SelectorProvider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
     在上面的几篇文章中，经常看到一个类SelectorProvider，一直不知道什么意思，再往下分析之前，我们先看一下SelectorProvider的定义，以便后面更好的理解通道和选择器。

package java.nio.channels.spi;

import java.io.IOException;
import java.net.ProtocolFamily;
import java.nio.channels.*;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.Iterator;
import java.util.ServiceLoader;
import java.util.ServiceConfigurationError;
import sun.security.action.GetPropertyAction;


/**
 * Service-provider class for selectors and selectable channels.
 *SelectorProvider主要是为创建选择器和可选择通道而服务的。
 *  A selector provider is a concrete subclass of this class that has a
 * zero-argument constructor and implements the abstract methods specified
 * below.  A given invocation of the Java virtual machine maintains a single
 * system-wide default provider instance, which is returned by the {@link
 * #provider() provider} method.  The first invocation of that method will locate
 * the default provider as specified below.
 *SelectorProvider的子类有一个无参的构造器，并实现了一些特殊的抽象方法，如下：
 通过#provider调用java虚拟机维护一个系统默认的SelectorProvider实例。
 * 
 * methods of the {@link java.nio.channels.DatagramChannel#open
 * DatagramChannel}, {@link java.nio.channels.Pipe#open Pipe}, {@link
 * java.nio.channels.Selector#open Selector}, {@link
 * java.nio.channels.ServerSocketChannel#open ServerSocketChannel}, and {@link
 * java.nio.channels.SocketChannel#open SocketChannel} classes.  It is also
 * used by the {@link java.lang.System#inheritedChannel System.inheritedChannel()}
 * method. A program may make use of a provider other than the default provider
 * by instantiating that provider and then directly invoking the <tt>open</tt>
 * methods defined in this class.
 *
 系统默认的SelectorProvider被用于DatagramChannel，Pipe，Selector，ServerSocketChannel，
 SocketChannel，System.inheritedChannel()的open方法中，用于创建相应的通道和选择器。
 * <p> All of the methods in this class are safe for use by multiple concurrent
 * threads.  

 *所有的方法可以并发安全访问。
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @since 1.4
 */

public abstract class SelectorProvider {

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

    /**
     * Initializes a new instance of this class.  
     *初始化SelectorProvider实例，如果不为null，检查运行时权限
     * @throws  SecurityException
     *          If a security manager has been installed and it denies
     *          {@link RuntimePermission}<tt>("selectorProvider")</tt>
     */
    protected SelectorProvider() {
        SecurityManager sm = System.getSecurityManager();
        if (sm != null)
            sm.checkPermission(new RuntimePermission("selectorProvider"));
    }
    //根据系统属性java.nio.channels.spi.SelectorProvider配置
    //加载SelectorProvider实例
    private static boolean loadProviderFromProperty() {
       //获取系统属性java.nio.channels.spi.SelectorProvider配置
        String cn = System.getProperty("java.nio.channels.spi.SelectorProvider");
        if (cn == null)
            return false;
        try {
            Class<?> c = Class.forName(cn, true,
                                       ClassLoader.getSystemClassLoader());
	   //加载SelectorProvider实例
            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);
        }
    }
    //获取系统加载路径下的所有SelectorProvider实现类的实现，以最后一个作为SelectorProvider
    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;
            }
        }
    }

    /**
     * Returns the system-wide default selector provider for this invocation of
     * the Java virtual machine.
     *返回JVM默认的SelectorProvider
     *  The first invocation of this method locates the default provider
     * object as follows: 

     *首先调用本地默认的SelectorProvider,过程如下：
     * [list=1]
     *
     *   <li> If the system property
     *   <tt>java.nio.channels.spi.SelectorProvider</tt> is defined then it is
     *   taken to be the fully-qualified name of a concrete provider class.
     *   The class is loaded and instantiated; if this process fails then an
     *   unspecified error is thrown.  
</li>
     *  如果java.nio.channels.spi.SelectorProvider系统属性被定义为一个具体的SelectorProvider
     实现类的唯一类名，则此类将会被加载，实例化，如果加载实例化失败，返回一个错误。
     *   <li> If a provider class has been installed in a jar file that is
     *   visible to the system class loader, and that jar file contains a
     *   provider-configuration file named
     *   <tt>java.nio.channels.spi.SelectorProvider</tt> in the resource
     *   directory <tt>META-INF/services</tt>, then the first class name
     *   specified in that file is taken.  The class is loaded and
     *   instantiated; if this process fails then an unspecified error is
     *   thrown.  
</li>
     *如果SelectorProvider的实现在Jar包中，且对系统类加载器可见，且Jar在资源文件META-INF/services
     的目录下，提供了provider-configuration文件java.nio.channels.spi.SelectorProvider，则文件的
     第一个class类将会被加载和实例化，如果加载实例化失败，返回一个错误。

     *   <li> Finally, if no provider has been specified by any of the above
     *   means then the system-default provider class is instantiated and the
     *   result is returned.  
</li>
     * 如果上面两步没有发现或实例化SelectorProvider成功，则系统默认的SelectorProvider类，将会实例化。
     * [/list]
     *
     *  Subsequent invocations of this method return the provider that was
     * returned by the first invocation.  

     *
     * @return  The system-wide default selector provider
     */
    public static SelectorProvider provider() {
        synchronized (lock) {
            if (provider != null)
                return provider;
	   //在与当前线程相同访问控制权限的环境中，加载SelectorProvider实例
            return AccessController.doPrivileged(
                new PrivilegedAction<SelectorProvider>() {
                    public SelectorProvider run() {
                            if (loadProviderFromProperty())
			        //获取系统配置的SelectorProvider
                                return provider;
                            if (loadProviderAsService())
			         //获取类加载路径下的SelectorProvider
                                return provider;
		            //加载默认的SelectorProvider
                            provider = sun.nio.ch.DefaultSelectorProvider.create();
                            return provider;
                        }
                    });
        }
    }

    /**
     * Opens a datagram channel.  </p>
     *打开一个DatagramChannel
     * @return  The new channel
     */
    public abstract DatagramChannel openDatagramChannel()
        throws IOException;

    /**
     * Opens a datagram channel.
     *根据协议，打开一个DatagramChannel
     * @param   family
     *          The protocol family
     *
     * @return  A new datagram channel
     *
     * @throws  UnsupportedOperationException
     *          If the specified protocol family is not supported
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @since 1.7
     */
    public abstract DatagramChannel openDatagramChannel(ProtocolFamily family)
        throws IOException;

    /**
     * Opens a pipe. </p>
     * 打开一个Pipe
     * @return  The new pipe
     */
    public abstract Pipe openPipe()
        throws IOException;

    /**
     * Opens a selector.  </p>
     *打开一个Selector
     * @return  The new selector
     */
    public abstract AbstractSelector openSelector()
        throws IOException;

    /**
     * Opens a server-socket channel.  </p>
     *打开一个ServerSocketChannel
     * @return  The new channel
     */
    public abstract ServerSocketChannel openServerSocketChannel()
        throws IOException;

    /**
     * Opens a socket channel. </p>
     *打开一个SocketChannel
     * @return  The new channel
     */
    public abstract SocketChannel openSocketChannel()
        throws IOException;

    /**
     * Returns the channel inherited from the entity that created this
     * Java virtual machine.
     *返回继承虚拟机创建实例的通道
     *  On many operating systems a process, such as a Java virtual
     * machine, can be started in a manner that allows the process to
     * inherit a channel from the entity that created the process. The
     * manner in which this is done is system dependent, as are the
     * possible entities to which the channel may be connected. For example,
     * on UNIX systems, the Internet services daemon (<i>inetd</i>) is used to
     * start programs to service requests when a request arrives on an
     * associated network port. In this example, the process that is started,
     * inherits a channel representing a network socket.
     *在许多操作系统中，一个进程允许从创建此进程的实体继承一个通道。这种方式依赖
     系统实现，也许实体到已连接的通道。在UNIX系统中，当在网络端口中，如果一个请求到达，
     后台网络服务将会启动程序处理请求。在这个例子中，进程被启动，继承的通道表示一个网络socket。
     * <p> In cases where the inherited channel represents a network socket
     * then the {@link java.nio.channels.Channel Channel} type returned
     * by this method is determined as follows:
     *此方法返回的具体通道的过程如下：
     * [list]
     *
     *  <li><p> If the inherited channel represents a stream-oriented connected
     *  socket then a {@link java.nio.channels.SocketChannel SocketChannel} is
     *  returned. The socket channel is, at least initially, in blocking
     *  mode, bound to a socket address, and connected to a peer.
     *  
</li>
     *如果继承通道表示一个面向流的连接Socket，则SocketChannel将会被返回。SocketChannel
     初始化为阻塞模式，绑定一个socket地址，连接一个peer
     *  <li> If the inherited channel represents a stream-oriented listening
     *  socket then a {@link java.nio.channels.ServerSocketChannel
     *  ServerSocketChannel} is returned. The server-socket channel is, at
     *  least initially, in blocking mode, and bound to a socket address.
     *  
</li>
     *如果继承通道表示一个面向流的监听socket，则ServerSocketChannel将会被返回。
     ServerSocketChannel初始化为阻塞模式，绑定一个socket地址。
     *  <li> If the inherited channel is a datagram-oriented socket
     *  then a {@link java.nio.channels.DatagramChannel DatagramChannel} is
     *  returned. The datagram channel is, at least initially, in blocking
     *  mode, and bound to a socket address.
     *  
</li>
     *如果继承的通道是一个面向报文的Socket，则DatagramChannel将会被返回，
     初始化为阻塞模式，绑定一个socket地址。
     * [/list]
     *
     *  In addition to the network-oriented channels described, this method
     * may return other kinds of channels in the future.
     *目前就这三种继承通道，将会可能种类更多。
     * <p> The first invocation of this method creates the channel that is
     * returned. Subsequent invocations of this method return the same
     * channel. 

     *第一次调用，则创建一个通道，后续的调用将会返回同一个通道
     * @return  The inherited channel, if any, otherwise <tt>null</tt>.
     *
     * @throws  IOException
     *          If an I/O error occurs
     *
     * @throws  SecurityException
     *          If a security manager has been installed and it denies
     *          {@link RuntimePermission}<tt>("inheritedChannel")</tt>
     *
     * @since 1.5
     */
   public Channel inheritedChannel() throws IOException {
        return null;
   }

}

总结：
SelectorProvider就是为了创建DatagramChannel，Pipe，Selector，ServerSocketChannel，SocketChannel，System.inheritedChannel()而服务的，在相应的通道和选择器的open方法中调用系统默认的SelectorProvider相关的open*方法，创建相应的通道和选择器。SelectorProvider的provider方法主要是实例化SelectorProvider，过程为：判断java.nio.channels.spi.SelectorProvider系统属性是否被定义为一个具体的SelectorProvider实现类的唯一类名，是则加载此类，实例化，如果加载实例化失败，返回一个错误。如果无没有选择器提供者属性配置，则在SelectorProvider的实现且对系统类加载器可见Jar包中，的资源文件META-INF/services的目录下，提供了provider-configuration文件java.nio.channels.spi.SelectorProvider，则文件的第一个class类将会被加载和实例化，如果加载实例化失败，返回一个错误。上两步失败，则加载系统默认的选择器提供者。inheritedChannel方法主要是更具系统网络服务，更具具体的网络请求，创建不同的可继承实例，如果继承通道表示一个面向流的连接Socket，则SocketChannel将会被返回。SocketChannel初始化为阻塞模式，绑定一个socket地址，连接一个peer。如果继承通道表示一个面向流的监听socket，则ServerSocketChannel将会被返回。ServerSocketChannel初始化为阻塞模式，绑定一个socket地址。如果继承的通道是一个面向报文的Socket，则DatagramChannel将会被返回，初始化为阻塞模式，绑定一个socket地址。

下面我们再来看一下SelectorProvider的provider方法系统默认的SelectorProvider：
//SelectorProvider

public static SelectorProvider provider() {
        synchronized (lock) {
            if (provider != null)
                return provider;
	   //在与当前线程相同访问控制权限的环境中，加载SelectorProvider实例
            return AccessController.doPrivileged(
                new PrivilegedAction<SelectorProvider>() {
                    public SelectorProvider run() {
                            if (loadProviderFromProperty())
			        //获取系统配置的SelectorProvider
                                return provider;
                            if (loadProviderAsService())
			         //获取类加载路径下的SelectorProvider
                                return provider;
		            //加载默认的SelectorProvider
                            provider = sun.nio.ch.DefaultSelectorProvider.create();
                            return provider;
                        }
                    });
        }
}

来看默认的DefaultSelectorProvider
//DefaultSelectorProvider

package sun.nio.ch;

import java.nio.channels.spi.SelectorProvider;

// Referenced classes of package sun.nio.ch:
//            WindowsSelectorProvider

public class DefaultSelectorProvider
{
    private DefaultSelectorProvider()
    {
    }
    public static SelectorProvider create()
    {
        //默认的WindowsSelectorProvider
        return new WindowsSelectorProvider();
    }
}

再来看WindowsSelectorProvider
//WindowsSelectorProvider

package sun.nio.ch;

import java.io.IOException;
import java.nio.channels.spi.AbstractSelector;

// Referenced classes of package sun.nio.ch:
//            SelectorProviderImpl, WindowsSelectorImpl

public class WindowsSelectorProvider extends SelectorProviderImpl
{

    public WindowsSelectorProvider()
    {
    }
    public AbstractSelector openSelector()
        throws IOException
    {
       //默认的选择器实现类
        return new WindowsSelectorImpl(this);
    }
}

再来看SelectorProviderImpl
//SelectorProviderImpl

package sun.nio.ch;

import java.io.IOException;
import java.net.ProtocolFamily;
import java.nio.channels.*;
import java.nio.channels.spi.AbstractSelector;
import java.nio.channels.spi.SelectorProvider;

// Referenced classes of package sun.nio.ch:
//            DatagramChannelImpl, PipeImpl, ServerSocketChannelImpl, SocketChannelImpl

public abstract class SelectorProviderImpl extends SelectorProvider
{

    public SelectorProviderImpl()
    {
    }
    //打开一个报文通道
    public DatagramChannel openDatagramChannel()
        throws IOException
    {
        return new DatagramChannelImpl(this);
    }
    //根据协议，打开一个报文通道
    public DatagramChannel openDatagramChannel(ProtocolFamily protocolfamily)
        throws IOException
    {
        return new DatagramChannelImpl(this, protocolfamily);
    }
     //打开一个管道
    public Pipe openPipe()
        throws IOException
    {
        return new PipeImpl(this);
    }
   //打开一个选择器，待子类扩展
    public abstract AbstractSelector openSelector()
        throws IOException;
   //打开一个监听socket通道
    public ServerSocketChannel openServerSocketChannel()
        throws IOException
    {
        return new ServerSocketChannelImpl(this);
    }
    //打开一个socket通道（连接）
    public SocketChannel openSocketChannel()
        throws IOException
    {
        return new SocketChannelImpl(this);
    }
}

从上面可以看出，WindowsSelectorProvider为系统默认选择器提供者，默认选择器为WindowsSelectorImpl，SelectorProviderImpl为默认的通道提供者，各类通道和管道的默认实现为：DatagramChannelImpl，ServerSocketChannelImpl，SocketChannelImpl，
PipeImpl。具体的通道，管道和选择器的实现，我们在相关文章会再次讲解。
我们来简单SocketChannel的open方法：
//SocketChannel

public abstract class ServerSocketChannel
    extends AbstractSelectableChannel
    implements NetworkChannel
{
	 public static ServerSocketChannel open() throws IOException {
		return SelectorProvider.provider().openServerSocketChannel();
	}
}

有了上面的分析，这个open应该很好理解。其他通道，选择器打开操作的思路是相同的。