Created
September 7, 2016 02:39
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Do removals from a java.util.concurrent.ConcurrentMap provide a happens-before guarantee against a retrieval that sees this occurrence?
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import java.util.concurrent.ConcurrentHashMap; | |
import java.util.concurrent.ConcurrentMap; | |
import com.google.common.util.concurrent.Uninterruptibles; | |
public class SomeProgram { | |
public static void main(String[] args) { | |
new SomeOperator(1).start(); | |
new SomeOperator(1).start(); | |
new SomeOperator(1).start(); | |
new SomeOperator(1).start(); | |
} | |
private static class SomeOperator extends Thread { | |
private static final ConcurrentMap<Integer, Thread> BLOCKING_OPERATORS = new ConcurrentHashMap<>(); | |
private final int id; | |
public SomeOperator(int id) { | |
this.id = id; | |
} | |
@Override | |
public void run() { | |
final Thread previous = BLOCKING_OPERATORS.put(id, this); | |
if (previous != null) { | |
previous.interrupt(); | |
Uninterruptibles.joinUninterruptibly(previous); | |
} | |
try { | |
// critical section | |
} finally { | |
// doing this breaks synchronization, not because two threads can enter | |
// the critical section, but because changes made to memory during the | |
// critical section may not be propagated to the subsequent thread. | |
// | |
// on the other hand, not doing this leads to a memory leak from the | |
// unbounded growing of the map. | |
// | |
// suffice to say, this code in its current state is broken. | |
BLOCKING_OPERATORS.remove(id); | |
} | |
} | |
@Override | |
public void interrupt() { | |
super.interrupt(); | |
// fire cancellation mechanism | |
} | |
} | |
} |
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