Copy that

Copy.java is one of several new samples included in JDK 7 to demonstrate its new API to the file system. If you have any recent snapshot installed then you'll find these samples in the $JAVA_HOME/sample/nio/file directory. Copy.java works like the Unix cp program to copy a file or file tree to a target location. It supports the -r, -p and -i options to do a recursive copy, preserve attributes, or work interactively to prompt whenever an existing file would be overwritten. The sample code is relatively simple and demonstrates many aspects of the API that are worth looking at.

At its heart, the copyTo method is used to do the copy. The Copy sample uses the COPY_ATTRIBUTES option when invoked with -p to preserve attributes. This is useful when you want the file attributes/meta-data copied to the target file. This means the file timestamps, permissions, Access Control List, extended attributes, etc. The other option used in the sample code is REPLACE_EXISTING; this is used to replace the target file if it exists. One thing to point out is that the copyTo method doesn't copy the contents of a directory. If you invoke it on a directory then it creates an empty directory. On the surface this might appearing limiting but work through the sample and it should becomes clear.

Another interesting thing to point out is that you'll see code like:


    target.resolve(source.relativize(file))

The relativize method used to compute a relative path between source and file. This is then resolved against target. For example, suppose we are doing a recusive copy from /users/gus to /users/joe/backup_of_gus. As we walk the tree we will need to copy /users/gus/stamps/rare/black_penny.html. In this sample source.relativize(file) yields the relative path stamps/rare/black_penny.html. Resolving this against the target directory (/users/joe/backup_of_gus) yields us the path in the target tree: /users/joe/backup_of_gus/stamps/rare/black_penny.html.

The other important API used in the sample is the Files.walkFileTree method. This method is used to implement recursive file operations, starting at a given starting point. The method is passed a FileVisitor that is invoked for each file or directory in the file tree. The Copy sample has an inner class TreeCopier that implements FileVisitor . TreeCopier's preVisitDirectory method is invoked for each directory before the entries in the directory are visited. It simply invokes the copyTo method to copy the directory (creating an empty directory as I mentioned above). When the copy succeeds or the directory already exists the preVisitDirectory returns CONTINUE to instruct the iterator to continue and visit the files in the directory. If the copy fails, then preVisitDirectory returns SKIP_SUBTREE to tell the iterator to skip the directory. You'll see that postVisitDirectory is also implemented. This is invoked after all of the entries in the directory have been visited. In this code it is implemented so that the directory's last-modified-time can be fixed up after we are done with the directory. Most recursive operations will only need to implement one of preVisitDirectory or postVisitDirectory. The visitFile method is very simple and just copies the file to the corresponding file in the target tree. One interesting thing to point out is that visitFile is invoked if a cycle is detected (a cycle being a symbolic link to a parent directory). Copying is one of the few cases where you want to follow symbolic links and so you need to be concerned with the possibility of a cycle.

Hopefully this sample will be useful when learning the new API. The $JAVA_HOME/sample/nio/file directory has a number of other examples that demonstrate other parts of the API.

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