Using the JVM Performance Counters

So, in JRockit there was this neat little dynamic MBean from which you could access all the JVM performance counters as attributes. Tonight I ended up in an e-mail thread leading me to think about how to retrieve the HotSpot ones. This is of course all very unsupported, and counter names/content and even the very API is subject to change at any given release. I am providing this information mostly as a reminder to myself. Who knows, someone might find this useful.

I will now go ahead and show how something similar to the PerformanceCounters MBean available in JRockit can be built. I recommend against doing this in any kind of production scenario, as the PerformanceCounter API is totally unsupported. Please don’t ask me what a certain counter means – you probably shouldn’t be using these in the first place. This is all for lolz. :)

Here comes the usual disclaimer:

The following blog entry will describe UNSUPPORTED functionality. This means that relying on the described APIs or functionality may BREAK your code/plugin with any given update of the JDK and/or Mission Control.

Performance Counter Hello World

If you ever used JRockit, you may fondly remember the old jrcmd utility, which now exists in a HotSpot incarnation as jcmd. Well, jrcmd had an option, –l, which would list all the the performance counters in the target JVM. The following code will let you implement something similar to the jrcmd –l command. Note that the PerformanceCounter API resides in a package most IDEs will restrict access to. You will need to change your project settings accordingly for the following to compile.

Anyways, here is a hack to list all the performance counters for a particular PID:

import java.io.IOException;
import java.nio.ByteBuffer;

import sun.management.counter.Counter;
import sun.management.counter.perf.PerfInstrumentation;
import sun.misc.Perf;

public class PerfCounterTest {

    public static void main(String[] args) throws IOException {
        if (args.length != 1) {
            System.out.println(“Usage: PerfCounterTest <PID>”);
            System.exit(2);
        }
        Perf p = Perf.getPerf();
        ByteBuffer buffer = p.attach(Integer.parseInt(args[0]), “r”);
        PerfInstrumentation perfInstrumentation = new PerfInstrumentation(buffer);
        for (Counter counter : perfInstrumentation.getAllCounters()) {
            System.out.println(String.format(
                    “%s = %s [Variability: %s, Units: %s]“, counter.getName(),
                    String.valueOf(counter.getValue()),
                    counter.getVariability(), counter.getUnits()));
        }

    }
}

Note that you will need the “sun.misc.Perf.getPerf” permission to be able to access the Perf instance when running with a security manager.

From here to a fully functional Dynamic MBean, the step is pretty short.

PerformanceCounterMBean á la JRockit for HotSpot

Here is an example of how code exposing the Performance Counters as a Dynamic MBean can look like:

import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.management.Attribute;
import javax.management.AttributeList;
import javax.management.AttributeNotFoundException;
import javax.management.DynamicMBean;
import javax.management.InvalidAttributeValueException;
import javax.management.MBeanAttributeInfo;
import javax.management.MBeanException;
import javax.management.MBeanInfo;
import javax.management.MalformedObjectNameException;
import javax.management.ObjectName;
import javax.management.ReflectionException;
import javax.management.RuntimeOperationsException;

import sun.misc.Perf;
import sun.management.counter.Counter;
import sun.management.counter.perf.PerfInstrumentation;

public class PerfCounters implements DynamicMBean {
    public final static ObjectName MBEAN_OBJECT_NAME;
    public final Map<String, Counter> counterMap;
    private final MBeanInfo info;

    static {
        MBEAN_OBJECT_NAME = createObjectName(“se.hirt.management:type=PerfCounters”);
    }

    public PerfCounters() {
        counterMap = setUpCounters();
        info = createMBeanInfo();
    }

    @Override
    public Object getAttribute(String attribute)
            throws AttributeNotFoundException, MBeanException,
            ReflectionException {
        if (attribute == null) {
            throw new RuntimeOperationsException(new IllegalArgumentException(
                    “The attribute name cannot be null.”),
                    “Cannot invoke getAttribute on ” + MBEAN_OBJECT_NAME
                            + ” with null as attribute name.”);
        }
        Counter c = counterMap.get(attribute);
        if (c == null) {
            throw new AttributeNotFoundException(
                    “Could not find the attribute ” + attribute);
        }
        return c.getValue();
    }

    @Override
    public void setAttribute(Attribute attribute)
            throws AttributeNotFoundException, InvalidAttributeValueException,
            MBeanException, ReflectionException {
        if (attribute == null) {
            throw new RuntimeOperationsException(new IllegalArgumentException(
                    “The attribute name cannot be null.”),
                    “Cannot invoke setAttribute on ” + MBEAN_OBJECT_NAME
                            + ” with null as attribute name.”);
        }
        Counter c = counterMap.get(attribute);
        if (c == null) {
            throw new AttributeNotFoundException(
                    “Could not find the attribute ” + attribute + “.”);
        }
        throw new RuntimeOperationsException(
                new UnsupportedOperationException(),
                “All attributes on the PerfCounters MBean are read only.”);
    }

    @Override
    public AttributeList getAttributes(String[] attributes) {
        AttributeList attributeList = new AttributeList();
        for (String attribute : attributes) {
            try {
                attributeList.add(new Attribute(attribute,
                        getAttribute(attribute)));
            } catch (AttributeNotFoundException | MBeanException
                    | ReflectionException e) {
                // Seems this one is not supposed to throw exceptions. Try to
                // get as many as possible.
            }
        }
        return attributeList;
    }

    @Override
    public AttributeList setAttributes(AttributeList attributes) {
        // Seems this one is not supposed to throw exceptions.
        // Just ignore.
        return null;
    }

    @Override
    public Object invoke(String actionName, Object[] params, String[] signature)
            throws MBeanException, ReflectionException {
        throw new MBeanException(new UnsupportedOperationException(
                MBEAN_OBJECT_NAME + ” does not have any operations.”));
    }

    @Override
    public MBeanInfo getMBeanInfo() {
        return info;
    }

    private static ObjectName createObjectName(String name) {
        try {
            return new ObjectName(name);
        } catch (MalformedObjectNameException e) {
            // This will not happen – known to be wellformed.
            e.printStackTrace();
        }
        return null;
    }

    private Map<String, Counter> setUpCounters() {
        Map<String, Counter> counters = new HashMap<>();
        Perf p = Perf.getPerf();
        try {
            ByteBuffer buffer = p.attach(0, “r”);
            PerfInstrumentation perfInstrumentation = new PerfInstrumentation(
                    buffer);
            for (Counter counter : perfInstrumentation.getAllCounters()) {
                counters.put(counter.getName(), counter);
            }
        } catch (IllegalArgumentException | IOException e) {
            System.err.println(“Failed to access performance counters. No counters will be available!”);
            e.printStackTrace();
        }
        return counters;
    }

    private MBeanInfo createMBeanInfo() {
        Collection<Counter> counters = counterMap.values();
        List<MBeanAttributeInfo> attributes = new ArrayList<>(counters.size());
        for (Counter c : counters) {
            if (!c.isVector()) {
                String typeName = “java.lang.String”;
                synchronized (c) {
                    Object value = c.getValue();
                    if (value != null) {
                        typeName = value.getClass().getName();
                    }
                }
                attributes.add(new MBeanAttributeInfo(c.getName(), typeName,
                        String.format(“%s [%s,%s]“, c.getName(), c.getUnits(),
                                c.getVariability()), true, false, false));
            }
        }
        MBeanAttributeInfo[] attributesArray = attributes.toArray(new MBeanAttributeInfo[attributes.size()]);
        return new MBeanInfo(
                this.getClass().getName(),
                “An MBean exposing the available JVM Performance Counters as attributes.”,
                attributesArray, null, null, null);
    }
}

And here is a little test application that registers the MBean with the platform MBean server and then waits around, giving you an opportunity to hook up with Mission Control and try it out:

import java.io.IOException;
import java.lang.management.ManagementFactory;

import javax.management.InstanceAlreadyExistsException;
import javax.management.MBeanRegistrationException;
import javax.management.NotCompliantMBeanException;

public class PerfCounterMBeanRunner {

    public static void main(String[] args) throws InstanceAlreadyExistsException, MBeanRegistrationException, NotCompliantMBeanException, IOException {
        ManagementFactory.getPlatformMBeanServer().registerMBean(new PerfCounters(), PerfCounters.MBEAN_OBJECT_NAME);
        System.out.println(“Press enter to quit!”);
        System.in.read();
    }

}

If you use the JMX Console to connect to a JVM where the PerfCounters MBean is registered, you should be able to find the PerfCounters MBean using the MBean Browser. There should be quite a few performance counters available:

JMC_perf_counters

Conclusion

Sample code for using the proprietary JVM performance counter API was provided, as well as example code on how to implement a version of the old jrcmd –l command, as well as the old PerformanceCounter MBean of JRockit. Again, these examples are using proprietary APIs that may break at any given release. Use responsibly!

jrockit_duke_love

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