JDK之Selector

    基於JDK8

    Selector在NIO部分會使用到，尤其是Netty中

    創建Selector

     如下List-1，使用靜態方法來創建，間接通過SelectorProvider來實現的

    List-1

public abstract class Selector implements Closeable {

    /**
     * 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();
    }

    如下List-2中，select()會一直阻塞，直到有channel返回或者Selector.wakeup()被調用；select(timeout)會阻塞timeout段時間；selectNow()會直接返回，不管是否有channel返回 

  List-2

    /**
     * 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 <tt>timeout</tt>
     *                  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.
     *
     * <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;

    如下List-3，provider()方法中

1. loadProviderFromProperty，從System.getProperty()中獲取類名稱，之後反射獲取實例
2. loadProviderAsService，同JDK的SPI方式，SPI返回的是一個集合，但是這裏只會取集合第一個元素
3. DefaultSelectorProvider，如果上面倆者都沒有，則使用DefaultSelectorProvider進行創建，如List-4所示，所以真正的SelectorProvider實現類是KQueueSelectorProvider

    由此可見，JDK中的設計，還是很留有餘地給使用者進行個性化實現的

    List-3

public abstract class SelectorProvider {

    private static final Object lock = new Object();
    private static SelectorProvider provider = null;

    private static boolean loadProviderFromProperty() {
        String cn = System.getProperty("java.nio.channels.spi.SelectorProvider");
        if (cn == null)
            return false;
        try {
            Class<?> c = Class.forName(cn, true,
                                       ClassLoader.getSystemClassLoader());
            provider = (SelectorProvider)c.newInstance();
            return true;
        } catch (ClassNotFoundException x) {
            throw new ServiceConfigurationError(null, x);
        } catch (IllegalAccessException x) {
            throw new ServiceConfigurationError(null, x);
        } catch (InstantiationException x) {
            throw new ServiceConfigurationError(null, x);
        } catch (SecurityException x) {
            throw new ServiceConfigurationError(null, x);
        }
    }

    private static boolean loadProviderAsService() {

        ServiceLoader<SelectorProvider> sl =
            ServiceLoader.load(SelectorProvider.class,
                               ClassLoader.getSystemClassLoader());
        Iterator<SelectorProvider> i = sl.iterator();
        for (;;) {
            try {
                if (!i.hasNext())
                    return false;
                provider = i.next();
                return true;
            } catch (ServiceConfigurationError sce) {
                if (sce.getCause() instanceof SecurityException) {
                    // Ignore the security exception, try the next provider
                    continue;
                }
                throw sce;
            }
        }
    }

    public static SelectorProvider provider() {
        synchronized (lock) {
            if (provider != null)
                return provider;
            return AccessController.doPrivileged(
                new PrivilegedAction<SelectorProvider>() {
                    public SelectorProvider run() {
                            if (loadProviderFromProperty())
                                return provider;
                            if (loadProviderAsService())
                                return provider;
                            provider = sun.nio.ch.DefaultSelectorProvider.create();
                            return provider;
                        }
                    });
        }
    }

    List-4

public class DefaultSelectorProvider {
    private DefaultSelectorProvider() {
    }

    public static SelectorProvider create() {
        return new KQueueSelectorProvider();
    }
}