Tuesday, May 22, 2007

Garbage collection example

A Java program can request the underlying JVM to perform garbage collection by calling the gc() method of the System class.

Note that System.gc() is a request to the underlying JVM. Garbage collection may not always happen. Hence we cannot depend on this method, however we can use it to optimize performance, with the understanding that it may not work as desired on all JVM's.

 


Study the program shown below. It ilustrates how we can invoke the garbage collector from a Java program.

01 /** This program creates instances of Bag objects in a loop which will run 100000 times.
02  * Whenever the program is in a loop that is a multiple of 1000, it requests the JVM
03  * to start the garbage collector. We will see that the JVM may not always fulfill the
04  * request.
05  */
06 public class GarbageCollectionDemo {
07   public static void main(String args[]) {
08     //loop 10000 times
09     for(int i=0;i<10000;i++) {
10       //if this loop is a multiple of 1000, then request for garbage collection
11       if(i%1000 == 0) {
12         System.out.println("Requesting for garbage collection");
13         System.gc();
14       }
15       System.out.println("Creating bag " + i);
16       Bag b = new Bag(String.valueOf(i));
17     }
18   }
19 }
20 
21 /** A placeholder object that is created in GarbageCollectionDemo
22  *
23  */
24 class Bag {
25   private String id;
26   public Bag(String id) {
27     this.id = id;
28   }
29 
30   /**We override the finalize method and put a print statement
31    * which will tell us when the object was garbage collected.
32    */
33   public void finalize() {
34     System.out.println("Garbage collecting bag " + id);
35   }
36 }
Java2html

Here is the output of the program. Since the output was very large, I have truncated most parts (which have been shown as dots ...). As is shown, we request for garbage collection after creating 999 objects [line 11]. The JVM complies and garbage collects all unused objects. Also note that the garbage collector again starts reclaiming objects on line 32, even though the program has not specifically asked for it to do so. It may have done so either because it ran short of resources (but that seems unlikely because the garbage collector had just freed resources some time back), or it was run in the normal course of the garbage collection algorithm (this seems more likely).

Now this JVM is really well behaved, it fulfills all our garbage collection requests, but do not take this behavior for granted. All JVM's may not be as well mannered.


02 Requesting for garbage collection
03 Creating bag 0
04 Creating bag 1
05 Creating bag 2
06 Creating bag 3
07 Creating bag 4
08 Creating bag 5
09 ...
10 Creating bag 999
11 Requesting for garbage collection
12 Garbage collecting bag 999
13 Garbage collecting bag 998
14 Garbage collecting bag 997
15 Garbage collecting bag 996
16 Garbage collecting bag 995
17 ...
18 Garbage collecting bag 5
19 Garbage collecting bag 4
20 Garbage collecting bag 3
21 Garbage collecting bag 2
22 Garbage collecting bag 1
23 Garbage collecting bag 0
24 Creating bag 1000
25 Creating bag 1001
26 Creating bag 1002
27 Creating bag 1003
28 ...
29 Creating bag 1452
30 Creating bag 1453
31 Creating bag 1454
32 Garbage collecting bag 1454
33 Garbage collecting bag 1001
34 ...
35 Garbage collecting bag 1228
36 Garbage collecting bag 1226
37 Garbage collecting bag 1227
38 Creating bag 1455
39 Creating bag 1456
40 ...
41 Creating bag 1998
42 Creating bag 1999
43 Requesting for garbage collection
44 Garbage collecting bag 1999
45 Garbage collecting bag 1998
46 ...
47 Garbage collecting bag 1456
48 Garbage collecting bag 1455
49 Creating bag 2000
50 ...
51 Creating bag 2813
52 Creating bag 2814
53 Garbage collecting bag 2814
54 Garbage collecting bag 2000
55 Garbage collecting bag 2813
56 ...
57 Garbage collecting bag 2413
58 Garbage collecting bag 2401
59 ...
60 Garbage collecting bag 2407
61 Creating bag 2815
62 Creating bag 2816
63 Creating bag 2817
64 ...
65 ...
66 ...
67 Creating bag 9872
68 ...
69 Creating bag 9999
Java2html

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