SelectableChannel.java

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package java.nio.channels;

import java.io.IOException;
import java.nio.channels.spi.AbstractInterruptibleChannel;
import java.nio.channels.spi.SelectorProvider;

/**
 * A channel that can be multiplexed via a {@link Selector}.
 *
 * <p> In order to be used with a selector, an instance of this class must
 * first be <i>registered</i> via the {@link #register(Selector, int, Object)
 * register} method.  This method returns a new {@link SelectionKey} object
 * that represents the channel's registration with the selector.
 *
 * <p> Once registered with a selector, a channel remains registered until it
 * is <i>deregistered</i>.  This involves deallocating whatever resources were
 * allocated to the channel by the selector.
 *
 * <p> A channel cannot be deregistered directly; instead, the key representing
 * its registration must be <i>cancelled</i>.  Cancelling a key requests that
 * the channel be deregistered during the selector's next selection operation.
 * A key may be cancelled explicitly by invoking its {@link
 * SelectionKey#cancel() cancel} method.  All of a channel's keys are cancelled
 * implicitly when the channel is closed, whether by invoking its {@link
 * Channel#close close} method or by interrupting a thread blocked in an I/O
 * operation upon the channel.
 *
 * <p> If the selector itself is closed then the channel will be deregistered,
 * and the key representing its registration will be invalidated, without
 * further delay.
 *
 * <p> A channel may be registered at most once with any particular selector.
 *
 * <p> Whether or not a channel is registered with one or more selectors may be
 * determined by invoking the {@link #isRegistered isRegistered} method.
 *
 * <p> Selectable channels are safe for use by multiple concurrent
 * threads. </p>
 *
 *
 * <a id="bm"></a>
 * <h2>Blocking mode</h2>
 *
 * A selectable channel is either in <i>blocking</i> mode or in
 * <i>non-blocking</i> mode.  In blocking mode, every I/O operation invoked
 * upon the channel will block until it completes.  In non-blocking mode an I/O
 * operation will never block and may transfer fewer bytes than were requested
 * or possibly no bytes at all.  The blocking mode of a selectable channel may
 * be determined by invoking its {@link #isBlocking isBlocking} method.
 *
 * <p> Newly-created selectable channels are always in blocking mode.
 * Non-blocking mode is most useful in conjunction with selector-based
 * multiplexing.  A channel must be placed into non-blocking mode before being
 * registered with a selector, and may not be returned to blocking mode until
 * it has been deregistered.
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @see SelectionKey
 * @see Selector
 * @since 1.4
 */
// 多路复用通道，且实现了中断接口。该通道通常应用在同步Socket通道，支持在非阻塞模式下运行
public abstract class SelectableChannel extends AbstractInterruptibleChannel implements Channel {
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Initializes a new instance of this class.
     */
    protected SelectableChannel() {
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 注册 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Registers this channel with the given selector, returning a selection
     * key.
     *
     * <p> An invocation of this convenience method of the form
     *
     * <blockquote>{@code sc.register(sel, ops)}</blockquote>
     *
     * behaves in exactly the same way as the invocation
     *
     * <blockquote>{@code sc.}{@link
     * #register(java.nio.channels.Selector, int, java.lang.Object)
     * register(sel, ops, null)}</blockquote>
     *
     * @param sel The selector with which this channel is to be registered
     * @param ops The interest set for the resulting key
     *
     * @return A key representing the registration of this channel with
     * the given selector
     *
     * @throws ClosedChannelException       If this channel is closed
     * @throws ClosedSelectorException      If the selector is closed
     * @throws IllegalBlockingModeException If this channel is in blocking mode
     * @throws IllegalSelectorException     If this channel was not created by the same provider
     *                                      as the given selector
     * @throws CancelledKeyException        If this channel is currently registered with the given selector
     *                                      but the corresponding key has already been cancelled
     * @throws IllegalArgumentException     If a bit in {@code ops} does not correspond to an operation
     *                                      that is supported by this channel, that is, if {@code set &
     *                                      ~validOps() != 0}
     */
    /*
     * 当前通道向指定的选择器selector发起注册操作，返回生成的"选择键"；
     * 完整的注册行为参见register(selector, ops, attachment)。
     */
    public final SelectionKey register(Selector selector, int ops) throws ClosedChannelException {
        // 附属对象(attachment)设置为null
        return register(selector, ops, null);
    }
    
    /**
     * Registers this channel with the given selector, returning a selection
     * key.
     *
     * <p> If this channel is currently registered with the given selector then
     * the selection key representing that registration is returned.  The key's
     * interest set will have been changed to {@code ops}, as if by invoking
     * the {@link SelectionKey#interestOps(int) interestOps(int)} method.  If
     * the {@code att} argument is not {@code null} then the key's attachment
     * will have been set to that value.  A {@link CancelledKeyException} will
     * be thrown if the key has already been cancelled.
     *
     * <p> Otherwise this channel has not yet been registered with the given
     * selector, so it is registered and the resulting new key is returned.
     * The key's initial interest set will be {@code ops} and its attachment
     * will be {@code att}.
     *
     * <p> This method may be invoked at any time.  If this method is invoked
     * while a selection operation is in progress then it has no effect upon
     * that operation; the new registration or change to the key's interest set
     * will be seen by the next selection operation.  If this method is invoked
     * while an invocation of {@link #configureBlocking(boolean) configureBlocking}
     * is in progress then it will block until the channel's blocking mode has
     * been adjusted.
     *
     * <p> If this channel is closed while this operation is in progress then
     * the key returned by this method will have been cancelled and will
     * therefore be invalid. </p>
     *
     * @param sel The selector with which this channel is to be registered
     * @param ops The interest set for the resulting key
     * @param att The attachment for the resulting key; may be {@code null}
     *
     * @return A key representing the registration of this channel with
     * the given selector
     *
     * @throws ClosedChannelException       If this channel is closed
     * @throws ClosedSelectorException      If the selector is closed
     * @throws IllegalBlockingModeException If this channel is in blocking mode
     * @throws IllegalSelectorException     If this channel was not created by the same provider
     *                                      as the given selector
     * @throws CancelledKeyException        If this channel is currently registered with the given selector
     *                                      but the corresponding key has already been cancelled
     * @throws IllegalArgumentException     If a bit in the {@code ops} set does not correspond to an
     *                                      operation that is supported by this channel, that is, if
     *                                      {@code set & ~validOps() != 0}
     */
    /*
     * 当前通道向指定的选择器selector发起注册操作，返回生成的"选择键"
     *
     * 具体的注册行为是：
     * 将通道(channel)、选择器(selector)、监听事件(ops)、附属对象(attachment)这四个属性打包成一个"选择键"对象，
     * 并将该对象分别存储到各个相关的"选择键"集合/队列中，涉及到的"选择键"集合/队列包括：
     *
     * AbstractSelectableChannel -> keys
     * SelectorImpl              -> keys
     * WindowsSelectorImpl       -> newKeys、updateKeys
     * AbstractSelector          -> cancelledKeys(出现异常时使用)
     *
     * 注：需要确保当前通道为非阻塞通道
     */
    public abstract SelectionKey register(Selector selector, int ops, Object attachment) throws ClosedChannelException;
    
    /**
     * Tells whether or not this channel is currently registered with any
     * selectors.  A newly-created channel is not registered.
     *
     * <p> Due to the inherent delay between key cancellation and channel
     * deregistration, a channel may remain registered for some time after all
     * of its keys have been cancelled.  A channel may also remain registered
     * for some time after it is closed.  </p>
     *
     * @return {@code true} if, and only if, this channel is registered
     */
    // 判断当前通道是否注册到了某个选择器上
    public abstract boolean isRegistered();
    
    /*▲ 注册 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 阻塞 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Adjusts this channel's blocking mode.
     *
     * <p> If this channel is registered with one or more selectors then an
     * attempt to place it into blocking mode will cause an {@link
     * IllegalBlockingModeException} to be thrown.
     *
     * <p> This method may be invoked at any time.  The new blocking mode will
     * only affect I/O operations that are initiated after this method returns.
     * For some implementations this may require blocking until all pending I/O
     * operations are complete.
     *
     * <p> If this method is invoked while another invocation of this method or
     * of the {@link #register(Selector, int) register} method is in progress
     * then it will first block until the other operation is complete. </p>
     *
     * @param block If {@code true} then this channel will be placed in
     *              blocking mode; if {@code false} then it will be placed
     *              non-blocking mode
     *
     * @return This selectable channel
     *
     * @throws ClosedChannelException       If this channel is closed
     * @throws IllegalBlockingModeException If {@code block} is {@code true} and this channel is
     *                                      registered with one or more selectors
     * @throws IOException                  If an I/O error occurs
     */
    // 将当前通道设置为阻塞/非阻塞模式
    public abstract SelectableChannel configureBlocking(boolean block) throws IOException;
    
    /**
     * Tells whether or not every I/O operation on this channel will block until it completes.
     * A newly-created channel is always in blocking mode.
     *
     * If this channel is closed then the value returned by this method is not specified.
     *
     * @return {@code true} if, and only if, this channel is in blocking mode
     */
    // 判断通道是否处于阻塞模式
    public abstract boolean isBlocking();
    
    /**
     * Retrieves the object upon which the {@link #configureBlocking
     * configureBlocking} and {@link #register register} methods synchronize.
     * This is often useful in the implementation of adaptors that require a
     * specific blocking mode to be maintained for a short period of time.
     *
     * @return The blocking-mode lock object
     */
    // 获取设置通道阻塞模式时使用的锁，常用于Socket适配器中
    public abstract Object blockingLock();
    
    /*▲ 阻塞 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    /**
     * Returns an <a href="SelectionKey.html#opsets">operation set</a>
     * identifying this channel's supported operations.  The bits that are set
     * in this integer value denote exactly the operations that are valid for
     * this channel.  This method always returns the same value for a given
     * concrete channel class.
     *
     * @return The valid-operation set
     */
    // 返回当前通道允许监听的事件，或称为允许注册的参数
    public abstract int validOps();
    
    /**
     * Retrieves the key representing the channel's registration with the given
     * selector.
     *
     * @param sel The selector
     *
     * @return The key returned when this channel was last registered with the
     * given selector, or {@code null} if this channel is not
     * currently registered with that selector
     */
    // 在已注册的SelectionKey集合中查找参数selector所在的SelectionKey
    public abstract SelectionKey keyFor(Selector sel);
    
    /**
     * Returns the provider that created this channel.
     *
     * @return The provider that created this channel
     */
    // 获取当前通道使用的选择器工厂
    public abstract SelectorProvider provider();
    
}