Re: Lock a file or somehow make it unwritable

From:
Lionel van den Berg <lionelv@gmail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Thu, 30 Jul 2009 22:55:44 -0700 (PDT)
Message-ID:
<4e89eee5-69b4-4da0-b912-8d8cf21d4ed2@y10g2000prf.googlegroups.com>
On Jul 31, 11:05 am, Arne Vajh=F8j <a...@vajhoej.dk> wrote:

Lionel van den Berg wrote:
Some experimentation shows that FileOutputStream does
not lock the file exclusively.

But .getChannel().lock() as in your code works (if used
for locking and not for test).

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.nio.channels.FileLock;

public class Lock {
     public static OutputStream uselessLock(File f) throws IOExcept=

ion {

         return new FileOutputStream(f, true);
     }
     public static FileLock workingLock(File f) throws IOException =

{

         return new FileOutputStream(f, true).getChannel().lock=

();

     }
     public static void main(String[] args) throws Exception {
         File f = new File("C:\\z.z");
         f.createNewFile();
         //uselessLock(f);
         workingLock(f);
         OutputStream os = new FileOutputStream(f, true);
         os.write(123);
         os.close();
      }

}

did result in:

Exception in thread "main" java.io.IOException: The process cannot
access the file because another process has locked a portion of the file

which I assume is what you want.


This is interesting. I tried you example, and it works as you said,
however, if you use a PrintStream as follows, there is no exception
and the text is not written:

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.PrintStream;
import java.nio.channels.FileLock;

/**
 *
 */
public class Main {

    public static FileLock workingLock(File f) throws IOException {
         return new FileOutputStream(f, true).getChannel().lock();
     }
     public static void main(String[] args) throws Exception {
         File f = new File("C:\\Test.txt");
         f.createNewFile();
         workingLock(f);
         PrintStream ps = new PrintStream(f);
         ps.print("Test");
         ps.close();
      }
}

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