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final class IPHolder {
private final List<InetAddress> ips = Collections.synchronizedList(new ArrayList<InetAddress>());
public void addAndPrintIPAddresses(InetAddress address) {
synchronized (ips) {
ips.add(address);
InetAddress[] addressCopy = (InetAddress[]) ips.toArray(new InetAddress[0]);
}
// Iterate through array addressCopy ...
}
}
}
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This technique is also called client-side locking \[[Goetz 06|AA. Java References#Goetz 06]\], because the class holds a lock on an object that might be accessible to other classes. Client-side locking is not always an appropriate strategy; see [CON34-J. Avoid client-side locking when using classes that do not commit to their locking strategy] for more information. |
The addressCopy array holds a copy of the IP addresses and can be safely iterated outside the synchronized block. This code does not violate CON11-J. Do not synchronize on a collection view if the backing collection is accessible, because while it does synchronize on a collection view (the synchronizedList), the backing collection is inaccessible, and therefore cannot be modified by any code.
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