AbstractSelectableChannel定义

Channel接口定义：http://donald-draper.iteye.com/blog/2369111
AbstractInterruptibleChannel接口定义：http://donald-draper.iteye.com/blog/2369238
SelectableChannel接口定义:http://donald-draper.iteye.com/blog/2369317
SelectionKey定义：http://donald-draper.iteye.com/blog/2369499
SelectorProvider定义：http://donald-draper.iteye.com/blog/2369615
前面我们看了可选择通道接口，选择key和通道、选择器提供服务者的相关定义，今天来看一下可选择通的基础实现AbstractSelectableChannel。

package java.nio.channels.spi;
import java.io.IOException;
import java.nio.channels.*;
/**
 * Base implementation class for selectable channels.
 *AbstractSelectableChannel可选择通道的基础实现类。
 *  This class defines methods that handle the mechanics of channel
 * registration, deregistration, and closing.  It maintains the current
 * blocking mode of this channel as well as its current set of selection keys.
 * It performs all of the synchronization required to implement the {@link
 * java.nio.channels.SelectableChannel} specification.  Implementations of the
 * abstract protected methods defined in this class need not synchronize
 * against other threads that might be engaged in the same operations.  

 *AbstractSelectableChannel提供通道的注册，反注册和关闭机制的实现。维护者与选择器key集合
 中通道对应的选择key相同的阻塞模式。实现了可选择通道需要实现所有同步操作。 AbstractSelectableChannel
 中所有的protect抽象方法，不需要同步，因为这些方法与其他线程相同的操作没有冲突。
 *
 * @author Mark Reinhold
 * @author Mike McCloskey
 * @author JSR-51 Expert Group
 * @since 1.4
 */

public abstract class AbstractSelectableChannel
    extends SelectableChannel
{

    // The provider that created this channel,通道提供者
    private final SelectorProvider provider;

    // Keys that have been created by registering this channel with selectors.
    // They are saved because if this channel is closed the keys must be
    // deregistered.  Protected by keyLock.
    //通道注册到选择器器的所有选择key。之所以保存的原因，是为了在通道关闭时，反注册通道的选择key。
    //通过keyLock保证选择key集合的线程安全访问
    private SelectionKey[] keys = null;
    private int keyCount = 0;//与通道相关选择key的数量

    // Lock for key set and count 选择key数组保护锁
    private final Object keyLock = new Object();

    // Lock for registration and configureBlocking operations
    //注册和阻塞配置同步锁
    private final Object regLock = new Object();

    // Blocking mode, protected by regLock，默认为阻塞模式
    //只有非阻塞模式的通道，才可以注册到选择器
    boolean blocking = true;

    /**
     * Initializes a new instance of this class.
     根据选择器服务提供者，构建AbstractSelectableChannel
     */
    protected AbstractSelectableChannel(SelectorProvider provider) {
        this.provider = provider;
    }

    /**
     * Returns the provider that created this channel.
     *返回创建通道的选择器提供者。
     * @return  The provider that created this channel
     */
    public final SelectorProvider provider() {
        return provider;
    }


    // -- Utility methods for the key set --
    //添加选择key
    private void addKey(SelectionKey k) {
        synchronized (keyLock) {
            int i = 0;
            if ((keys != null) && (keyCount < keys.length)) {
	        //如果选择key数组已经创建，且数组未满，遍历数组中第一个为null的选择key，
		//存在，记录位置
                // Find empty element of key array
                for (i = 0; i < keys.length; i++)
                    if (keys[i] == null)
                        break;
            } else if (keys == null) {
	        //如果选择key数组为创建，则创建选择key数组，默认容量为3，（读写连接事件/接受连接事件）
                keys =  new SelectionKey[3];
            } else {
	        //如果选择key数组已满，则扩容为原来容量的2倍
                // Grow key array
                int n = keys.length * 2;
                SelectionKey[] ks =  new SelectionKey[n];
                for (i = 0; i < keys.length; i++)
                    ks[i] = keys[i];
                keys = ks;
                i = keyCount;
            }
            keys[i] = k;
            keyCount++;
        }
    }
    //判断通道是否与指定的选择器是否有关联，
    //换一种说法为，通道是否注册到选择器
    private SelectionKey findKey(Selector sel) {
        synchronized (keyLock) {
            if (keys == null)
                return null;
	   //遍历通道选择key数组，匹配选择key的选择器
            for (int i = 0; i < keys.length; i++)
                if ((keys[i] != null) && (keys[i].selector() == sel))
                    return keys[i];
            return null;
        }
    }
    //移除通道的指定选择key
    void removeKey(SelectionKey k) {                    // package-private
        synchronized (keyLock) {
	    //遍历通道选择key数组，匹配选择key，相等则置null
            for (int i = 0; i < keys.length; i++)
                if (keys[i] == k) {
                    keys[i] = null;
                    keyCount--;
                }
	    //设置选择key状态为无效
            ((AbstractSelectionKey)k).invalidate();
        }
    }
   //判断通道选择key是否存在有效的，即是否注册到通道
    private boolean haveValidKeys() {
        synchronized (keyLock) {
            if (keyCount == 0)
                return false;
	   //遍历通道选择key数组，判断选择key是否有效，存在一个有效，则返回true
            for (int i = 0; i < keys.length; i++) {
                if ((keys[i] != null) && keys[i].isValid())
                    return true;
            }
            return false;
        }
    }


    // -- Registration --
    //是否注册到通道，选择key数组实际数量不为0，则已注册
    public final boolean isRegistered() {
        synchronized (keyLock) {
            return keyCount != 0;
        }
    }
    //通道注册到指定选择器的选择key
    public final SelectionKey keyFor(Selector sel) {
        return findKey(sel);
    }

    /**
     * Registers this channel with the given selector, returning a selection key.
     *注册通道到选择器，返回通道与选择器的映射选择key
     *   This method first verifies that this channel is open and that the
     * given initial interest set is valid.
     *方法首先验证通道是否打开，关注的操作事件是否有效
     * <p> If this channel is already registered with the given selector then
     * the selection key representing that registration is returned after
     * setting its interest set to the given value.
     *如果通道已经注册到选择器，则更新兴趣操作事件集，和附件对象
     * <p> Otherwise this channel has not yet been registered with the given
     * selector, so the {@link AbstractSelector#register register} method of
     * the selector is invoked while holding the appropriate locks.  The
     * resulting key is added to this channel's key set before being returned.
     如果还没有注册到选择器，则注册通道到选择器，并将返回的选择key添加到通道
     的选择key数组中。
     * 

     *
     * @throws  ClosedSelectorException {@inheritDoc}
     *
     * @throws  IllegalBlockingModeException {@inheritDoc}
     *
     * @throws  IllegalSelectorException {@inheritDoc}
     *
     * @throws  CancelledKeyException {@inheritDoc}
     *
     * @throws  IllegalArgumentException {@inheritDoc}
     */
    public final SelectionKey register(Selector sel, int ops,
                                       Object att)
        throws ClosedChannelException
    {
        if (!isOpen())
	    //通道已经关闭，则抛出ClosedChannelException
            throw new ClosedChannelException();
        if ((ops & ~validOps()) != 0)
	    //如果注册的操作事件非通道所支持的操作事件，则抛出IllegalArgumentException
            throw new IllegalArgumentException();
        synchronized (regLock) {
            if (blocking)
	        //如果通道是阻塞模式，则抛出IllegalBlockingModeException
                throw new IllegalBlockingModeException();
            SelectionKey k = findKey(sel);
            if (k != null) {
	        //通道已经注册到选择器，更新兴趣操作事件集和附加对象
                k.interestOps(ops);
                k.attach(att);
            }
            if (k == null) {
                // New registration
		//否则，注册通道到选择器，具体注册流程，我们在以后具体详说
                k = ((AbstractSelector)sel).register(this, ops, att);
		//将注册返回的选择key，添加到通道的选择key集合中
                addKey(k);
            }
            return k;
        }
    }


    // -- Closing --

    /**
     * Closes this channel.
     *
     *  This method, which is specified in the {@link
     * AbstractInterruptibleChannel} class and is invoked by the {@link
     * java.nio.channels.Channel#close close} method, in turn invokes the
     * {@link #implCloseSelectableChannel implCloseSelectableChannel} method in
     * order to perform the actual work of closing this channel.  It then
     * cancels all of this channel's keys.  

     */
    protected final void implCloseChannel() throws IOException {
        //关闭可选择通达
        implCloseSelectableChannel();
        synchronized (keyLock) {
            int count = (keys == null) ? 0 : keys.length;
	    //遍历通道的选择key数组，取消选择key
            for (int i = 0; i < count; i++) {
                SelectionKey k = keys[i];
                if (k != null)
                    k.cancel();
            }
        }
    }

    /**
     * Closes this selectable channel.
     *关闭可选择通道
     *  This method is invoked by the {@link java.nio.channels.Channel#close
     * close} method in order to perform the actual work of closing the
     * channel.  This method is only invoked if the channel has not yet been
     * closed, and it is never invoked more than once.
     *这个方法在通道关闭方法中执行实际的通道关闭工作，在通道还没有完全关闭时，调用，
     最多调用一次。
     * <p> An implementation of this method must arrange for any other thread
     * that is blocked in an I/O operation upon this channel to return
     * immediately, either by throwing an exception or by returning normally.
     此方法的实现必须安排其他阻塞在通道IO操作的线程立刻返回，或抛出一个异常，或正常返回。
     * 

     */
    protected abstract void implCloseSelectableChannel() throws IOException;


    // -- Blocking --
    //判断阻塞模式
    public final boolean isBlocking() {
        synchronized (regLock) {
            return blocking;
        }
    }
    //获取阻塞锁，即注册锁regLock
    public final Object blockingLock() {
        return regLock;
    }

    /**
     * Adjusts this channel's blocking mode.
     *
     *  If the given blocking mode is different from the current blocking
     * mode then this method invokes the {@link #implConfigureBlocking
     * implConfigureBlocking} method, while holding the appropriate locks, in
     * order to change the mode.  

     */
    public final SelectableChannel configureBlocking(boolean block)
        throws IOException
    {
        if (!isOpen())
	    //如果通道关闭，则抛出ClosedChannelException
            throw new ClosedChannelException();
        synchronized (regLock) {
            if (blocking == block)
	        //通道阻塞模式相同，则直接返回
                return this;
            if (block && haveValidKeys())
	       //已注册到通道，只能是非阻塞模式，配置阻塞模式，抛出IllegalBlockingModeException
                throw new IllegalBlockingModeException();
            implConfigureBlocking(block);
            blocking = block;
        }
        return this;
    }

    /**
     * Adjusts this channel's blocking mode.
     *调整通道阻塞模式
     *  This method is invoked by the {@link #configureBlocking
     * configureBlocking} method in order to perform the actual work of
     * changing the blocking mode.  This method is only invoked if the new mode
     * is different from the current mode.  

     *此方方法在#configureBlocking方法中调用，主要执行实际的阻塞模式切换工作。
     此方只有在配置的阻塞模式与当前阻塞模式不同时，才会被调用。
     * @throws IOException
     *         If an I/O error occurs
     */
    protected abstract void implConfigureBlocking(boolean block)
        throws IOException;

}

总结：
AbstractSelectableChannel有一个SelectorProvider类型的变量provider，主要是为创建通道而服务。一个选择key数组keys，保存与通道相关的选择key，一个key计数器keyCount，记录当前通道注册到选择器，生成的选择key。一个布尔blocking记录当前通道的阻塞模式。一个keyLock拥有控制选择key数据的线程安全访问。同时还有一个regLock控制通道注册选择器和配置通道阻塞模式的线程安全访问。提供了选择key集合keys的添加和移除，判断通道是否注册到选择器，及获取注册到选择器的选择key。注册通道到选择器过程为：首先验证通道是否打开，关注的操作事件是否有效，如果通道打开且事件有效，判断通道是注册到选择器，如果通道已经注册到选择器，则更新兴趣操作事件集，和附件对象，否则调用选择器的注册方法，并将返回的选择key添加到通道选择key集合。关闭通道所做的工作主要是，遍历通道的选择key数组，取消选择key。