What throws an IOException in Java?

java.io.IOException seems to be the most common type of exception, and coincidentally, it seems to also be the most ambiguous.

I keep seeing the throws IOException whenever writing with sockets, files, etc. I've never actually had one fired on me, however, so I'm wondering what it is that is supposed to fire the exception. The documentation isn't very helpful in explaining what's going on:

> Signals that an I/O exception of some sort has occurred. This class is the general class of exceptions produced by failed or interrupted I/O operations.

What are some instances where an IOException would be thrown, and how is it supposed to be used?

Assume you were:

1. Reading a network file and got disconnected.
2. Reading a local file that was no longer available.
3. Using some stream to read data and some other process closed the stream.
4. Trying to read/write a file, but don't have permission.
5. Trying to write to a file, but disk space was no longer available.

There are many more examples, but these are the most common, in my experience.

In general, I/O means Input or Output. Those methods throw the IOException whenever an input or output operation is failed or interpreted. Note that this won't be thrown for reading or writing to memory as Java will be handling it automatically.

Here are some cases which result in IOException.

• Reading from a closed inputstream
• Try to access a file on the Internet without a network connection

Java documentation is helpful to know the root cause of a particular IOException.

Just have a look at the direct known sub-interfaces of IOException from the documentation page:

ChangedCharSetException, CharacterCodingException, CharConversionException, ClosedChannelException, EOFException, FileLockInterruptionException, FileNotFoundException, FilerException, FileSystemException, HttpRetryException, IIOException, InterruptedByTimeoutException, InterruptedIOException, InvalidPropertiesFormatException, JMXProviderException, JMXServerErrorException, MalformedURLException, ObjectStreamException, ProtocolException, RemoteException, SaslException, SocketException, SSLException, SyncFailedException, UnknownHostException, UnknownServiceException, UnsupportedDataTypeException, UnsupportedEncodingException, UserPrincipalNotFoundException, UTFDataFormatException, ZipException

Most of these exceptions are self-explanatory.

A few IOExceptions with root causes:

EOFException: Signals that an end of file or end of stream has been reached unexpectedly during input. This exception is mainly used by data input streams to signal the end of the stream.

SocketException: Thrown to indicate that there is an error creating or accessing a Socket.

RemoteException: A RemoteException is the common superclass for a number of communication-related exceptions that may occur during the execution of a remote method call. Each method of a remote interface, an interface that extends java.rmi.Remote, must list RemoteException in its throws clause.

UnknownHostException: Thrown to indicate that the IP address of a host could not be determined (you may not be connected to Internet).

MalformedURLException: Thrown to indicate that a malformed URL has occurred. Either no legal protocol could be found in a specification string or the string could not be parsed.