How can I see which garbage collector java is using

Question

The Java Virtual Machine supports several garbage collection strategies.

This article explains them.

Now I am wondering which (automatically selected) strat

Answer 1

http://java.sun.com/j2se/1.5.0/docs/guide/vm/gc-ergonomics.html which is applicable for J2SE 6 as well states that the default is the Parallel Collector.

We tested this once on a JVM 1.5 by setting only

-server -Xms3g -Xmx3g -XX:PermSize=128m -XX:LargePageSizeInBytes=4m -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps

and the output showed

41359.597: [GC [PSYoungGen: 90499K->32K(377344K)] 268466K->181862K(2474496K), 0.0183138 secs]
41359.615: [Full GC [PSYoungGen: 32K->0K(377344K)] [PSOldGen: 181830K->129760K(2097152K)] 181862K->129760K(2474496K) [PSPermGen: 115335K->115335K(131072K)], 4.4590942 secs]

where PS stands for Parallel Scavenging

Answer 2

As Joachim already pointed out, the article you refer to describes the VM strategies offered by Sun's VM. The VM specification itself does not mandate specific GC algorithms and hence it won't make sense to have e.g. enumerated values for these in the API.

You can however get some infos from the Management API:

List<GarbageCollectorMXBean> beans = 
    ManagementFactory.getGarbageCollectorMXBeans();

Iterating through these beans, you can get the name of the GC (although only as a string) and the names of the memory pools, which are managed by the different GCs.

Answer 3

Looks like, we have more convenient way to define the version of GC at runtime. Always use tools, my suggestion. To define GC version we need two tools that come with JVM (placed in your jdk/bin directory):

1. VisualVM - start it and try to profile some process (for example you can profile VisualVM itself). Your profile will show you a PID of process (see the green rectangles at a screenshot).
2. jMap - start this tool with -heap <PID> options and find a string dedicated to a Garbage Collector type (see a pink line at a screenshot)

Answer 4

You can write simple progam which connects via jmx to your java process:

public class PrintJMX {
  public static void main(String[] args) throws Exception {
    String rmiHostname = "localhost";
    String defaultUrl = "service:jmx:rmi:///jndi/rmi://" + rmiHostname + ":1099/jmxrmi";
    JMXServiceURL jmxServiceURL = new JMXServiceURL(defaultUrl);

    JMXConnector jmxConnector = JMXConnectorFactory.connect(jmxServiceURL);
    MBeanServerConnection mbsc = jmxConnector.getMBeanServerConnection();


    ObjectName gcName = new ObjectName(ManagementFactory.GARBAGE_COLLECTOR_MXBEAN_DOMAIN_TYPE + ",*");

    for (ObjectName name : mbsc.queryNames(gcName, null)) {
      GarbageCollectorMXBean gc = ManagementFactory.newPlatformMXBeanProxy(mbsc,
        name.getCanonicalName(),
        GarbageCollectorMXBean.class);

      System.out.println(gc.getName());
    }

  }
}

Answer 5

Best way to get this is : Go to command Line and enter the following command.

java -XX:+PrintCommandLineFlags -version

It will show you result like :

C:\windows\system32>java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=132968640 -XX:MaxHeapSize=2127498240 -XX:+PrintCommandLineFl
ags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:-UseLargePagesInd
**ividualAllocation -XX:+UseParallelGC**
java version "1.8.0_66"
Java(TM) SE Runtime Environment (build 1.8.0_66-b17)
Java HotSpot(TM) 64-Bit Server VM (build 25.66-b17, mixed mode)`enter code here`

Answer 6

Put this in the JAVA_OPTS:

-XX:+UseSerialGC -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCTimeStamps

For the UseSerialGC we will see in the log:

 7.732: [GC 7.732: [DefNew: 419456K->47174K(471872K), 0.1321800 secs] 419456K->47174K(1520448K), 0.1322500 secs] [Times: user=0.10 sys=0.03, real=0.14 secs]

For the UseConcMarkSweepGC we will see in the log:

 5.630: [GC 5.630: ['ParNew: 37915K->3941K(38336K), 0.0123210 secs] 78169K->45163K(1568640K), 0.0124030 secs] [Times: user=0.02 sys=0.00, real=0.01 secs]

For the UseParallelGC we will see in the log:

30.250: [GC [PSYoungGen: 441062K->65524K(458752K)] 441062K->76129K(1507328K), 0.1870880 secs] [Times: user=0.33 sys=0.03, real=0.19 secs]