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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