What is the easiest way to call a Windows kernel function from Java?

Question

While searching for how to do this, I found some vague discussion about different options, like JNI vs JNA, but not much in the way of concrete examples.

Context: if Java's File.renameTo() cannot do it's job (for whatever reason; it is a little problematic), I'd like to fall back to directly using this native Windows function, which is defined in kernel32.dll (from this answer):

BOOL WINAPI MoveFile(
  __in  LPCTSTR lpExistingFileName,
  __in  LPCTSTR lpNewFileName
);

So, using whatever approach, how exactly would you call that function from within Java code? I'm looking for the simplest way, with the minimum amount of non-Java code or extra steps (e.g. in compilation or deployment).

Answer 1

If you go with JNA, consider invoking MoveFileW directly - it saves having to provide configuration info to choose between Unicode and ANSI calls.

import java.io.*;
import com.sun.jna.*;

public class Ren {

  static interface Kernel32 extends Library {
    public static Kernel32 INSTANCE = (Kernel32) Native
        .loadLibrary("Kernel32", Kernel32.class);

    public static int FORMAT_MESSAGE_FROM_SYSTEM = 4096;
    public static int FORMAT_MESSAGE_IGNORE_INSERTS = 512;

    public boolean MoveFileW(WString lpExistingFileName,
        WString lpNewFileName);

    public int GetLastError();

    public int FormatMessageW(int dwFlags,
        Pointer lpSource, int dwMessageId,
        int dwLanguageId, char[] lpBuffer, int nSize,
        Pointer Arguments);
  }

  public static String getLastError() {
    int dwMessageId = Kernel32.INSTANCE.GetLastError();
    char[] lpBuffer = new char[1024];
    int lenW = Kernel32.INSTANCE.FormatMessageW(
        Kernel32.FORMAT_MESSAGE_FROM_SYSTEM
            | Kernel32.FORMAT_MESSAGE_IGNORE_INSERTS, null,
        dwMessageId, 0, lpBuffer, lpBuffer.length, null);
    return new String(lpBuffer, 0, lenW);
  }

  public static void main(String[] args) throws IOException {
    String from = ".\\from.txt";
    String to = ".\\to.txt";
    new FileOutputStream(from).close();
    if (!Kernel32.INSTANCE.MoveFileW(new WString(from),
        new WString(to))) {
      throw new IOException(getLastError());
    }
  }
}

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EDIT: I've edited my answer after checking the code - I was mistaken about using char[] in the signature - it is better to use WString.

Answer 2

If this is really necessary (renameTo doesn't work and you're sure MoveFile will), I would use JNA. It looks like most of the work is already done in com.mucommander.file.util.Kernel32.java/Kernel32API.java.

Answer 3

Based on the NativeCall library I did the following POC Application.

It uses the MoveFileA function from kernel32.dll

It comes as complete working sample with a run.bat and all jar and dlls in place.

It moves the included test.txt to test2.txt

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If you don't like the NativeCall library version I did another POC Application based on/resuing on the Java Native Access (JNA) library. This time MoveFileA and MoveFileW are implemented and demonstrated.