How to use JNI to call MEMCMP from Java

Question

Hi,

I need to compare 2 byte arrays and know which one is bigger or if they are equal (just equal or different is not enough). The byte arrays represent a String value of 15 characters or more. This comparison is repeated considerably in my code.

I would like to improve the bye array compare by using an equivalent of C++ memcmp method in Java (hopefully by JNI). I found an example to use DLLImport in C#, so I hope a JNI call can be applied as well.

Here is the C# code segment:

[DllImport("msvcrt.dll")]
    unsafe static extern int memcmp(void* b1, void* b2, long count);

    unsafe static int ByteArrayCompare1(byte[] b1, int b1Index, int b1Length, byte[] b2, int b2Index, int b2Length)
    {
        CompareCount++;
        fixed (byte* p1 = b1)
        fixed (byte* p2 = b2)
        {
            int cmp = memcmp(p1 + b1Index, p2 + b2Index, Math.Min(b1Length, b2Length));
            if (cmp == 0)
            {
                cmp = b1Length.CompareTo(b2Length);
            }

            return cmp;
        }
    }

Does anyone know how to implement this in Java?

Thanks in advance,

Diana

Answer 1

You can use JNI, but Java has a variation on JNI called JNA (Java Native Access), which lets you access shared libraries directly without needing a JNI interface wrapped around them, so you can use that to access memcmp directly:

import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Pointer;

public class Test {
    public interface CStdLib extends Library {
        int memcmp(Pointer s1, Pointer s2, int n);
    }

    public static void main(String[] args) {
        CStdLib c = (CStdLib)Native.loadLibrary("msvcrt", CStdLib.class);
        c.memcmp(...);
    }
}

I've not tested the above, and I'm not sure in particular about the memcmp signature, since it takes void* and size_t, which both don't have obvious Java equivalents, but some variation of that should work correctly

_{(Attribution: I pulled some of the JNA info from another answer of mine)}

Answer 2

Are you certain your code is spending significant time on those comparisons? I would suggest calling a Java function for now, and then timing it; if you still need to, you can add JNI/JNA.

Remember that by adding JNI you increase your chance of bugs significantly, and you limit your program to only the architectures that you compile the library for.

Answer 3

Just take the following code and see if it is fast enough.

package so3883485;

import java.util.concurrent.atomic.AtomicLong;

public class ByteArrayUtils {

  static final AtomicLong COMPARE_COUNT = new AtomicLong(0);

  public static int compare(byte[] b1, int b1Index, int b1Length, byte[] b2, int b2Index, int b2Length) {
    COMPARE_COUNT.incrementAndGet();

    final int commonLength = Math.min(b1Length, b2Length);
    for (int i = 0; i < commonLength; i++) {
      final byte byte1 = b1[b1Index + i];
      final byte byte2 = b2[b2Index + i];
      if (byte1 != byte2) {
        return (byte1 < byte2) ? -1 : 1;
      }
    }

    if (b1Length != b2Length) {
      return (b1Length < b2Length) ? -2 : 2;
    }

    return 0;
  }

}

And some unit test to make sure the basic cases work as expected.

package so3883485;

import static org.junit.Assert.*;
import static so3883485.ByteArrayUtils.*;

import org.junit.Test;

public class ByteArrayUtilsTest {

  @Test
  public void test() {
    byte[] bytes = { 1, 2, 3, 4, 5 };
    assertEquals(0, compare(bytes, 0, bytes.length, bytes, 0, bytes.length));
    assertEquals(0, compare(bytes, 0, 0, bytes, 0, 0));
    assertEquals(-2, compare(bytes, 0, 0, bytes, 0, 1));
    assertEquals(2, compare(bytes, 0, 1, bytes, 0, 0));
    assertEquals(-1, compare(bytes, 1, 1, bytes, 2, 1));
    assertEquals(1, compare(bytes, 2, 1, bytes, 1, 1));
  }
}