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(), etc. Something like number of cycles done, average time, etc.

We have systems running on customer machines where there is a suspicion that misconfigured memory pools are causing excessive GC frequency and length - it occurs to me that it would be good in general to periodically report the basic GC activity.

Is there any platform independent way to do this?

EDIT: I specifically want to output this data to the system log (the console), while running; this is not something I want to connect to the JVM for, as would be with JConsole or JVisualVM.

Edit2: The MX bean looks like what I want - does anyone have a working code example which obtains one of these?

Answer 1

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

Answer 2

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 3

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

Answer 4

See GarbageCollectorMXBean.