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	/*
	* Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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	*/

	package java.nio.channels;

	import java.io.Closeable;
	import java.io.IOException;
	import java.nio.channels.spi.SelectorProvider;
	import java.util.Set;


	/**
	* A multiplexor of {@link SelectableChannel} objects.
	*
	* <p> A selector may be created by invoking the {@link #open open} method of
	* this class, which will use the system's default {@link
	* java.nio.channels.spi.SelectorProvider selector provider} to
	* create a new selector. A selector may also be created by invoking the
	* {@link java.nio.channels.spi.SelectorProvider#openSelector openSelector}
	* method of a custom selector provider. A selector remains open until it is
	* closed via its {@link #close close} method.
	*
	* <a name="ks"></a>
	*
	* <p> A selectable channel's registration with a selector is represented by a
	* {@link SelectionKey} object. A selector maintains three sets of selection
	* keys:
	*
	* <ul>
	*
	* <li><p> The <i>key set</i> contains the keys representing the current
	* channel registrations of this selector. This set is returned by the
	* {@link #keys() keys} method. </p></li>
	*
	* <li><p> The <i>selected-key set</i> is the set of keys such that each
	* key's channel was detected to be ready for at least one of the operations
	* identified in the key's interest set during a prior selection operation.
	* This set is returned by the {@link #selectedKeys() selectedKeys} method.
	* The selected-key set is always a subset of the key set. </p></li>
	*
	* <li><p> The <i>cancelled-key</i> set is the set of keys that have been
	* cancelled but whose channels have not yet been deregistered. This set is
	* not directly accessible. The cancelled-key set is always a subset of the
	* key set. </p></li>
	*
	* </ul>
	*
	* <p> All three sets are empty in a newly-created selector.
	*
	* <p> A key is added to a selector's key set as a side effect of registering a
	* channel via the channel's {@link SelectableChannel#register(Selector,int)
	* register} method. Cancelled keys are removed from the key set during
	* selection operations. The key set itself is not directly modifiable.
	*
	* <p> A key is added to its selector's cancelled-key set when it is cancelled,
	* whether by closing its channel or by invoking its {@link SelectionKey#cancel
	* cancel} method. Cancelling a key will cause its channel to be deregistered
	* during the next selection operation, at which time the key will removed from
	* all of the selector's key sets.
	*
	* <a name="sks"></a><p> Keys are added to the selected-key set by selection
	* operations. A key may be removed directly from the selected-key set by
	* invoking the set's {@link java.util.Set#remove(java.lang.Object) remove}
	* method or by invoking the {@link java.util.Iterator#remove() remove} method
	* of an {@link java.util.Iterator iterator} obtained from the
	* set. Keys are never removed from the selected-key set in any other way;
	* they are not, in particular, removed as a side effect of selection
	* operations. Keys may not be added directly to the selected-key set. </p>
	*
	*
	* <a name="selop"></a>
	* <h2>Selection</h2>
	*
	* <p> During each selection operation, keys may be added to and removed from a
	* selector's selected-key set and may be removed from its key and
	* cancelled-key sets. Selection is performed by the {@link #select()}, {@link
	* #select(long)}, and {@link #selectNow()} methods, and involves three steps:
	* </p>
	*
	* <ol>
	*
	* <li><p> Each key in the cancelled-key set is removed from each key set of
	* which it is a member, and its channel is deregistered. This step leaves
	* the cancelled-key set empty. </p></li>
	*
	* <li><p> The underlying operating system is queried for an update as to the
	* readiness of each remaining channel to perform any of the operations
	* identified by its key's interest set as of the moment that the selection
	* operation began. For a channel that is ready for at least one such
	* operation, one of the following two actions is performed: </p>
	*
	* <ol>
	*
	* <li><p> If the channel's key is not already in the selected-key set then
	* it is added to that set and its ready-operation set is modified to
	* identify exactly those operations for which the channel is now reported
	* to be ready. Any readiness information previously recorded in the ready
	* set is discarded. </p></li>
	*
	* <li><p> Otherwise the channel's key is already in the selected-key set,
	* so its ready-operation set is modified to identify any new operations
	* for which the channel is reported to be ready. Any readiness
	* information previously recorded in the ready set is preserved; in other
	* words, the ready set returned by the underlying system is
	* bitwise-disjoined into the key's current ready set. </p></li>
	*
	* </ol>
	*
	* If all of the keys in the key set at the start of this step have empty
	* interest sets then neither the selected-key set nor any of the keys'
	* ready-operation sets will be updated.
	*
	* <li><p> If any keys were added to the cancelled-key set while step (2) was
	* in progress then they are processed as in step (1). </p></li>
	*
	* </ol>
	*
	* <p> Whether or not a selection operation blocks to wait for one or more
	* channels to become ready, and if so for how long, is the only essential
	* difference between the three selection methods. </p>
	*
	*
	* <h2>Concurrency</h2>
	*
	* <p> Selectors are themselves safe for use by multiple concurrent threads;
	* their key sets, however, are not.
	*
	* <p> The selection operations synchronize on the selector itself, on the key
	* set, and on the selected-key set, in that order. They also synchronize on
	* the cancelled-key set during steps (1) and (3) above.
	*
	* <p> Changes made to the interest sets of a selector's keys while a
	* selection operation is in progress have no effect upon that operation; they
	* will be seen by the next selection operation.
	*
	* <p> Keys may be cancelled and channels may be closed at any time. Hence the
	* presence of a key in one or more of a selector's key sets does not imply
	* that the key is valid or that its channel is open. Application code should
	* be careful to synchronize and check these conditions as necessary if there
	* is any possibility that another thread will cancel a key or close a channel.
	*
	* <p> A thread blocked in one of the {@link #select()} or {@link
	* #select(long)} methods may be interrupted by some other thread in one of
	* three ways:
	*
	* <ul>
	*
	* <li><p> By invoking the selector's {@link #wakeup wakeup} method,
	* </p></li>
	*
	* <li><p> By invoking the selector's {@link #close close} method, or
	* </p></li>
	*
	* <li><p> By invoking the blocked thread's {@link
	* java.lang.Thread#interrupt() interrupt} method, in which case its
	* interrupt status will be set and the selector's {@link #wakeup wakeup}
	* method will be invoked. </p></li>
	*
	* </ul>
	*
	* <p> The {@link #close close} method synchronizes on the selector and all
	* three key sets in the same order as in a selection operation.
	*
	* <a name="ksc"></a>
	*
	* <p> A selector's key and selected-key sets are not, in general, safe for use
	* by multiple concurrent threads. If such a thread might modify one of these
	* sets directly then access should be controlled by synchronizing on the set
	* itself. The iterators returned by these sets' {@link
	* java.util.Set#iterator() iterator} methods are <i>fail-fast:</i> If the set
	* is modified after the iterator is created, in any way except by invoking the
	* iterator's own {@link java.util.Iterator#remove() remove} method, then a
	* {@link java.util.ConcurrentModificationException} will be thrown. </p>
	*
	*
	* @author Mark Reinhold
	* @author JSR-51 Expert Group
	* @since 1.4
	*
	* @see SelectableChannel
	* @see SelectionKey
	*/

	public abstract class Selector implements Closeable {

	/**
	* Initializes a new instance of this class.
	*/
	protected Selector() { }

	/**
	* Opens a selector.
	*
	* <p> The new selector is created by invoking the {@link
	* java.nio.channels.spi.SelectorProvider#openSelector openSelector} method
	* of the system-wide default {@link
	* java.nio.channels.spi.SelectorProvider} object. </p>
	*
	* @return A new selector
	*
	* @throws IOException
	* If an I/O error occurs
	*/
	public static Selector open() throws IOException {
	return SelectorProvider.provider().openSelector();
	}

	/**
	* Tells whether or not this selector is open.
	*
	* @return <tt>true</tt> if, and only if, this selector is open
	*/
	public abstract boolean isOpen();

	/**
	* Returns the provider that created this channel.
	*
	* @return The provider that created this channel
	*/
	public abstract SelectorProvider provider();

	/**
	* Returns this selector's key set.
	*
	* <p> The key set is not directly modifiable. A key is removed only after
	* it has been cancelled and its channel has been deregistered. Any
	* attempt to modify the key set will cause an {@link
	* UnsupportedOperationException} to be thrown.
	*
	* <p> The key set is <a href="#ksc">not thread-safe</a>. </p>
	*
	* @return This selector's key set
	*
	* @throws ClosedSelectorException
	* If this selector is closed
	*/
	public abstract Set<SelectionKey> keys();

	/**
	* Returns this selector's selected-key set.
	*
	* <p> Keys may be removed from, but not directly added to, the
	* selected-key set. Any attempt to add an object to the key set will
	* cause an {@link UnsupportedOperationException} to be thrown.
	*
	* <p> The selected-key set is <a href="#ksc">not thread-safe</a>. </p>
	*
	* @return This selector's selected-key set
	*
	* @throws ClosedSelectorException
	* If this selector is closed
	*/
	public abstract Set<SelectionKey> selectedKeys();

	/**
	* Selects a set of keys whose corresponding channels are ready for I/O
	* operations.
	*
	* <p> This method performs a non-blocking <a href="#selop">selection
	* operation</a>. If no channels have become selectable since the previous
	* selection operation then this method immediately returns zero.
	*
	* <p> Invoking this method clears the effect of any previous invocations
	* of the {@link #wakeup wakeup} method. </p>
	*
	* @return The number of keys, possibly zero, whose ready-operation sets
	* were updated by the selection operation
	*
	* @throws IOException
	* If an I/O error occurs
	*
	* @throws ClosedSelectorException
	* If this selector is closed
	*/
	public abstract int selectNow() throws IOException;

	/**
	* Selects a set of keys whose corresponding channels are ready for I/O
	* operations.
	*
	* <p> This method performs a blocking <a href="#selop">selection
	* operation</a>. It returns only after at least one channel is selected,
	* this selector's {@link #wakeup wakeup} method is invoked, the current
	* thread is interrupted, or the given timeout period expires, whichever
	* comes first.
	*
	* <p> This method does not offer real-time guarantees: It schedules the
	* timeout as if by invoking the {@link Object#wait(long)} method. </p>
	*
	* @param timeout If positive, block for up to <tt>timeout</tt>
	* milliseconds, more or less, while waiting for a
	* channel to become ready; if zero, block indefinitely;
	* must not be negative
	*
	* @return The number of keys, possibly zero,
	* whose ready-operation sets were updated
	*
	* @throws IOException
	* If an I/O error occurs
	*
	* @throws ClosedSelectorException
	* If this selector is closed
	*
	* @throws IllegalArgumentException
	* If the value of the timeout argument is negative
	*/
	public abstract int select(long timeout)
	throws IOException;

	/**
	* Selects a set of keys whose corresponding channels are ready for I/O
	* operations.
	*
	* <p> This method performs a blocking <a href="#selop">selection
	* operation</a>. It returns only after at least one channel is selected,
	* this selector's {@link #wakeup wakeup} method is invoked, or the current
	* thread is interrupted, whichever comes first. </p>
	*
	* @return The number of keys, possibly zero,
	* whose ready-operation sets were updated
	*
	* @throws IOException
	* If an I/O error occurs
	*
	* @throws ClosedSelectorException
	* If this selector is closed
	*/
	public abstract int select() throws IOException;

	/**
	* Causes the first selection operation that has not yet returned to return
	* immediately.
	*
	* <p> If another thread is currently blocked in an invocation of the
	* {@link #select()} or {@link #select(long)} methods then that invocation
	* will return immediately. If no selection operation is currently in
	* progress then the next invocation of one of these methods will return
	* immediately unless the {@link #selectNow()} method is invoked in the
	* meantime. In any case the value returned by that invocation may be
	* non-zero. Subsequent invocations of the {@link #select()} or {@link
	* #select(long)} methods will block as usual unless this method is invoked
	* again in the meantime.
	*
	* <p> Invoking this method more than once between two successive selection
	* operations has the same effect as invoking it just once. </p>
	*
	* @return This selector
	*/
	public abstract Selector wakeup();

	/**
	* Closes this selector.
	*
	* <p> If a thread is currently blocked in one of this selector's selection
	* methods then it is interrupted as if by invoking the selector's {@link
	* #wakeup wakeup} method.
	*
	* <p> Any uncancelled keys still associated with this selector are
	* invalidated, their channels are deregistered, and any other resources
	* associated with this selector are released.
	*
	* <p> If this selector is already closed then invoking this method has no
	* effect.
	*
	* <p> After a selector is closed, any further attempt to use it, except by
	* invoking this method or the {@link #wakeup wakeup} method, will cause a
	* {@link ClosedSelectorException} to be thrown. </p>
	*
	* @throws IOException
	* If an I/O error occurs
	*/
	public abstract void close() throws IOException;

	}

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