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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 id="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 id="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

 *

 * <a id="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> 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>

 *

 *

 * <a id="ksc"></a>

 *

 *

 * @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 {@code true} 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.

     *

     *

     * @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.

     *

     *

     * @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 {@code timeout}

     *                  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

     */

    /**

     * 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;

}