When I compile and run the following very simple Java program using OpenJDK 11 (Zulu distribution on Windows 10):
public class GCTest {
public static void main(String[] args) {
System.out.println("Free memory before garbage collection: " + Runtime.getRuntime().freeMemory());
Runtime.getRuntime().gc();
System.out.println("Free memory after garbage collection: " + Runtime.getRuntime().freeMemory());
}
}
it looks like garbage collection is decreasing the amount of free memory:
Free memory before garbage collection: 266881496
Free memory after garbage collection: 7772200
This does not happen when I run it with Oracle's Java 8:
Free memory before garbage collection: 254741016
Free memory after garbage collection: 255795064
Why is that?
The answer is: The GC of Java 11 (when explicitly called e.g. via System.gc()) can reduce the used memory of the Java process (in Java known as totalMemory).
In Java 8 the default garbage collector was not able to reduce the used memory of the Java process. Memory occupied by the Java process was never ever released. Only if you switch to the G1GC garbage collector (option '-XX:+UseG1GC') Java 8 is able to reduce the used memory of the Java process (if you manually call System.gc()).
The "free memory" is the memory occupied by the Java process but that is currently not used. Therefore if you perform a garbage collection and the memory occupied by Java is reduced the amount of free memory is reduced, too.
Therefore the common way to calculate the "free memory" of a Java process is to use
Runtime r = Runtime.getRuntime();
long free = r.maxMemory() - r.totalMemory() + r.freeMemory();
This way is independent of the current memory occupied by the Java process.