为什么声明InputStream.close（）会抛出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.

Answer 2

I'm looking through the Java source code, and have found something interesting which is indicative of the IOException reason. InputStream is an abstract class . We therefore can't predict the kind of input which will be closed, so it's good to keep information flowing.

Whatever code uses that input stream needs to be able to throw an IOException , because there is a chance that closing the input stream may fail. If it fails, whatever's using the implementation needs to know about it, because there's a good chance it needs to be handled.

It's important to understand the layout of the Exception structure in Java. Every exception, of course, extends Exception . However, there are also broader categories of exceptions: java.lang.IOException is one of these, and covers all possible input/output exceptions. When we say there has been an I/O error, we're referencing anything which falls under IOException . As a result, many exceptions extends this one, eg FileNotFoundException , EOFException , etc. as it's important to have a broad, overarching exception to manage these.

As a result, any IO class will need to be able to throw any of the various IOExceptions on close. close() therefore must throw IOException - this gives its implementation the ability to throw any of the extensions of IOException as well. This is why close() throws IOException - exceptions are inherited, and you need to be able to any of the IOExceptions when closing a stream.

---

Here are a couple scenarios worth noting:

• You can't close an IOStream twice, though this generally doesn't throw an exception if you do
• The content becomes inaccessible (eg a disk was unmounted) (The close() is actually critical for the operating system, as it needs to have an indicator of when the file is no longer busy)
• The generic source has closed
• Any generic failure not covered by all other subclasses of IOException (eg FileNotFoundException )

You can check what caused an IOException by running Exception.getMessage() .

Answer 3

The underlying close system call will have to be made eventually, eg on linux http://linux.die.net/man/2/close . That call is documented to fail with EIO : "An I/O error occurred." So the reason is, that the underlying file system close call can fail.

Answer 4

I have wondered about this myself and have done a little research on this topic few years ago. This is what I know....

If you look at the javadoc in the link you provided, it clearly states that "The close method of InputStream does nothing", so there is no way it would throw an exception, correct? But then looking at all of the subclasses of IOException you will see that there are many situations where subclasses of inputstream may not be able to close the stream. So my best guess is that this exception is designed for subclasses to make use of it.

http://docs.oracle.com/javase/6/docs/api/java/io/IOException.html

In some cases, it is nothing more than a nuisance, and in others it clearly indicates that something went wrong. It all depends on what type of inputstream implementation you are making use of.