An Serviceability-Agent based tool to see stats of NIO direct memory for JDK8

DirectMemorySize.java

import java.lang.reflect.*;
import java.lang.reflect.Method;
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 {
	private boolean exactMallocMode;
	private boolean verbose;

	public DirectMemorySize(boolean exactMallocMode, boolean verbose) {
		this.exactMallocMode = exactMallocMode;
		this.verbose = verbose;
	}

	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: (in bytes)");
			System.out.printf("  reserved size = %f MB (%d bytes)\n", toM(reservedMemory), reservedMemory);
			System.out.printf("  max size      = %f MB (%d bytes)\n", toM(directMemory), directMemory);

			if (verbose) {
				System.out.println("Currently allocated direct buffers:");
			}

			if (exactMallocMode || verbose) {
				final long pageSize = getStaticIntFieldValue("java.nio.Bits", "pageSize");
				ObjectHeap heap = VM.getVM().getObjectHeap();
				InstanceKlass deallocatorKlass =
						SystemDictionaryHelper.findInstanceKlass("java.nio.DirectByteBuffer$Deallocator");
				final LongField addressField = (LongField) deallocatorKlass.findField("address", "J");
				final IntField capacityField = (IntField) deallocatorKlass.findField("capacity", "I");
				final int[] countHolder = new int[1];

				heap.iterateObjectsOfKlass(new DefaultHeapVisitor() {
					public boolean doObj(Oop oop) {
						long address = addressField.getValue(oop);
						if (address == 0) return false; // this deallocator has already been run

						long capacity = capacityField.getValue(oop);
						long mallocSize = capacity + pageSize;
						countHolder[0]++;

						if (verbose) {
							System.out.printf("  0x%016x: capacity = %f MB (%d bytes),"
											+ " mallocSize = %f MB (%d bytes)\n",
									address, toM(capacity), capacity,
									toM(mallocSize), mallocSize);
						}

						return false;
					}
				}, deallocatorKlass, false);

				if (exactMallocMode) {
					long totalMallocSize = reservedMemory + pageSize * countHolder[0];
					System.out.printf("NIO direct memory malloc'd size: %f MB (%d bytes)\n",
							toM(totalMallocSize), totalMallocSize);
				}
			}
		} 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); // on JDK7 use: field.getValue(klass.getJavaMirror());
	}

	public static int getStaticIntFieldValue(String className, String fieldName) {
		InstanceKlass klass = SystemDictionaryHelper.findInstanceKlass(className);
		IntField field = (IntField) klass.findField(fieldName, "I");
		return field.getValue(klass); // on JDK7 use: field.getValue(klass.getJavaMirror());
	}

	public static double toM(long value) {
		return value / (1024 * 1024.0);
	}

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

	protected void printFlagsUsage() {
		System.out.println("    -e\tto print the actual size malloc'd");
		System.out.println("    -v\tto print verbose info of every live DirectByteBuffer allocated from Java");
		super.printFlagsUsage();
	}

	public static void main(String[] args) {
		boolean exactMallocMode = false;
		boolean verbose = false;

		// argument processing logic copied from sun.jvm.hotspot.tools.JStack
		int used = 0;
		for (String arg : args) {
			if ("-e".equals(arg)) {
				exactMallocMode = true;
				used++;
			} else if ("-v".equals(arg)) {
				verbose = true;
				used++;
			}
		}

		if (used != 0) {
			args = Arrays.copyOfRange(args, used, args.length);
		}

		DirectMemorySize tool = new DirectMemorySize(exactMallocMode, verbose);
		try {
			Method start = Tool.class.getDeclaredMethod("start", Class.forName("[Ljava.lang.String;"));
			start.setAccessible(true);
			start.invoke(tool, new Object[]{args});
		} catch (Exception e) {
			e.printStackTrace();
		}
		tool.stop();
	}
}

Caused by: java.lang.NullPointerException
at DirectMemorySize.getStaticLongFieldValue(DirectMemorySize.java:79)