Selector定义

Channel接口定义：[url]http://donald-draper.iteye.com/blog/2369111[/url]
AbstractInterruptibleChannel接口定义：[url]http://donald-draper.iteye.com/blog/2369238[/url]
SelectableChannel接口定义:[url]http://donald-draper.iteye.com/blog/2369317[/url]
SelectionKey定义：[url]http://donald-draper.iteye.com/blog/2369499[/url]
SelectorProvider定义：[url]http://donald-draper.iteye.com/blog/2369615[/url]
AbstractSelectableChannel定义：[url]http://donald-draper.iteye.com/blog/2369742[/url]
NetworkChannel接口定义：[url]http://donald-draper.iteye.com/blog/2369773[/url]
ServerSocketChannel定义：[url]http://donald-draper.iteye.com/blog/2369836[/url]
本身打算讲ServerSocketChannel的具体实现的，看了ServerSocketChannelImp的方法中设计到选择器，所以我们先看一下选择器，再看ServerSocketChannelImp：

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.
 *Selector是可选择通道的多路复用器
 *  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.
 *选择器通过系统默认的选择器open实现创建，或者SelectorProvider的#openSelector方法。
 在选择器没关闭之前，都处于打开状态
 * 
 *
 *  A selectable channel's registration with a selector is represented by a
 * {@link SelectionKey} object.  A selector maintains three sets of selection
 * keys:
 *可选择通道注册到选择器的token，我们用SelectionKey表示。选择器主要维护3个选择key集合。
 * [list]
 *
 *   
 The key set contains the keys representing the current
 *   channel registrations of this selector.  This set is returned by the
 *   {@link #keys() keys} method. 

 *key set集合表示当前注册到选择的通道对应的选择key。这个集合通过keys方法返回。
 *   
 The selected-key set 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. 

 *selected-key set集合表示在一个选择操作后，注册到选择器的通道已经准备就绪的
 选择key集合。通过#selectedKeys方法返回，selected-key set集合总是key set的子集。
 *   
 The cancelled-key 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. 

 *cancelled-key集合，表示通道还没有反注册，但选择key已经取消的选择key。这个集合不能直接访问。
cancelled-key集合总是key set的子集。
 * [/list]
 *
 *  All three sets are empty in a newly-created selector.
 *在选择器创建时，三个集合都为空。
 * 
 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.
 *key set只会通过选择通道的注册方法添加选择key到key集合中。已取消的key在选择操作中被移除。
 key set集合自己直接修改。
 * 
 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.
 *无论是由于通道关闭，还是选择key取消，key被取消后，将会添加到选择器的取消key集合。
 取消一个key将会使通道在下一次选择操作中，从选择器反注册，同时取消key将会从选择器的key集合中移除。
 * 
 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. 

 *选择操作将会把已经就绪的通道对应的选择key添加到 selected-key set 集合中。
 通过set的remove和迭代器的remove操作，将会从取消key集合中，移除选择key。
 除选择操作可以从key集合中移除key，其他的方式或方法不能从选择key中移除key。
 key不能直接添加的可选择key集合。
 *
 * 
 * 
Selection

 *
 *  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:
 * 

 *在每一个选择操作过程中，可能从可选择key集合中添加或者移除可以，可能从key集合或者key取消集合中
 移除一个key。选择器执行#select，select(long)和#selectNow方法，将会涉及3步：
 * [list=1]
 *
 *   
 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. 

 * 1.将会清空取消key集合，与取消key相关的通道将会反注册。这一步取消key集合将为空。
 *   
 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: 

 *2.在每一次选择操作开始，底层操作系统都将检查和更显通道已经准备就绪的操作事件。
 在通道准备好至少一个的操作事件下，将会执行以下两个动作：
 *   

 *
 *     
 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.  

 *2.1 如果通道相关的选择key不在可选择key集合中，则添加key到可选择key集合，并更新就绪的
 操作事件，先前的就绪事件集将会被遗弃。
 *     
 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. 

 *2.2 如果通道相关key已经在可选择key集合中，更新就绪操作集，并保留先前准备就绪的操作事件，
 换句话，底层操作系统的返回的bitwise-disjoined就绪集将会更新到当前就绪操作事件集。
 *   [/list]
 *
 *   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.
 *3.如果在这一步开始时，key集合中的所有key的兴趣操作事件为空，可选择key集合或其他任何key的
 就绪操作事件集没有更新（这一步不太明白，想表示什么意思？，理解的网友给我留言，共同成长。
 这个要结合具体的选择方法的实现，这个在以后的文章中，再看）
 *   
 If any keys were added to the cancelled-key set while step (2) was
 *   in progress then they are processed as in step (1). 

 *在第二步的过程中，key被添加到取消key集合，将会返回第一步移除取消key
 * 
 *
 *  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. 

 *无论一个选择操作在阻塞等待一个或多个通道准备就绪，到目前为止等待多久，
 三个选方法基本上不同。
 *
 * 
Concurrency
 *并发性
 *  Selectors are themselves safe for use by multiple concurrent threads;
 * their key sets, however, are not.
 *选择器是线程安全的，而他的几个key集合不是线程安全的
 * 
 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.
 *选择操作同步选择器，key集合，可选择key集合，同时在第一步和第三步中同步取消key集合。
 * 
 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.
 *当一个选择key的兴趣操作事件集在选择操作的过程中改变时，对当前选择操作没有影响，
 当下一次选择操作中，才能被看到。
 * 
 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.
 *在任何时候，通道都有可能关闭，key都有可能取消。一个key存在与选择器的key集合中，
 不意味着key有效或key关联的通道打开。如果其他线程有可能取消key或关闭通道，
 应用必须小心这些同步，并检查需要的条件。
 * 
 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:
 *一个线程阻塞在选择操作中，可以被其他线程以一下三种方式中断：
 * [list]
 *
 *   
 By invoking the selector's {@link #wakeup wakeup} method,
 *   

 *选择器调用#wakeup方法
 *   
 By invoking the selector's {@link #close close} method, or
 *   

 *选择器调用#close方法
 *   
 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. 

 *调用阻塞线程的中断方法Thread#interrupt，线程中断位被设置，选择器的#wakeup方法将会
 被调用。
 * [/list]
 *
 *  The {@link #close close} method synchronizes on the selector and all
 * three key sets in the same order as in a selection operation.
 *关闭方法将会同步到选择操作中的选择器和关联的3个key集合。
 * 
 *
 * 
 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 fail-fast: 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. 

 * key集合和可选择key集合，一般情况下多线程访问时不是线程安全的。如果线程想要直接修改
 相关集合应该控制集合同步。在迭代器创建时，如果集合被修改，Set#iterator将会fail-fast。
 任何调用迭代器的Iterator#remove方法，都将抛出ConcurrentModificationException。
 *
 * @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.
     *用系统默认的SelectorProvider实例打开一个选择器
     *  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.  

     *
     * @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.  
     *判断选择器是否打开
     * @return true if, and only if, this selector is open
     */
    public abstract boolean isOpen();

    /**
     * Returns the provider that created this channel.  
     *返回创建选择器的选取器服务提供者。
     * @return  The provider that created this channel
     */
    public abstract SelectorProvider provider();

    /**
     * Returns this selector's key set.
     *返回选择器的选择key集合
     *  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.
     *选择key集合不能直接地修改。在通道反注册或选择key取消时，相关的选择key将会
     从选择key集合中移除。任何尝试修改选择key集合的操作，将抛出UnsupportedOperationException
     * 
 The key set is [url=#ksc]not thread-safe[/url]. 

     *
     * @return  This selector's key set
     *
     * @throws  ClosedSelectorException 通道已关闭，则抛出ClosedSelectorException
     *          If this selector is closed
     */
    public abstract Set keys();

    /**
     * Returns this selector's selected-key set.
     *返回选择器的已选择的key集合，及操作事件已经就绪的key集合
     *  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.
     *选择key不可以直接添加到已选择的key集合，但可以移除。
     任何尝试添加选择key到key集合的操作，将抛出UnsupportedOperationException
     * 
 The selected-key set is [url=#ksc]not thread-safe[/url]. 

     *
     * @return  This selector's selected-key set
     *
     * @throws  ClosedSelectorException
     *          If this selector is closed
     */
    public abstract Set selectedKeys();

    /**
     * Selects a set of keys whose corresponding channels are ready for I/O
     * operations.
     *选择一个关联通道已经准备好ＩＯ操作的选择key数量。
     *  This method performs a non-blocking selection
     * operation.  If no channels have become selectable since the previous
     * selection operation then this method immediately returns zero.
     *这个方法以非阻塞模式执行。由于先前已经进行选择操作，没有通道可选择，此方法，
     将立即返回为0.
     * 
 Invoking this method clears the effect of any previous invocations
     * of the {@link #wakeup wakeup} method.  

     *调用此方法，将清除wakeup方法的效果。
     * @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.
     *选择一个关联通道已经准备好ＩＯ操作的选择key数量。
     *  This method performs a blocking selection
     * operation.  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.
     *这个方法以非阻塞模式执行。在至少有一个通道可选择，选择器wakeup方法被调用，
     当前线程被中断，或超时，无论这几种情况，那一个先发生，都将直接返回。
     * 
 This method does not offer real-time guarantees: It schedules the
     * timeout as if by invoking the {@link Object#wait(long)} method. 

     *此方法不能保证事实的保证，因为通过Object#wait(long)去调度超时时间。
     * @param  timeout  If positive, block for up to 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
     */
    public abstract int select(long timeout)
        throws IOException;

    /**
     * Selects a set of keys whose corresponding channels are ready for I/O
     * operations.
     *选择一个关联通道已经准备好ＩＯ操作的选择key数量。
     *  This method performs a blocking selection
     * operation.  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.  

     *这个方法以阻塞模式执行。在至少有一个通道可选择，选择器wakeup方法被调用，
     当前线程被中断，无论这几种情况，那一个先发生，都将直接返回。
     * @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.
     *唤醒一个等待选取key可用的线程。
     *  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.
     *如果其他线程当前正在阻塞或超时等待选择操作，wakeup方法调用，将唤醒上述线程。
     如果当前没有选择操作在进程中，下一个调用选择操作相关方法将立即返回，除非#selectNow
     方法在同时调用，#selectNow是非阻塞的。在wakeup之后，任何调用select操作的都将返回非0。
    #select和select(long)将会向平常一样阻塞，除非wakeup方法同时被调用。
     * 
 Invoking this method more than once between two successive selection
     * operations has the same effect as invoking it just once.  

     *在多次选择操作中，wakeup只起效一次。
     * @return  This selector
     */
    public abstract Selector wakeup();

    /**
     * Closes this selector.
     *关闭选择器
     *  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.
     *如果当前线程阻塞在选择器的选择操作上，调用选择器的wakeup方法，将会消除中断位。
     * 
 Any uncancelled keys still associated with this selector are
     * invalidated, their channels are deregistered, and any other resources
     * associated with this selector are released.
     *所有与选择器相关联的通道的选择key将会无效，通道将被反注册，与选择器关联的
     任何资源都将会被释放
     * 
 If this selector is already closed then invoking this method has no
     * effect.
     *如果选择器已关闭，再次调用无效
     * 
 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. 

     *在选择器关闭后，任何尝试wakeup方法调用的，将抛出ClosedSelectorException
     * @throws  IOException
     *          If an I/O error occurs
     */
    public abstract void close() throws IOException;

}