Java/Security/File Read Write

Authenticate and login context

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.security.PrivilegedAction;
import javax.security.auth.Subject;
import javax.security.auth.login.LoginContext;
import javax.security.auth.login.LoginException;
public class AuthenticateNT {
  public static void main(String[] args) {
    try {
      LoginContext loginContext = new LoginContext("AuthenticateNT");
      loginContext.login();
      System.out.println("Login Successful");
      Subject subject = loginContext.getSubject();
      System.out.println(subject);
      Subject.doAs(subject, new WriteFileAction());
      loginContext.logout();
      System.exit(0);
    } catch (LoginException loginException) {
      loginException.printStackTrace();
      System.exit(-1);
    }
  }
}

class WriteFileAction implements PrivilegedAction {
  public Object run() 
  {
     try {
        File file = new File( "D:/", "privilegedFile.txt" );
        FileWriter fileWriter = new FileWriter( file );
        
        fileWriter.write( "Welcome to JAAS!" );
        fileWriter.close();
     }    
     catch ( IOException ioException ) {
        ioException.printStackTrace();
     }
     
     return null;
     
  } 
}
/*
// java.policy
// Policy file that grants AllPermission to JAAS modules and specific permissions to the D:\Projects\Java codebase.
grant codebase "file:/D:/jdk1.3.1/jre/lib/ext/jaas.jar" {
   permission java.security.AllPermission;
}; 
grant codebase "file:/D:/JavaProjects/src/-" {
   permission javax.security.auth.AuthPermission 
      "createLoginContext";
   permission javax.security.auth.AuthPermission "doAs";
   permission java.io.FilePermission "D:/privilegedFile.txt", 
      "write";
   permission java.io.FilePermission "D:/privilegedFile.txt", 
      "read";
};
*/
/*
// jaas.policy
// Policy file defining the permissions for the named Principal
grant codeBase "file:D:/", 
   Principal com.sun.security.auth.NTUserPrincipal "santry" {
   permission java.io.FilePermission "D:/privilegedFile.txt", 
      "write";
   permission java.io.FilePermission "D:/privilegedFile.txt", 
      "read";
};
*/
/*
// jaas.config 
// Configures JAAS to use NTLoginModule
// for authentication.
AuthenticateNT {
   com.sun.security.auth.module.NTLoginModule required debug=false;
};
*/

Checking permissions

 
import java.io.File;
public class Permissions {
  public static void main(String[] args) {
    if (args.length != 1) {
      System.err.println("usage: java Permissions filespec");
      return;
    }
    File file = new File(args[0]);
    System.out.println("Checking permissions for " + args[0]);
    System.out.println("  Execute = " + file.canExecute());
    System.out.println("  Read = " + file.canRead());
    System.out.println("  Write = " + file.canWrite());
  }
}

Writable, read only file

 
import java.io.File;
public class ROW {
  public static void main(String[] args) {
    File filespec = new File("c:\text.txt");
    if (filespec.setWritable(false))
      System.out.println(filespec + " made read-only");
    else
      System.out.println("Permission denied");
    if (filespec.setWritable(true))
      System.out.println(filespec + " made writable");
    else
      System.out.println("Permission denied");
    System.out.println(filespec + " is currently "
        + (filespec.canWrite() ? "writable" : "read-only"));
  }
}