An Serviceability-Agent based tool to see stats of NIO direct memory, as an alternative on JDKs without JMX support for direct memory monitoring.

command_prompt

$ java -version
java version "1.6.0_30"
Java(TM) SE Runtime Environment (build 1.6.0_30-b12)
Java HotSpot(TM) 64-Bit Server VM (build 20.5-b03, mixed mode)
$ javac -classpath $JAVA_HOME/lib/sa-jdi.jar DirectMemorySize.java 
$ jps
18486 GroovyStarter
23135 Jps
$ java -classpath .:$JAVA_HOME/lib/sa-jdi.jar DirectMemorySize `pgrep java`
Attaching to process ID 18486, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 20.5-b03
NIO direct memory: (in bytes)
  reserved size = 0.000000 MB (0 bytes)
  max size      = 4069.000000 MB (4266655744 bytes)

DirectMemorySize.java

import java.io.*;
import java.util.*;
import sun.jvm.hotspot.memory.*;
import sun.jvm.hotspot.oops.*;
import sun.jvm.hotspot.debugger.*;
import sun.jvm.hotspot.runtime.*;
import sun.jvm.hotspot.tools.*;
import sun.jvm.hotspot.utilities.*;

public class DirectMemorySize extends Tool {
    public void run() {
        // Ready to go with the database...
        try {
            long reservedMemory = getStaticLongFieldValue("java.nio.Bits", "reservedMemory");
            long directMemory = getStaticLongFieldValue("sun.misc.VM", "directMemory");
            
            System.out.println("NIO direct memory:");
            System.out.printf("  reserved size = %fMB (%d bytes)\n", toM(reservedMemory), reservedMemory);
            System.out.printf("  max size = %fMB (%d bytes)\n", toM(directMemory), directMemory);
        } catch (AddressException e) {
            System.err.println("Error accessing address 0x"
                               + Long.toHexString(e.getAddress()));
            e.printStackTrace();
        }
    }

    public static long getStaticLongFieldValue(String className, String fieldName) {
        InstanceKlass klass = SystemDictionaryHelper.findInstanceKlass(className);
        LongField field = (LongField) klass.findField(fieldName, "J");
        return field.getValue(klass);
    }
    
    public static double toM(long value) {
        return value / (1024 * 1024.0);
    }

    public String getName() {
        return "directMemorySize";
    }

    public static void main(String[] args) {
        DirectMemorySize tool = new DirectMemorySize();
        tool.start(args);
        tool.stop();
    }
}