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How to use truncate with FileChannel?

Dear Experts,

I want to truncate a certain number of bytes from the end of a file. I'm trying this:

FileChannel srcChannel = new FileInputStream(absoluteFileName).getChannel();
srcChannel.lock();
srcChannel.truncate(srcChannel.size() - someNumberOfBytes);
srcChannel.close();

However, I get this error:

java.nio.channels.NonWritableChannelException
      at sun.nio.ch.FileChannelImpl.lock(FileChannelImpl.java:739)
      at java.nio.channels.FileChannel.lock(FileChannel.java:865)
      at ChopHeaderAndFooter.getLastLineAndCount(ChopHeaderAndFooter.java:60)
      at ChopHeaderAndFooter.doMain(ChopHeaderAndFooter.java:26)
at ChopHeaderAndFooter.main(ChopHeaderAndFooter.java:18)

Any ideas how to make this work? (I'd rather not write the entire file out to another file...)

Thanks!
BrianMc1958

Mick Barry

Use a FileOutputStream instead of FIS

BrianMc1958

ASKER

I'm getting:

java.io.FileNotFoundException: C:\MyFile (Access is denied)

which was the whole point of trying FileChannel in the first place. I've done everything I can to be certain that there on no restrictions of C:\MyFile, but I can't get anything to modify it!

Any more ideas?

Thanks again...
BrianMc1958

ASKER CERTIFIED SOLUTION

Mick Barry

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BrianMc1958

ASKER

It's failing on the first statement below:

FileChannel srcChannel = new FileOutputStream(absoluteFileName).getChannel();
srcChannel.lock();
srcChannel.truncate(srcChannel.size() - lastLine.length());
srcChannel.close();

I'm as sure as I can be that I have access to the file. However, there might be something I don't understand about the security. As far as I can tell, it's an ordinary file with full access granted to everyone.

Mick Barry

van u post a strack trace for the exception
how is absoluteFileName set?

CEHJ

Use a RandomAccessFile to get the channel

Mick Barry

> Use a RandomAccessFile to get the channel

that won't make any difference.

Mick Barry

i suspect there may be a problem with the file name, try getting rid of the c:

CEHJ

>> Use a RandomAccessFile to get the channel

(Opened in rw mode)

Mick Barry

> (Opened in rw mode)

Still won't change anything :)

SOLUTION

Manikandan Thiagarajan

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CEHJ

>>open in rw mode as per ojects told.

You mean as *i* said ...

Manikandan Thiagarajan

sorry,use randam acces file ,open in rw mode as per CEHJ told.

Mick Barry

I said it won't work, it'll get the same error :)

BrianMc1958

ASKER

Sorry, folks. Actually took a weekend off.

I'm afraid objects is correct here. I started with randomAccess, using the "rw" option, and I do get the same error. It works with just the "r", but then fails with "rw". That was why I tried switching over to the Channel technique in the first place. The fact that it does read the file OK with the "r" option tells me it's not a question of finding the file.

Thanks again for all your responses. If anyone else has any ideas, I'd be happy to try them. I should say, though, that I've found a klutzy way around the problem for now, using the Channel's transferFrom method to copy everything but the bytes I want into a second file...

--BrianMc1958

SOLUTION

CEHJ

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Mick Barry

> I'm afraid objects is correct here.

;)

you need access to the file, without that you cannot change it.

CEHJ

>>you need access to the file

Except that wasn't necessarily the error:

>>java.nio.channels.NonWritableChannelException

BrianMc1958

ASKER

Dear Everybody,

I'm afraid I can't pursue this anymore. (I had to put my tail between my legs and take the longer route around the problem.) To anyone reading this, I'm afraid I didn't get an answer this time. But to close the question, I'll split the points.

Thank you all for your help anyway!

--BrianMc1958