unsafe_vs_array.java

import java.lang.reflect.Field;
 
import sun.misc.Unsafe;
 
public class Test {
	private static final int N = 128 * 1024 * 1024;
 
	public static void main(String[] args) throws NoSuchFieldException, SecurityException, IllegalArgumentException, IllegalAccessException {
		{
			Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
			theUnsafe.setAccessible(true);
			Unsafe unsafe = (Unsafe) theUnsafe.get(null);
			
			long astart = System.nanoTime();
			final long ptr = unsafe.allocateMemory(N);
			long aend = System.nanoTime();
			System.out.printf("Unsafe alloc took %10s nano seconds%n", aend - astart);
			
			long wstart = System.nanoTime();
			for (int i = 0; i < N; ++i) {
				unsafe.putByte(ptr + i, (byte) 1);
			}
			long wend = System.nanoTime();
			System.out.printf("Unsafe write took %10s nano seconds%n", wend - wstart);
 
			long rstart = System.nanoTime();
			int s = 0;
			for (int i = 0; i < N; ++i) {
				s += unsafe.getByte(ptr + i);
			}
			long rend = System.nanoTime();
			System.out.printf("Unsafe  read took %10s nano seconds%n", rend - rstart);
 
			unsafe.freeMemory(ptr);
			
			System.out.println(s);
		}
		
		{
			long astart = System.nanoTime();
			final byte[] array = new byte[N];
			long aend = System.nanoTime();
			System.out.printf("Array  alloc took %10s nano seconds%n", aend - astart);
			
			long wstart = System.nanoTime();
			for (int i = 0; i < N; ++i) {
				array[i] = (byte) 1;
			}
			long wend = System.nanoTime();
			System.out.printf("Array  write took %10s nano seconds%n", wend - wstart);
			
			long rstart = System.nanoTime();
			int s = 0;
			for (int i = 0; i < N; ++i) {
				s += array[i];
			}
			long rend = System.nanoTime();
			System.out.printf("Array   read took %10s nano seconds%n", rend - rstart);
			
			System.out.println(s);
		}
		
//		Unsafe alloc took      16142 nano seconds
//		Unsafe write took  120145208 nano seconds
//		Unsafe  read took  114300970 nano seconds
//		134217728
//		Array  alloc took   42636826 nano seconds
//		Array  write took   20694570 nano seconds
//		Array   read took   63825587 nano seconds
//		134217728
	}
}