Selector.java

/*
 * Copyright (c) 2000, 2018, 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.Objects;
import java.util.Set;
import java.util.function.Consumer;

/**
 * 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
 * that adds keys or updates keys in the set.
 * 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
 * of an {@link java.util.Iterator iterator} obtained from the set.
 * All keys may be removed from the selected-key set by invoking the set's
 * {@link java.util.Set#clear() clear} method.  Keys may not be added directly
 * to the selected-key set. </p>
 *
 * <a id="selop"></a>
 * <h2>Selection</h2>
 *
 * <p> A selection operation queries the underlying operating system for an
 * update as to the readiness of each registered channel to perform any of the
 * operations identified by its key's interest set.  There are two forms of
 * selection operation:
 *
 * <ol>
 *
 * <li><p> The {@link #select()}, {@link #select(long)}, and {@link #selectNow()}
 * methods add the keys of channels ready to perform an operation to the
 * selected-key set, or update the ready-operation set of keys already in the
 * selected-key set. </p></li>
 *
 * <li><p> The {@link #select(Consumer)}, {@link #select(Consumer, long)}, and
 * {@link #selectNow(Consumer)} methods perform an <i>action</i> on the key
 * of each channel that is ready to perform an operation.  These methods do
 * not add to the selected-key set. </p></li>
 *
 * </ol>
 *
 * <h3>Selection operations that add to the selected-key set</h3>
 *
 * <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>
 *
 *
 * <h3>Selection operations that perform an action on selected keys</h3>
 *
 * <p> During each selection operation, keys may be removed from the selector's
 * key, selected-key, and cancelled-key sets.  Selection is performed by the
 * {@link #select(Consumer)}, {@link #select(Consumer, long)}, and {@link
 * #selectNow(Consumer)} 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.
 *
 * <p> For a channel that is ready for at least one such operation, the
 * ready-operation set of the channel's key is set to identify exactly the
 * operations for which the channel is ready and the <i>action</i> specified
 * to the {@code select} method is invoked to consume the channel's key.  Any
 * readiness information previously recorded in the ready set is discarded
 * prior to invoking the <i>action</i>.
 *
 * <p> Alternatively, where a channel is ready for more than one operation,
 * the <i>action</i> may be invoked more than once with the channel's key and
 * ready-operation set modified to a subset of the operations for which the
 * channel is ready.  Where the <i>action</i> is invoked more than once for
 * the same key then its ready-operation set never contains operation bits
 * that were contained in the set at previous calls to the <i>action</i>
 * in the same selection operation.  </p></li>
 *
 * <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>
 *
 *
 * <h2>Concurrency</h2>
 *
 * <p> A Selector and its key set are safe for use by multiple concurrent
 * threads.  Its selected-key set and cancelled-key set, however, are not.
 *
 * <p> The selection operations synchronize on the selector itself, 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 a selection operation 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 its
 * selected-key set in the same order as in a selection operation.
 *
 * <a id="ksc"></a>
 * <p> A Selector's key set is safe for use by multiple concurrent threads.
 * Retrieval operations from the key set do not generally block and so may
 * overlap with new registrations that add to the set, or with the cancellation
 * steps of selection operations that remove keys from the set.  Iterators and
 * spliterators return elements reflecting the state of the set at some point at
 * or since the creation of the iterator/spliterator.  They do not throw
 * {@link java.util.ConcurrentModificationException ConcurrentModificationException}.
 *
 * <a id="sksc"></a>
 * <p> A selector's selected-key set is not, in general, safe for use by
 * multiple concurrent threads.  If such a thread might modify the set directly
 * then access should be controlled by synchronizing on the set itself.  The
 * iterators returned by the set's {@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
 * @see SelectableChannel
 * @see SelectionKey
 * @since 1.4
 */
// 通道选择器，完成对通道的多路复用
public abstract class Selector implements Closeable {
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Initializes a new instance of this class.
     */
    protected Selector() {
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    // 构造一个选择器Selector
    public static Selector open() throws IOException {
        return SelectorProvider.provider().openSelector();
    }
    
    /*▲ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 选择就绪通道 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    /*
     * 选择可用的已就绪通道，返回本轮select()中找到的所有【可用的】"已就绪键"(已就绪通道)的数量
     *
     * 注：
     * 1.会将可用的"已就绪键"存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)
     * 2.本地(native层)没有相关的变动事件时，一直阻塞(参见SubSelector#poll())
     */
    public abstract int select() throws IOException;
    
    /**
     * 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
     */
    /*
     * 选择可用的已就绪通道，返回本轮select()中找到的所有【可用的】"已就绪键"(已就绪通道)的数量
     *
     * 注：
     * 1.会将可用的"已就绪键"存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)
     * 2.本地(native层)没有相关的变动事件时，立即返回(参见SubSelector#poll())
     */
    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
     */
    /*
     * 选择可用的已就绪通道，返回本轮select()中找到的所有【可用的】"已就绪键"(已就绪通道)的数量
     *
     * 注：会将可用的"已就绪键"存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)
     *
     * timeout: 监听等待中的超时设置(参见SubSelector#poll())：
     *          timeout<=0表示一直阻塞，直到本地被新来的事件唤醒选择器线程，然后传导到Java层；
     *          timeout为其他值表示阻塞timeout毫秒。
     */
    public abstract int select(long timeout) throws IOException;
    
    /**
     * Selects and performs an action on the keys whose corresponding channels
     * are ready for I/O operations.
     *
     * <p> This method performs a blocking <a href="#selop">selection
     * operation</a>.  It wakes up from querying the operating system only when
     * 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> This method is equivalent to invoking the 2-arg
     * {@link #select(Consumer, long) select} method with a timeout of {@code 0}
     * to block indefinitely.  </p>
     *
     * @param action The action to perform
     *
     * @return The number of unique keys consumed, possibly zero
     *
     * @throws IOException             If an I/O error occurs
     * @throws ClosedSelectorException If this selector is closed or is closed by the action
     * @implSpec The default implementation invokes the 2-arg {@code select}
     * method with a timeout of {@code 0}.
     * @since 11
     */
    /*
     * 选择可用的已就绪通道，返回本轮select()中找到的所有【可用的】"已就绪键"(已就绪通道)的数量
     *
     * action : 如果为null，则会将可用的"已就绪键"存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)；
     *          如果不为null，则用来处理可用的"已就绪键"，即【不会】将其存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)。
     *          这就意味着使用selectedKeys()时就无法获取到"已就绪键"了。
     *
     * 注：本地(native层)没有相关的变动事件时，一直阻塞(参见SubSelector#poll())
     */
    public int select(Consumer<SelectionKey> action) throws IOException {
        return select(action, 0);
    }
    
    /**
     * Selects and performs an action on the keys whose corresponding channels
     * are ready for I/O operations.
     *
     * <p> This method performs a non-blocking <a href="#selop">selection
     * operation</a>.
     *
     * <p> Invoking this method clears the effect of any previous invocations
     * of the {@link #wakeup wakeup} method.  </p>
     *
     * @param action The action to perform
     *
     * @return The number of unique keys consumed, possibly zero
     *
     * @throws IOException             If an I/O error occurs
     * @throws ClosedSelectorException If this selector is closed or is closed by the action
     * @implSpec The default implementation removes all keys from the
     * selected-key set, invokes {@link #selectNow() selectNow()} and then
     * performs the action for each key added to the selected-key set.  The
     * default implementation does not detect the action performing a reentrant
     * selection operation.  The selected-key set may or may not be empty on
     * completion of the default implementation.
     * @since 11
     */
    /*
     * 选择可用的已就绪通道，返回本轮select()中找到的所有【可用的】"已就绪键"(已就绪通道)的数量
     *
     * action : 如果为null，则会将可用的"已就绪键"存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)；
     *          如果不为null，则用来处理可用的"已就绪键"，即【不会】将其存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)。
     *          这就意味着使用selectedKeys()时就无法获取到"已就绪键"了。
     *
     * 注：本地(native层)没有相关的变动事件时，立即返回(参见SubSelector#poll())
     */
    public int selectNow(Consumer<SelectionKey> action) throws IOException {
        return doSelect(Objects.requireNonNull(action), -1);
    }
    
    /**
     * Selects and performs an action on the keys whose corresponding channels
     * are ready for I/O operations.
     *
     * <p> This method performs a blocking <a href="#selop">selection
     * operation</a>.  It wakes up from querying the operating system only when
     * 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> The specified <i>action</i>'s {@link Consumer#accept(Object) accept}
     * method is invoked with the key for each channel that is ready to perform
     * an operation identified by its key's interest set.  The {@code accept}
     * method may be invoked more than once for the same key but with the
     * ready-operation set containing a subset of the operations for which the
     * channel is ready (as described above).  The {@code accept} method is
     * invoked while synchronized on the selector and its selected-key set.
     * Great care must be taken to avoid deadlocking with other threads that
     * also synchronize on these objects.  Selection operations are not reentrant
     * in general and consequently the <i>action</i> should take great care not
     * to attempt a selection operation on the same selector.  The behavior when
     * attempting a reentrant selection operation is implementation specific and
     * therefore not specified.  If the <i>action</i> closes the selector then
     * {@code ClosedSelectorException} is thrown when the action completes.
     * The <i>action</i> is not prohibited from closing channels registered with
     * the selector, nor prohibited from cancelling keys or changing a key's
     * interest set.  If a channel is selected but its key is cancelled or its
     * interest set changed before the <i>action</i> is performed on the key
     * then it is implementation specific as to whether the <i>action</i> is
     * invoked (it may be invoked with an {@link SelectionKey#isValid() invalid}
     * key).  Exceptions thrown by the action are relayed to the caller.
     *
     * <p> This method does not offer real-time guarantees: It schedules the
     * timeout as if by invoking the {@link Object#wait(long)} method.
     *
     * @param action  The action to perform
     * @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 unique keys consumed, possibly zero
     *
     * @throws IOException              If an I/O error occurs
     * @throws ClosedSelectorException  If this selector is closed or is closed by the action
     * @throws IllegalArgumentException If the value of the timeout argument is negative
     * @implSpec The default implementation removes all keys from the
     * selected-key set, invokes {@link #select(long) select(long)} with the
     * given timeout and then performs the action for each key added to the
     * selected-key set.  The default implementation does not detect the action
     * performing a reentrant selection operation.  The selected-key set may
     * or may not be empty on completion of the default implementation.
     * @since 11
     */
    /*
     * 选择可用的已就绪通道，返回本轮select()中找到的所有【可用的】"已就绪键"(已就绪通道)的数量
     *
     * action : 如果为null，则会将可用的"已就绪键"存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)；
     *          如果不为null，则用来处理可用的"已就绪键"，即【不会】将其存储到"已就绪键集合"中(参见SelectorImpl#selectedKeys)。
     *          这就意味着使用selectedKeys()时就无法获取到"已就绪键"了。
     *
     * timeout: 监听等待中的超时设置(参见SubSelector#poll())：
     *          timeout<=0表示一直阻塞，直到本地被新来的事件唤醒选择器线程，然后传导到Java层；
     *          timeout为其他值表示阻塞timeout毫秒。
     */
    public int select(Consumer<SelectionKey> action, long timeout) throws IOException {
        if(timeout<0) {
            throw new IllegalArgumentException("Negative timeout");
        }
        
        return doSelect(Objects.requireNonNull(action), timeout);
    }
    
    /**
     * Default implementation of select(Consumer) and selectNow(Consumer).
     */
    /*
     * 选择可用的已就绪通道，返回本轮select()中找到的所有【可用的】"已就绪键"(已就绪通道)的数量
     * 注：与WindowsSelectorImpl中的实现不同
     */
    private int doSelect(Consumer<SelectionKey> action, long timeout) throws IOException {
        synchronized(this) {
            // 获取已就绪通道selectedKeys的视图，与selectedKeys共享元素，允许删除，但不允许增加
            Set<SelectionKey> selectedKeys = selectedKeys();
            
            synchronized(selectedKeys) {
                selectedKeys.clear();
                
                int numKeySelected;
                if(timeout<0) {
                    numKeySelected = selectNow();
                } else {
                    numKeySelected = select(timeout);
                }
                
                // copy selected-key set as action may remove keys
                Set<SelectionKey> keysToConsume = Set.copyOf(selectedKeys);
                assert keysToConsume.size() == numKeySelected;
                selectedKeys.clear();
                
                // invoke action for each selected key
                keysToConsume.forEach(k -> {
                    action.accept(k);
                    if(!isOpen()) {
                        throw new ClosedSelectorException();
                    }
                });
                
                return numKeySelected;
            }
        }
    }
    
    /*▲ 选择就绪通道 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 打开/关闭 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Tells whether or not this selector is open.
     *
     * @return {@code true} if, and only if, this selector is open
     */
    // 判断选择器是否处于开启状态
    public abstract boolean isOpen();
    
    
    /**
     * 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
     */
    // 关闭选择器
    public abstract void close() throws IOException;
    
    /*▲ 打开/关闭 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 唤醒 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Causes the first selection operation that has not yet returned to return immediately.
     *
     * <p> If another thread is currently blocked in a selection operation then
     * that invocation will return immediately.  If no selection operation is
     * currently in progress then the next invocation of a selection operation
     * will return immediately unless {@link #selectNow()} or {@link
     * #selectNow(Consumer)} is invoked in the meantime.  In any case the value
     * returned by that invocation may be non-zero.  Subsequent selection
     * operations 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
     */
    // 通过"哨兵"元素唤醒所有阻塞的辅助线程，并设置interruptTriggered = true，后续这些辅助线程将会结束运行
    public abstract Selector wakeup();
    
    /*▲ 唤醒 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 视图 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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 set is <a href="#ksc">safe</a> for use by multiple concurrent
     * threads.  </p>
     *
     * @return This selector's key set
     *
     * @throws ClosedSelectorException If this selector is closed
     */
    // 获取新注册通道keys的视图，与keys共享元素，但只读
    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="#sksc">not thread-safe</a>.  </p>
     *
     * @return This selector's selected-key set
     *
     * @throws ClosedSelectorException If this selector is closed
     */
    // 获取已就绪通道selectedKeys的视图，与selectedKeys共享元素，允许删除，但不允许增加
    public abstract Set<SelectionKey> selectedKeys();
    
    /*▲ 视图 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    /**
     * Returns the provider that created this channel.
     *
     * @return The provider that created this channel
     */
    // 返回构造当前选择器的选择器工厂
    public abstract SelectorProvider provider();
    
}