CODEWITHSUNDEEP
javagarbage-collectionmonitoring
Florin Andrei
Florin Andrei

Reputation: 1095

How do I find the time elapsed since the last garbage collection with a JVM?

This is mostly about the Oracle Java 8 JVM. Assume I don't have access to logs. Java is not running in verbose mode and stdin / stderr are not being logged.

Is there any place within the running JVM where it keeps the timestamp of the last GC event? Or anything else that might indicate when the last GC event took place?

I just want to know how long ago the last GC event happened. Bonus points for differentiating between different types of GC events.

Upvotes: 2

Views: 456

Answers (1)

Holger
Holger

Reputation: 298599

When you are fine with a solution that works with Oracle’s Java 8 JVM and might work with others, you can use the GarbageCollectorMXBean platform extension. I linked to the JDK 11 documentation, as this class is not included in JDK 8’s documentation, but this type is present and the following solution works.

final RuntimeMXBean rt = ManagementFactory.getRuntimeMXBean();
long base = rt.getStartTime();

for(GarbageCollectorMXBean gc: ManagementFactory.getGarbageCollectorMXBeans()) {
  if(gc instanceof com.sun.management.GarbageCollectorMXBean) {
    com.sun.management.GcInfo info =
        ((com.sun.management.GarbageCollectorMXBean)gc).getLastGcInfo();

    if(info != null) {
      System.out.printf("%-15s%tF %2$tT.%2$tL  (%6dms ago)%n",
        gc.getName(), base + info.getStartTime(), rt.getUptime()-info.getStartTime());
    }
  }
}

When I prepended

System.gc();
System.out.println(System.getProperty("java.version"));

for a simple run, I got

1.8.0_131
PS Scavenge    2019-04-09 11:58:25.808  (    50ms ago)
PS MarkSweep   2019-04-09 11:58:25.809  (    74ms ago)

This information can also be queried remotely, see also Ways to Access MXBeans. It works if the JVM has the Management Agent already running or is reachable via the Attach API (usually implies running on the same computer), so the Management Agent can be started later on. These options are also what tools like VisualVM use.

Upvotes: 1

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