So I was looking at building a simple object cache that expires the objects after a given time. The obvious mechanism for this is the use the DelayedQueue class from the concurrency package in Java; but I wanted to know if it way possible to update the delay after an object has been added to the queue. Looking at the Delayed interface there didn’t seem to be a good reason not to in the docs so I thought it was time to experiment.
So first of all you need to to create an instance of Delayed, this is a very simple implementation that with the switch of a flag you can basically invert the timeout order in the list. (And add a suitable offset so things happen in the right order
static int COUNT=100;
class DelayedSwap implements Delayed, Comparable<Delayed> {
int index = 0;
volatile boolean swap = false;
long starttime;
public DelayedSwap(int index, long starttime) {
super();
this.index = index;
this.starttime = starttime;
}
private long getDelay() {
return (swap ? starttime + (2*COUNT - index) * 100 :
starttime + index * 100) - System.currentTimeMillis();
}
public String toString()
{
return index + ' swapped ' + swap + ' delay ' + getDelay();
}
@Override
public long getDelay(TimeUnit unit) {
return unit.convert(getDelay(), TimeUnit.MILLISECONDS);
}
@Override
public int compareTo(Delayed delayed) {
if (delayed == this)
return 0;
return (int)(getDelay(TimeUnit.MILLISECONDS) - delayed.getDelay(TimeUnit.MILLISECONDS));
}
}
So to test this I created a method that would create a bunch of the DelayedSwap objects and half way through processing the list switch the flag so altering the order of expiration.
public static void main(String[] args) throws InterruptedException {
long start = System.currentTimeMillis();
final List
delayed = new ArrayList
();
for (int i = 1; i < COUNT; i++) {
delayed.add(new DelayedSwap(i, start));
}
final DelayQueue dq = new DelayQueue();
dq.addAll(delayed);
new Thread(new Runnable() {
@Override
public void run() {
try {
TimeUnit.SECONDS.sleep(5);
} catch (InterruptedException e) {
}
for (DelayedSwap d : delayed) {
d.swap = true;
}
}
}).start();
while (!dq.isEmpty()) {
System.out.println(dq.take());
}
}So what I was expecting was the elements 1-50 ish written out in the correct order but instead after the swap over the elements are coming out in an arbitrary order quite far away from the request delay time.
1 swapped false delay -19 2 swapped false delay -4 3 swapped false delay -4 4 swapped false delay -4 5 swapped false delay -4 6 swapped false delay -4 7 swapped false delay -4 8 swapped false delay -4 9 swapped false delay -4 10 swapped false delay -4 11 swapped false delay -4 12 swapped false delay -4 13 swapped false delay -4 14 swapped false delay -4 15 swapped false delay -4 16 swapped false delay -4 17 swapped false delay -4 18 swapped false delay -4 19 swapped false delay -4 20 swapped false delay -4 21 swapped false delay -4 22 swapped false delay -4 23 swapped false delay -4 24 swapped false delay -4 25 swapped false delay -4 26 swapped false delay -4 27 swapped false delay -4 28 swapped false delay -4 29 swapped false delay -4 30 swapped false delay -4 31 swapped false delay -4 32 swapped false delay -4 33 swapped false delay -4 34 swapped false delay -4 35 swapped false delay -4 36 swapped false delay -4 37 swapped false delay -4 38 swapped false delay -4 39 swapped false delay -5 40 swapped false delay -4 41 swapped false delay -4 42 swapped false delay -5 43 swapped false delay -4 44 swapped false delay -5 45 swapped false delay -5 46 swapped false delay -5 47 swapped false delay -5 48 swapped false delay -5 49 swapped false delay -5 50 swapped false delay -5 51 swapped true delay -6 94 swapped true delay -4306 96 swapped true delay -4506 87 swapped true delay -3606 91 swapped true delay -4006 97 swapped true delay -4606 95 swapped true delay -4406 98 swapped true delay -4706 92 swapped true delay -4106 82 swapped true delay -3106 80 swapped true delay -2906 90 swapped true delay -3906 93 swapped true delay -4206 74 swapped true delay -2306 99 swapped true delay -4806 70 swapped true delay -1906 69 swapped true delay -1806 66 swapped true delay -1506 83 swapped true delay -3206 62 swapped true delay -1107 61 swapped true delay -1007 58 swapped true delay -707 71 swapped true delay -2007 89 swapped true delay -3807 85 swapped true delay -3407 78 swapped true delay -2707 86 swapped true delay -3507 81 swapped true delay -3007 88 swapped true delay -3707 84 swapped true delay -3307 79 swapped true delay -2807 76 swapped true delay -2507 72 swapped true delay -2107 68 swapped true delay -1707 65 swapped true delay -1407 60 swapped true delay -907 57 swapped true delay -608 55 swapped true delay -408 75 swapped true delay -2408 77 swapped true delay -2608 73 swapped true delay -2208 63 swapped true delay -1208 67 swapped true delay -1608 64 swapped true delay -1308 59 swapped true delay -808 56 swapped true delay -508 54 swapped true delay -308 53 swapped true delay -208 52 swapped true delay -108 Process exited with exit code 0.
So the trick is when you know you are going to modify the delay is to remove and then re-add the element to the queue.
// Replacement swap loop
for (DelayedSwap d : delayed) {
if (dq.remove(d))
{
d.swap = true;
dq.add(d);
}
}This run produces a more sensible set of results:
1 swapped false delay -4 2 swapped false delay -8 3 swapped false delay -14 4 swapped false delay -8 5 swapped false delay -4 6 swapped false delay -4 7 swapped false delay -4 8 swapped false delay -4 9 swapped false delay -4 10 swapped false delay -4 11 swapped false delay -4 12 swapped false delay -4 13 swapped false delay -4 14 swapped false delay -4 15 swapped false delay -4 16 swapped false delay -4 17 swapped false delay -4 18 swapped false delay -8 19 swapped false delay -4 20 swapped false delay -4 21 swapped false delay -4 22 swapped false delay -4 23 swapped false delay -4 24 swapped false delay -4 25 swapped false delay -4 26 swapped false delay -4 27 swapped false delay -4 28 swapped false delay -4 29 swapped false delay -4 30 swapped false delay -4 31 swapped false delay -4 32 swapped false delay -4 33 swapped false delay -4 34 swapped false delay -4 35 swapped false delay -4 36 swapped false delay -4 37 swapped false delay -4 38 swapped false delay -4 39 swapped false delay -5 40 swapped false delay -5 41 swapped false delay -5 42 swapped false delay -4 43 swapped false delay -4 44 swapped false delay -5 45 swapped false delay -5 46 swapped false delay -5 47 swapped false delay -5 48 swapped false delay -5 49 swapped false delay -5 50 swapped false delay -5 99 swapped true delay -5 98 swapped true delay -5 97 swapped true delay -11 96 swapped true delay -1 95 swapped true delay -5 94 swapped true delay -9 93 swapped true delay -5 92 swapped true delay -5 91 swapped true delay -5 90 swapped true delay -5 89 swapped true delay -5 88 swapped true delay -5 87 swapped true delay -5 86 swapped true delay -5 85 swapped true delay -5 84 swapped true delay -5 83 swapped true delay -5 82 swapped true delay -5 81 swapped true delay -5 80 swapped true delay -5 79 swapped true delay -5 78 swapped true delay -5 77 swapped true delay -5 76 swapped true delay -5 75 swapped true delay -5 74 swapped true delay -5 73 swapped true delay -5 72 swapped true delay -6 71 swapped true delay -5 70 swapped true delay -5 69 swapped true delay -5 68 swapped true delay -5 67 swapped true delay -5 66 swapped true delay -5 65 swapped true delay -5 64 swapped true delay -5 63 swapped true delay -6 62 swapped true delay -5 61 swapped true delay -6 60 swapped true delay -6 59 swapped true delay -6 58 swapped true delay -6 57 swapped true delay -6 56 swapped true delay -6 55 swapped true delay -6 54 swapped true delay -6 53 swapped true delay -6 52 swapped true delay -6 51 swapped true delay -6 Process exited with exit code 0.
I don’t think this is a bug in the object itself, as you wouldn’t expect a HashTable to orders it’s self when the key changes, but I was a little bit surprise by the behaviour.
Happy coding and don’t forget to share!
Reference: Changing delay, and hence the order, in a DelayQueue from our JCG partner Gerard Davison at the Gerard Davison’s blog blog.
