G's Blog

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

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.