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The previous compliant solution is safe for multithreaded use but does not scale because of excessive synchronization, which can lead to contention and deadlockdecreased performance.
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The {{ConcurrentHashMap}} class used in this compliant solution provides several utility methods for performing atomic operations and is often a good choice for algorithms that must scale \[[Lee 2009|AA. References#Lee 09]\]. |
Note that this compliant solution still requires synchronization, because without it, the test to prevent overflow and the increment will not happen atomically, so two threads calling increment() can still cause overflow. The synchronization block is smaller, and does not include the lookup or addition of new values, so it has less of an impact on performance as the previous compliant solution.
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final class KeyedCounter {
private final ConcurrentMap<String, AtomicInteger> map =
new ConcurrentHashMap<String, AtomicInteger>();
private final Object lock = new Object();
public void increment(String key) {
AtomicInteger value = new AtomicInteger();
AtomicInteger old = map.putIfAbsent(key, value);
if (old != null) {
value = old;
}
}
synchronized (lock) {
if (value.get() == Integer.MAX_VALUE) {
throw new ArithmeticException("Out of range");
}
value.incrementAndGet(); // Increment the value atomically
}
}
public Integer getCount(String key) {
AtomicInteger value = map.get(key);
return (value == null) ? null : value.get();
}
// Other accessors ...
}
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| Section 5.2.1, ConcurrentHashMap | |||
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<ac:structured-macro ac:name="unmigrated-wiki-markup" ac:schema-version="1" ac:macro-id="326ca6010c70624b-0b02537b-4872481b-8ea98bc7-c630f03ac9f299127e9271d0"><ac:plain-text-body><![CDATA[ | [[Lee 2009 | AA. References#Lee 09]] | Map & Compound Operation | ]]></ac:plain-text-body></ac:structured-macro> |
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