Listing 1
micah/util/UID.java

package micah.util;

public class UID {

    public static final int SUCCESS = 0;
    public static final int FAILURE = -1;

    public static native int setuid(int uid);
    public static native int seteuid(int uid);
    public static native int setgid(int gid);
    public static native int setegid(int gid);
 
    static {
 System.loadLibrary("uid");
    }
}

Listing 2
micah/util/micah_util_UID.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class micah_util_UID */

#ifndef _Included_micah_util_UID
#define _Included_micah_util_UID
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     micah_util_UID
 * Method:    setuid
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_micah_util_UID_setuid
  (JNIEnv *, jclass, jint);

/*
 * Class:     micah_util_UID
 * Method:    seteuid
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_micah_util_UID_seteuid
  (JNIEnv *, jclass, jint);

/*
 * Class:     micah_util_UID
 * Method:    setgid
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_micah_util_UID_setgid
  (JNIEnv *, jclass, jint);

/*
 * Class:     micah_util_UID
 * Method:    setegid
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_micah_util_UID_setegid
  (JNIEnv *, jclass, jint);

#ifdef __cplusplus
}
#endif
#endif

Listing 3
micah/util/unix_uid.c

#include <jni.h>
#include "micah_util_UID.h"
#include <sys/types.h>
#include <unistd.h>
 
JNIEXPORT jint JNICALL
Java_micah_util_UID_setuid (JNIEnv * jnienv,
jclass j, jint uid)
{
    return((jint)setuid((uid_t)uid));
}

JNIEXPORT jint JNICALL
Java_micah_util_UID_seteuid (JNIEnv * jnienv,
jclass j, jint uid)
{
    return((jint)seteuid((uid_t)uid));
}

JNIEXPORT jint JNICALL
Java_micah_util_UID_setgid (JNIEnv * jnienv,
jclass j, jint gid)
{
    return((jint)setgid((uid_t)gid));
}

JNIEXPORT jint JNICALL
Java_micah_util_UID_setegid (JNIEnv * jnienv,
jclass j, jint gid)
{
    return((jint)setegid((uid_t)gid));
}

Listing 4
micah/util/win_uid.c

#include <jni.h>
#include "micah_util_UID.h"

JNIEXPORT jint JNICALL
Java_micah_util_UID_setuid (JNIEnv * jn, jclass j,
jint uid)
{
 return(0);
}

JNIEXPORT jint JNICALL
Java_micah_util_UID_seteuid (JNIEnv * jn, jclass j,
jint uid)
{
 return(0);
}

JNIEXPORT jint JNICALL
Java_micah_util_UID_setgid (JNIEnv * jn, jclass j,
jint gid)
{
 return(0);
}

JNIEXPORT jint JNICALL
Java_micah_util_UID_setegid (JNIEnv * jn, jclass j,
jint gid)
{
 return(0);
}

Listing 5
testchUID.java

import java.io.*;
import micah.util.*;

class testchUID {
 public static void main(String[] args) {
  try {
   System.in.read();
  }
  catch (IOException ioe) {}
  int result=UID.setuid(1010);
  if (result == UID.SUCCESS) {
      System.out.println("Success!");
  }
  else if (result == UID.FAILURE) {
      System.out.println("Failure!");
  }
  try {
   System.in.read();
  }
  catch (IOException ioe) {}
 }
}