Base implementation class for interruptible channels.

 This class encapsulates the low-level machinery required to implement
the asynchronous closing and interruption of channels.  A concrete channel
class must invoke the begin and end methods
before and after, respectively, invoking an I/O operation that might block
indefinitely.  In order to ensure that the end method is always
invoked, these methods should be used within a
try ... finally block:



boolean completed = false;
try {
    begin();
    completed = ...;    // Perform blocking I/O operation
    return ...;         // Return result
} finally {
    end(completed);
}

 The completed argument to the end method tells
whether or not the I/O operation actually completed, that is, whether it had
any effect that would be visible to the invoker.  In the case of an
operation that reads bytes, for example, this argument should be
true if, and only if, some bytes were actually transferred into the
invoker's target buffer.

 A concrete channel class must also implement the implCloseChannel method in such a way that if it is
invoked while another thread is blocked in a native I/O operation upon the
channel then that operation will immediately return, either by throwing an
exception or by returning normally.  If a thread is interrupted or the
channel upon which it is blocked is asynchronously closed then the channel's
end method will throw the appropriate exception.

 This class performs the synchronization required to implement the Channel specification.  Implementations of the implCloseChannel method need not synchronize against
other threads that might be attempting to close the channel.

Base implementation class for selectable channels.

 This class defines methods that handle the mechanics of channel
registration, deregistration, and closing.  It maintains the current
blocking mode of this channel as well as its current set of selection keys.
It performs all of the synchronization required to implement the SelectableChannel specification.  Implementations of the
abstract protected methods defined in this class need not synchronize
against other threads that might be engaged in the same operations.

Base implementation class for selection keys.

This class tracks the validity of the key and implements cancellation.

Base implementation class for selectors.

 This class encapsulates the low-level machinery required to implement
the interruption of selection operations.  A concrete selector class must
invoke the begin and end methods before and
after, respectively, invoking an I/O operation that might block
indefinitely.  In order to ensure that the end method is always
invoked, these methods should be used within a
try ... finally block:



try {
    begin();
    // Perform blocking I/O operation here
    ...
} finally {
    end();
}

 This class also defines methods for maintaining a selector's
cancelled-key set and for removing a key from its channel's key set, and
declares the abstract register method that is invoked by a
selectable channel's register
method in order to perform the actual work of registering a channel.

Service-provider class for asynchronous channels.

 An asynchronous channel provider is a concrete subclass of this class that
has a zero-argument constructor and implements the abstract methods specified
below.  A given invocation of the Java virtual machine maintains a single
system-wide default provider instance, which is returned by the provider method.  The first invocation of that method will locate
the default provider as specified below.

 All of the methods in this class are safe for use by multiple concurrent
threads.

Service-provider class for selectors and selectable channels.

 A selector provider is a concrete subclass of this class that has a
zero-argument constructor and implements the abstract methods specified
below.  A given invocation of the Java virtual machine maintains a single
system-wide default provider instance, which is returned by the provider method.  The first invocation of that method will locate
the default provider as specified below.

 The system-wide default provider is used by the static open
methods of the DatagramChannel, Pipe, Selector, ServerSocketChannel, and SocketChannel classes.  It is also
used by the System.inheritedChannel()
method. A program may make use of a provider other than the default provider
by instantiating that provider and then directly invoking the open
methods defined in this class.

 All of the methods in this class are safe for use by multiple concurrent
threads.