Using ConcurrentHashMap

[James Yong]

Hi,

I have google and couldn't find any tutorial/examples on using
ConcurrentHashMap.
Suppose I have the following three scenarios, what should I do to ensure
thread-safety?


if (!map.containsKey(key))
map.put(key, value);


for (int i=0; i<list.size(); i++) {
doSomething(list.get(i));
}


for (Iterator i=list.iterator(); i.hasNext(); ) {
doSomething(i.next());
}



Regards,
James Yong

 

[Robert Klemme]

Hi,

I have google and couldn't find any tutorial/examples on using
ConcurrentHashMap.
Suppose I have the following three scenarios, what should I do to
ensure thread-safety?


if (!map.containsKey(key))
map.put(key, value);

In this case a normal HashMap is sufficient because you need external
synchronization anyway:

synchronize ( map ) {
if (!map.containsKey(key))
map.put(key, value);
}

for (int i=0; i<list.size(); i++) {
doSomething(list.get(i));
}


for (Iterator i=list.iterator(); i.hasNext(); ) {
doSomething(i.next());
}

These are no maps. Also it depends on what you want to allow. The easiest
again is external synchronization on the complete container for the complete
iteration. Difficult to answer without further info on your requirements.

Regards

robert

 

[James Yong]

Hi Robert,

In this case a normal HashMap is sufficient because you need external
synchronization anyway:

Thanks for the explanation above ;-)

synchronize ( map ) {
if (!map.containsKey(key))
map.put(key, value);
}


These are no maps. Also it depends on what you want to allow. The easiest
again is external synchronization on the complete container for the complete
iteration. Difficult to answer without further info on your requirements.

Regards

robert

Yeah, you are right that the 2nd and 3rd scenario has nothing to do with
Maps.
I am trying to use Doug Lea's util.concurrent package with Java 1.4. I am
not sure if I need to implement
a different Iterator if I were to use from the package to replace List and
Maps
I took the examples blindly from
http://www-128.ibm.com/developerworks/java/library/j-jtp07233.html


Guess I have to spend some time looking into the source code of the
packages. Thanks

Regards,
James

 

[Thomas Hawtin]

I have google and couldn't find any tutorial/examples on using
ConcurrentHashMap.
Suppose I have the following three scenarios, what should I do to ensure
thread-safety?


if (!map.containsKey(key))
map.put(key, value);

map.putIfAbsent(key, value);

for (int i=0; i<list.size(); i++) {
doSomething(list.get(i));
}

for (Iterator i=list.iterator(); i.hasNext(); ) {
doSomething(i.next());
}

I assume this is CopyOnWriteArrayList.

for (E e : list) {
doSomething(e);
}

The iterator works on a constant version of the list.

Synchronizing to the whole ConcurrentMap/CopyOnWriteArrayList wont help
as the point is to avoid collection-global locks.

Tom Hawtin

 

[James Yong]

map.putIfAbsent(key, value);

Thanks Thomas. I think you have more or less clear some doubts I have ;-)

I assume this is CopyOnWriteArrayList.

for (E e : list) {
doSomething(e);
}

The iterator works on a constant version of the list.

Synchronizing to the whole ConcurrentMap/CopyOnWriteArrayList wont help
as the point is to avoid collection-global locks.

Yes, I agree on that too ;-)

Regards,
James

 

[javabudy]

Guys when should we use synchronizedHashMap and ConcurrenthashMap ?