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Java Forum / General / March 2007Accessing file while it is being copied
My app processes files as they get dropped into a directory. There's a loop that keeps checking if a file is available, and if it is, then the file gets read. The trouble is that if the app detects a new file before it has been fully copied from another location, and it starts processing, then I get errors. Somehow, I need to be able to detect when the copy process has completed. I tried to test for an exclusive file lock using NIO, but it did not work. In order to lock a file, you have to open it first, and I get a FileNotFoundException when I create a FileInputStream on the file. java.io.FileNotFoundException: C:\mydir\product.xml (The process cannot access the file because it is being used by another process) at java.io.FileInputStream.open(Native Method) at java.io.FileInputStream.<init>(FileInputStream.java:106) How can I know when the copy process is complete. (Also, this has to work on both Windows and FreeBSD). Possible solutions: A) make a second directory for links to files D) create a sentinel file for every file copied in; that is say file getting copied in is named "lots_o_data.text" have the process performing copy put in another file named something like "lots_o_data.text.finished". opalpa opalpa@gmail.com http://opalpa.info/ opalpa opalpa@gmail.com http://opalpa.info wrote On 03/05/07 16:13,: > Possible solutions: > A) make a second directory for links to files > D) create a sentinel file for every file copied in; that is say file > getting copied in is named "lots_o_data.text" have the process > performing copy put in another file named something like > "lots_o_data.text.finished". G) Copy the file using the name "lots_o_data.temporary" and after copying rename it to "lots_o_data.text". Teach your program to ignore files named "....temporary".  SignatureEric.Sosman@sun.com > opalpa opalpa@gmail.com http://opalpa.info wrote On 03/05/07 16:13,: > > Possible solutions: [quoted text clipped - 7 lines] > and after copying rename it to "lots_o_data.text". Teach > your program to ignore files named "....temporary". J) Use a self-delimiting format for the file contents (checksum, end-of-file marker, anything which comes "for free" with the internal file format (such as well-formdedness), MIME-style encapsulation of the "real" contents...) -- chris > My app processes files as they get dropped into a directory. There's a > loop that keeps checking if a file is available, and if it is, then the [quoted text clipped - 16 lines] > How can I know when the copy process is complete. (Also, this has to > work on both Windows and FreeBSD). It stops throwing the exception. What's the problem here? If it throws this exception just wait for a few seconds and try again. SignatureKnute Johnson email s/nospam/knute/ >> My app processes files as they get dropped into a directory. There's a >> loop that keeps checking if a file is available, and if it is, then [quoted text clipped - 19 lines] > It stops throwing the exception. What's the problem here? If it throws > this exception just wait for a few seconds and try again. Because it's almost always a bad idea to have exceptions thrown in the normal course of execution. Exceptions are just that -- unusual cases. Besides, there's no guarantee that this method will work in a cross-platform way. Maybe a particular operation system doesn't throw the exception. Then my app would start processing a partially-copied file, leading to all kinds of problems. I believe BSD, being a unix, would not throw the exception. As for throwing exceptions, I've personally changed my mind on throwing exceptions in course of execution. I used to treat exception as a synonym for error. Currently I occasionally treat exceptions as a control flow mechanism. opalpa opalpa@gmail.com http://opalpa.info/ > I believe BSD, being a unix, would not throw the exception. The possible absence of this exception is another reason to keep the operation of coping the files and the operation of processing the files seperate in sequence. If the format of the file permits a subsequence to be a valid file then reading a file too early could chop of information. --- I'm suprised to learn FileNotFoundException covers inaccseibility to file. I would have guessed that IOException gets thrown. opalpa opalpa@gmail.com http://opalpa.info/ >>> My app processes files as they get dropped into a directory. There's >>> a loop that keeps checking if a file is available, and if it is, then [quoted text clipped - 22 lines] > Because it's almost always a bad idea to have exceptions thrown in the > normal course of execution. Exceptions are just that -- unusual cases. I think an incomplete file would be an unusual situation, it's certainly not what you want to find. > Besides, there's no guarantee that this method will work in a > cross-platform way. Maybe a particular operation system doesn't throw > the exception. Then my app would start processing a partially-copied > file, leading to all kinds of problems. I think you're toast. The only other idea I have is to check periodically to see if the file size stops changing. But I'm not sure that is good either because I have seen file systems that set the length of the file before completing the write. You might try reading it again and again until it's checksum becomes stable. You might just try using both the exception and the lock. See if that works on most OS. SignatureKnute Johnson email s/nospam/knute/ Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. 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