The I/O Package - The Java Programming Language - page 761

it is interrupted is problematic. For example, suppose you use a particu-
lar stream to read HTTP requests across the network. If a thread reading
the next request is interrupted after reading two bytes of the header
field in the request packet, the next thread reading from that stream
will get invalid data unless the stream takes steps to prevent this. Given
the effort involved in writing classes that can deal effectively with these
sorts of situations, most implementations do not allow a thread to be in-
terrupted until the main I/O operation has completed, so you cannot rely
on blocking I/O being interruptible. The interruptible channels provided
in the java.nio package support interruption by closing the stream when
any thread using the stream is interruptedthis ensures that there are no
issues about what would next be read.
Even when interruption cannot be responded to during an I/O operation
many systems will check for interruption at the start and/or end of the
operation and throw the InterruptedIOException then. Also, if a thread is
blocked on a stream when the stream is closed by another thread, most
implementations will unblock the blocked thread and throw an IOExcep-
tion .
20.5.2. Filter Streams
Filter streams FilterInputStream , FilterOutputStream , FilterReader , and
FilterWriter help you chain streams to produce composite streams of
greater utility. Each filter stream is bound to another stream to which
it delegates the actual input or output actions. Filter streams get their
power from the ability to filterprocesswhat they read or write, trans-
forming the data in some way.
Filter byte streams add new constructors that accept a stream of the
appropriate type (input or output) to which to connect. Filter character
streams similarly add a new constructor that accepts a character stream
of the appropriate type (reader or writer). However, many character
streams already have constructors that take another character stream,
so those Reader and Writer classes can act as filters even if they do not
extend FilterReader or FilterWriter .