The Telio Blog

Getting the file owner in Java using JNI

Since i needed this for a project, I thought it was worth a post.

Basically I needed to watch a directory for incoming files and perform a given action based on the owner of the file, in Java.

For the polling part, no problem, but how can I get the owner of a file in Java ? Well after 1 hour searching the internet and concluding that :

• It is not possible in pure Java at least up to 1.5
• It should be done using native code through JNI

I decided to give it a shot since I had another hour in front of me. Here a the steps I followed :

1. Try to find some code that does this in C++ : http://msdn2.microsoft.com/en-us/library/aa446629.aspx

2. Try to compile this code Since I have no Visual Studio or other C++ IDE, let’s go find one .. :

http://www.bloodshed.net/devcpp.html

3. Install Dev++ and compile the code .. and to my great surprise, it works (I must say I have bad college memories about C++, compilers, make files, compiler directives, etc…)

4. Now let’s tackle the JNI part .. since I never used JNI before (but know what it’s about), I type ‘jni tutorial’ in google .. and find :

http://www.codetoad.com/java_simpleJNI.asp

I chose this page because it seemed to have an example of passing a String parameter and getting back a String result – which is exactly what I need since I will pass a file path and get back the file owner.

5. Create a java class

import java.io.*;
public class getowner
{
	private static native String getowner(String arg);
        public static void main(String[] args)
	{
	 	System.load("c:/temp/java/getowner.dll");
		File f = new File(args[0]);
		File[] children = f.listFiles();
		for (int i=0; i < children.length;i++)
		{
			if (children[i].isFile())
				System.out.println(getowner(children[i].getPath()));
	 	}
	}
}

6. Compile the class and generate the header file using javah

javac getowner.java

javah getowner

Which generates the following header file :

/* DO NOT EDIT THIS FILE - it is machine generated */
#include 
/* Header for class getowner */

#ifndef _Included_getowner
#define _Included_getowner
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     getowner
 * Method:    getowner
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_getowner_getowner
  (JNIEnv *, jclass, jstring);

#ifdef __cplusplus
}
#endif
#endif

7. Write the implementation by modifying the code used in step 2

(see attachment for the complete source code)

8. Compile the whole thing, don’t forget in the project settings to add 2 include directories pointing to the JDK include and include/win32 folder

If you’re lucky you should get a nice getowner.dll file

9. You can now run the java program :

java getowner c:windows

which will output the owner of all the file in you windows directory.

You can download the source code zip getowner.zip that contains :

• the project file
• the cpp source file
• the generated header file
• the java class

Have fun !

P.