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.