Why is InputStream.close() declared to throw IOException?

Question

The java.io.InputStream.close() method is declared to throw an IOException. Under what circumstances would such an exception actually be thrown?

Edit: Yes I have read the javadoc. Can anybody be more specific than "when an I/O error occurs"? What I/O error can occur while closing an InputStream?

Answer 1

In the case of input stream reading from a file system, an error can be raised while the file system itself updates the last access time metadata, or some other metadata, on a file during closure. Anyway, this happens almost never in practice.

In the case of an input stream reading from a network connection, an error on closure is a bit more conceivable. A normal closure of a network socket actually involves a closure request (TCP/IP FIN packet) being sent over the connection and waiting for the other end to acknowledge this closure request. (In fact, the other end of the connection then in turn sends a closure request, which the closing end acknowledges.) So in the case of a socket input stream, a closure operation actually involves sending traffic over the connection and the closure can thus fail with an error.

Note that in many implementations, close() generally doesn't throw an IOException if the stream is already closed; it simply fails silently to close the stream again.