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
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 | 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.
01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 | 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.
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 | 1 swapped false delay -192 swapped false delay -43 swapped false delay -44 swapped false delay -45 swapped false delay -46 swapped false delay -47 swapped false delay -48 swapped false delay -49 swapped false delay -410 swapped false delay -411 swapped false delay -412 swapped false delay -413 swapped false delay -414 swapped false delay -415 swapped false delay -416 swapped false delay -417 swapped false delay -418 swapped false delay -419 swapped false delay -420 swapped false delay -421 swapped false delay -422 swapped false delay -423 swapped false delay -424 swapped false delay -425 swapped false delay -426 swapped false delay -427 swapped false delay -428 swapped false delay -429 swapped false delay -430 swapped false delay -431 swapped false delay -432 swapped false delay -433 swapped false delay -434 swapped false delay -435 swapped false delay -436 swapped false delay -437 swapped false delay -438 swapped false delay -439 swapped false delay -540 swapped false delay -441 swapped false delay -442 swapped false delay -543 swapped false delay -444 swapped false delay -545 swapped false delay -546 swapped false delay -547 swapped false delay -548 swapped false delay -549 swapped false delay -550 swapped false delay -551 swapped true delay -694 swapped true delay -430696 swapped true delay -450687 swapped true delay -360691 swapped true delay -400697 swapped true delay -460695 swapped true delay -440698 swapped true delay -470692 swapped true delay -410682 swapped true delay -310680 swapped true delay -290690 swapped true delay -390693 swapped true delay -420674 swapped true delay -230699 swapped true delay -480670 swapped true delay -190669 swapped true delay -180666 swapped true delay -150683 swapped true delay -320662 swapped true delay -110761 swapped true delay -100758 swapped true delay -70771 swapped true delay -200789 swapped true delay -380785 swapped true delay -340778 swapped true delay -270786 swapped true delay -350781 swapped true delay -300788 swapped true delay -370784 swapped true delay -330779 swapped true delay -280776 swapped true delay -250772 swapped true delay -210768 swapped true delay -170765 swapped true delay -140760 swapped true delay -90757 swapped true delay -60855 swapped true delay -40875 swapped true delay -240877 swapped true delay -260873 swapped true delay -220863 swapped true delay -120867 swapped true delay -160864 swapped true delay -130859 swapped true delay -80856 swapped true delay -50854 swapped true delay -30853 swapped true delay -20852 swapped true delay -108Process 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.
1 2 3 4 5 6 7 8 | // 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:
001 002 003 004 005 006 007 008 009 010 011 012 013 014 015 016 017 018 019 020 021 022 023 024 025 026 027 028 029 030 031 032 033 034 035 036 037 038 039 040 041 042 043 044 045 046 047 048 049 050 051 052 053 054 055 056 057 058 059 060 061 062 063 064 065 066 067 068 069 070 071 072 073 074 075 076 077 078 079 080 081 082 083 084 085 086 087 088 089 090 091 092 093 094 095 096 097 098 099 100 | 1 swapped false delay -42 swapped false delay -83 swapped false delay -144 swapped false delay -85 swapped false delay -46 swapped false delay -47 swapped false delay -48 swapped false delay -49 swapped false delay -410 swapped false delay -411 swapped false delay -412 swapped false delay -413 swapped false delay -414 swapped false delay -415 swapped false delay -416 swapped false delay -417 swapped false delay -418 swapped false delay -819 swapped false delay -420 swapped false delay -421 swapped false delay -422 swapped false delay -423 swapped false delay -424 swapped false delay -425 swapped false delay -426 swapped false delay -427 swapped false delay -428 swapped false delay -429 swapped false delay -430 swapped false delay -431 swapped false delay -432 swapped false delay -433 swapped false delay -434 swapped false delay -435 swapped false delay -436 swapped false delay -437 swapped false delay -438 swapped false delay -439 swapped false delay -540 swapped false delay -541 swapped false delay -542 swapped false delay -443 swapped false delay -444 swapped false delay -545 swapped false delay -546 swapped false delay -547 swapped false delay -548 swapped false delay -549 swapped false delay -550 swapped false delay -599 swapped true delay -598 swapped true delay -597 swapped true delay -1196 swapped true delay -195 swapped true delay -594 swapped true delay -993 swapped true delay -592 swapped true delay -591 swapped true delay -590 swapped true delay -589 swapped true delay -588 swapped true delay -587 swapped true delay -586 swapped true delay -585 swapped true delay -584 swapped true delay -583 swapped true delay -582 swapped true delay -581 swapped true delay -580 swapped true delay -579 swapped true delay -578 swapped true delay -577 swapped true delay -576 swapped true delay -575 swapped true delay -574 swapped true delay -573 swapped true delay -572 swapped true delay -671 swapped true delay -570 swapped true delay -569 swapped true delay -568 swapped true delay -567 swapped true delay -566 swapped true delay -565 swapped true delay -564 swapped true delay -563 swapped true delay -662 swapped true delay -561 swapped true delay -660 swapped true delay -659 swapped true delay -658 swapped true delay -657 swapped true delay -656 swapped true delay -655 swapped true delay -654 swapped true delay -653 swapped true delay -652 swapped true delay -651 swapped true delay -6Process 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.
