A good concise guide to all the jdk7 enhancements can be found here. Some interesting ones are –
Scalable file I/O support
Many of the java.io.file methods did not scale. Requesting a large directory listing over a server could result in a hang. Large directories could also cause memory resource problems, resulting in a denial of service. The new file I/O package is implemented so that it works regardless of the file system’s size.
Watching a directory for changes
The Watch Service api allows you to register a directory for file change notifications. An application can register for creation, deletion, or modification events. This feature can be implemented using the FileVisitor API to recursively watch an entire file tree for changes. For more information, see Watching a Directory for Changes (Java Tutorial).