Hi experts,
I found this article
https://www.ibm.com/developerworks/library/j-hashmap/index.html and stuck
upon that statement
static volatile Map currentMap = null; // this must be volatile
static Object lockbox = new Object();
public static void buildNewMap() { // this is called by the producer
Map newMap = new HashMap(); // when the data needs to be
updated
synchronized (lockbox) { // this must be synchronized
because of the Java memory model
// .. do stuff to put things in newMap
newMap.put(....);
newMap.put(....);
}
/* After the above synchronization block, everything that is in the HashMap
is visible outside this thread */
/* Now make the updated set of values available to the consumer threads.
As long as this write operation can complete without being interrupted,
and is guaranteed to be written to shared memory, and the consumer can
live with the out of date information temporarily, this should work fine */
currentMap = newMap;
}
public static Object getFromCurrentMap(Object key) {
Map m = null;
Object result = null;
m = currentMap; // no locking around this is required
if (m != null) { // should only be null during
initialization
Object result = m.get(key); // get on a HashMap is not synchronized
// Do any additional processing needed using the result
}
return(result);
}
The synchronized block and the volatile keyword in Listing 1 are required
> because no happens-before relationship exists between the writes to the
> currentMap and the reads from currentMap.
I don't understand why we need a synchronized block in this case. isn't it
enough, in this case, to publish a newly created and populated hashmap
through a volatile variable to make other thread see its contents?
Thanks in advance
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