String[] args)
  {
   HelloThread hello=new HelloThread();
   hello.go();
  }
}

but getting exception like this

Exception in thread "Thread-1" java.util.ConcurrentModificationException

anything wrong in my approach ?

Answer 1

You may not modify a Collection that you are iterating. You can work around this by accessing the array entries by index, not through an Iterator. I can provide more advice if you tell me the problem that you are trying to solve with this code.

Answer 2

Iterator of synchronizedList is not (and can't be) synchronized, you need to synchronize on the list manually while iterating (see javadoc):

synchronized(arrayList) {
    Iterator it=arrayList.iterator(); 
    while(it.hasNext()) { 
        System.out.println(it.next()); 
   } 
}

Another approach is to use a CopyOnWriteArrayList instead of Collections.synchronizedList(). It implements a copy-on-write semantic and therefore doesn't require synchronization.

Answer 3

As Spike said, you can't modify a collection while iterating it. However, I think the solution is to lock the list while iterating.

class HelloThread  
{

 int i=1;
 List arrayList;
  public  void go()
  {
 arrayList=Collections.synchronizedList(new ArrayList());
 Thread thread1=new Thread(new Runnable() {

  public void run() {
  while(i<=10)
  {
synchronized(someLock) {
   arrayList.add(i);
}
   i++;
  }
  }
 });
 thread1.start();
 Thread thred2=new Thread(new Runnable() {
  public void run() {
     while(true)
     {
synchronized(someLock) {
   Iterator it=arrayList.iterator();
      while(it.hasNext())
      {
       System.out.println(it.next());
      }
}
     }
  }
 });
 thred2.start();
  }
 }

public class test
{
  public static void main(String[] args)
  {
   HelloThread hello=new HelloThread();
   hello.go();
  }
}

I'm not sure what you're trying to do, so I hope this doesn't break the functionality of your code.

Answer 4

The java.util.ConcurrentModificationException occurs when you manipulate (add,remove) a collection while iterating over the same collection.

You probably want to consume the create entries in your second thread while after they have been created by your first thread. So you could use ArrayLists get( index ) and size() for control

Answer 5

Consider using a CopyOnWriteArrayList which is thread-safe. Every time you add an item, a fresh copy of the underlying array is created. However, the iterator will not reflect additions to the list since the iterator was created, but is guaranteed not to throw ConcurrentModificationException.

arrayList=new CopyOnWriteArrayList();

Answer 6

Other answers have identified the problem:

• The iterators for synchronized collections are not synchronized. In fact, they are simply the iterators returned by the collection objects inside the wrapper classes.

• Many collection classes (including ArrayList) use a fail-fast mechanism to detect concurrent modifications during iteration. This behavior is clearly documented in the javadocs for the respective classes. This is what you are seeing.

Not all collection classes do this. For example, many of the java.util.Concurrent... collection classes allow concurrent modification during iteration, but relax the semantics of the iteration sequence so that the results of the modifications may or may not be apparent in the objects returned by the iterator.

The javadoc for the Collections.synchronizedList() explains how to synchronize the iterator. Basically you do this:

List list = Collections.synchronizedList(new ArrayList());
  ...
synchronized (list) {
    Iterator i = list.iterator(); // Must be in synchronized block
    while (i.hasNext())
        foo(i.next());
}

(Aside: normally it is not safe to assume that doing something like this would work. In theory, the synchronized list could use a private lock object, and the synchronized statement would not lock out concurrent modifications. However the javadocs say that this is what to do in this case ... so it is safe.)

The problem with doing that is that locking the collection creates a potential concurrency bottleneck. The alternative to is to use a copy-on-write data structure that internally makes a copy of the relevant parts of the collection. This approach means that an iterator sees sees a snapshot of the collection. Modifications may be made to the collection concurrent with an iteration, but the iterator does not see them. The problem with copy-on-write is that modifications are potentially a lot more expensive.

Ultimately, you need to balance the characteristics and costs of the different collection types wrt concurrent modification versus your actual requirements. Can you get away with the iterator not seeing all concurrent modifications?