Can you get basic GC stats in Java?

Question

I would like to have some long-running server applications periodically output general GC performance numbers in Java, something like the GC equivalent of Runtime.freeMemory

Answer 1

Try -XX:-PrintGC and -XX:-PrintGCDetails; see VM Debugging Options.

Answer 2

Slightly off topic but you can hook up VisualVM and JConsole to running applications and see useful stats.

Answer 3

Here's an example using GarbageCollectorMXBean to print out GC stats. Presumably you would call this method periodically, e.g. scheduling using a ScheduledExecutorService.

public void printGCStats() {
    long totalGarbageCollections = 0;
    long garbageCollectionTime = 0;

    for(GarbageCollectorMXBean gc :
            ManagementFactory.getGarbageCollectorMXBeans()) {

        long count = gc.getCollectionCount();

        if(count >= 0) {
            totalGarbageCollections += count;
        }

        long time = gc.getCollectionTime();

        if(time >= 0) {
            garbageCollectionTime += time;
        }
    }

    System.out.println("Total Garbage Collections: "
        + totalGarbageCollections);
    System.out.println("Total Garbage Collection Time (ms): "
        + garbageCollectionTime);
}

Answer 4

See GarbageCollectorMXBean.