CODEWITHSUNDEEP
javacollectionshashmaphashtablesynchronized
Pushparaj
Pushparaj

Reputation: 1049

synchronizing on Collections.synchronizedmap

In Collections javadoc it is mentioned like below..

It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views:

Map m = Collections.synchronizedMap(new HashMap());
      ...
  Set s = m.keySet();  // Needn't be in synchronized block
      ...
  synchronized (m) {  // Synchronizing on m, not s!
      Iterator i = s.iterator(); // Must be in synchronized block
      while (i.hasNext())
          foo(i.next());
  }

My Question is, if I declared an Hashtable and can I use it without synchronizing on it. Like below

  Hashtable ht = new Hashtable();
     Set s = m.keySet();

 Iterator i = s.iterator();
          while (i.hasNext())
              foo(i.next());

Upvotes: 1

Views: 396

Answers (4)

Aaditya Gavandalkar
Aaditya Gavandalkar

Reputation: 824

As you are doing every operation in a single thread, there should be no concurrency issue observed in this case. Though I must say that the example you provided from JavaDoc is not related to example you have question regarding. Please modify the question with more details.

Upvotes: 0

Karthik Bose
Karthik Bose

Reputation: 36064

if I declared an Hashtable and can I use it without synchronizing on it?

No, you need to synchronize it.

Reason:

  • Iteration over [any] collections in Java is not thread safe.

  • This is the implementation of the Hashtable#keyset() method: public Set<K> keySet() { if (keySet == null) keySet = Collections.synchronizedSet(new KeySet(), this); return keySet; } If you notice, it returns synchronizedSet. As it's mentioned in the javadoc It is imperative that the user manually synchronize on the returned set when iterating over it because the iterator method in the SyncronizedSet is not synchronized.

Upvotes: 1

Peter Lawrey
Peter Lawrey

Reputation: 533492

if I declared an Hashtable and can I use it without synchronizing on it.

You can but you shouldn't if you want to avoid a ConcurrentModificationException. I.e. it uses synchronization just like a synchronized HashMap as has the same limitation.

Note: if you use a ConcurrentHashMap, you don't need to lock it (and in fact you can't)

Upvotes: 3

yshavit
yshavit

Reputation: 43391

Yes.

From the docs:

The iterators returned by the iterator method of the collections returned by all of this class's "collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException.

So, if you want to go through an iterator from a Hashtable, you have to ensure that nobody else modifies the Hashtable object in the meanwhile. If that object is shared among multiple threads, the best way to do that is to synchronize on a common object, such as that very same Hashtable you're iterating over.

Upvotes: 3

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