DatagramChannelImpl 解析一（初始化）

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
AbstractSelectableChannel定义：http://donald-draper.iteye.com/blog/2369742
NetworkChannel接口定义：http://donald-draper.iteye.com/blog/2369773
Selector定义：http://donald-draper.iteye.com/blog/2370015
AbstractSelector定义：http://donald-draper.iteye.com/blog/2370138
SelectorImpl分析 :http://donald-draper.iteye.com/blog/2370519
WindowsSelectorImpl解析一（FdMap，PollArrayWrapper）：
http://donald-draper.iteye.com/blog/2370811
WindowsSelectorImpl解析二（选择操作，通道注册，通道反注册，选择器关闭等）：
http://donald-draper.iteye.com/blog/2370862
MembershipKey定义:http://donald-draper.iteye.com/blog/2372947
MulticastChanne接口定义：http://donald-draper.iteye.com/blog/2373009
MembershipKeyImpl 简介：http://donald-draper.iteye.com/blog/2373066
DatagramChannel定义：http://donald-draper.iteye.com/blog/2373046
引言：
    先来回顾一下报文通道相关的概念。MembershipKeyImpl内部有一个多播关系key关联的多播通道和多播分组地址，及多播报文源地址，及一个地址阻塞集。MembershipKeyImpl主要操作为drop关系key，直接委托个多播通道drop方法；block地址，首先判断多播关系key中的阻塞Set中是否包含对应的地址，有，则直接返回，否则委托给DatagramChannelImpl的block方法，完成实际的阻塞工作，然后添加地址的多播关系key阻塞set；unblock，首先判断多播关系key中的阻塞Set中是否包含对应的地址，无，则直接返回，有则委托给DatagramChannelImpl的unblock方法，完成实际的的解除阻塞工作，并从多播关系key中的阻塞Set移除对应的地址。
MulticastChanne定义一个通道加入多播组的接口方法join。DatagramChannel的send和receive方法是不需要进行网络连接的，而read和write方法有与不能接受和返回socket地址。
通道必须建立连接。
    在上面一篇DatagramChannel文章中我们看了一下报文通道抽象类的方法的定义，
今天这篇我们来看报文通道的具体实现我们需要关注的方法为drop，block，unblock，join，
send，receive，read和write。
我们从DatagramChannel的open方法看起

//DatagramChannel
  public static DatagramChannel open() throws IOException {
        return SelectorProvider.provider().openDatagramChannel();
    }

SelectorProvider.provider()这个过程我们就不详说了实际是加载系统默认的SelectorProvider实例，则个我们在SelectorProvider定义有提过，简单看一下：
//SelectorProviderImpl

public abstract class SelectorProviderImpl extends SelectorProvider
{
    public DatagramChannel openDatagramChannel()
        throws IOException
    {
        return new DatagramChannelImpl(this);
    }

    public DatagramChannel openDatagramChannel(ProtocolFamily protocolfamily)
        throws IOException
    {
        return new DatagramChannelImpl(this, protocolfamily);
    }
}

从上面可以看出报文通道的具体实现为DatagramChannelImpl，下面来看DatagramChannelImpl
//DatagramChannelImpl

package sun.nio.ch;
import java.io.FileDescriptor;
import java.io.IOException;
import java.net.*;
import java.nio.ByteBuffer;
import java.nio.channels.*;
import java.nio.channels.spi.SelectorProvider;
import java.util.*;
import sun.net.ResourceManager;

// Referenced classes of package sun.nio.ch:
//            DatagramDispatcher, DatagramSocketAdaptor, DirectBuffer, IOStatus, 
//            IOUtil, MembershipKeyImpl, MembershipRegistry, NativeDispatcher, 
//            NativeThread, Net, SelChImpl, SelectionKeyImpl, 
//            SelectorImpl, Util

class DatagramChannelImpl extends DatagramChannel
    implements SelChImpl
{
   private static NativeDispatcher nd = new DatagramDispatcher();//报文分发器
    private final FileDescriptor fd;//报文通道的文件描述
    private final int fdVal;//文件描述值
    private final ProtocolFamily family;//网络协议
    private volatile long readerThread;//读线程
    private volatile long writerThread;//写线程
    private InetAddress cachedSenderInetAddress;//缓存的发送者InetAddress
    private int cachedSenderPort;//缓存的发送者port
    private final Object readLock;//读锁
    private final Object writeLock;//写锁
    private final Object stateLock;//状态锁
    private static final int ST_UNINITIALIZED = -1;//未初始化
    private static final int ST_UNCONNECTED = 0;//未连接
    private static final int ST_CONNECTED = 1;//已连接
    private static final int ST_KILLED = 2;//关闭
    private int state;//状态
    private SocketAddress localAddress;//本地SocketAddress
    private SocketAddress remoteAddress;//远端SocketAddress
    private DatagramSocket socket;//报文Sockeet
    private MembershipRegistry registry;//多播关系注册器
    private SocketAddress sender;//报文发送者的SocketAddress
    static final boolean $assertionsDisabled = !sun/nio/ch/DatagramChannelImpl.desiredAssertionStatus();

    static 
    {
        //加载net和nio资源库
        Util.load();
        initIDs();
    }
}
//初始化ID
private static native void initIDs();

我们来看一下多播关系注册器
//MembershipRegistry

class MembershipRegistry
{
    private Map groups;//HashMap<InetAddress,LinkedList<MembershipKeyImpl>>
    MembershipRegistry()
    {
        groups = null;
    }
    //注册多播关系到注册器
     void add(MembershipKeyImpl membershipkeyimpl)
    {
        InetAddress inetaddress = membershipkeyimpl.group();
        Object obj;
	//从当前注册器获取多播地址对应的obj-LinkedList<membershipkeyimpl>
        if(groups == null)
        {
            groups = new HashMap();
            obj = null;
        } else
        {
            obj = (List)groups.get(inetaddress);
        }
	//如果obj为null，则创建LinkedList，并将多播地址与obj的映射关系添加到注册器的Group中
        if(obj == null)
        {
            obj = new LinkedList();
            groups.put(inetaddress, obj);
        }
	//添加多播关系key到多播地址对应的多播关系key集合中
        ((List) (obj)).add(membershipkeyimpl);
    }
    //移除多播关系key
     void remove(MembershipKeyImpl membershipkeyimpl)
    {
        //根据多播关系key的多播组地址，获取对应的多播关系key集合
        InetAddress inetaddress = membershipkeyimpl.group();
        List list = (List)groups.get(inetaddress);
        if(list != null)
        {
            Iterator iterator = list.iterator();
	    //遍历多播关系key集合，找多播关系key，则移除
            do
            {
                if(!iterator.hasNext())
                    break;
                if(iterator.next() != membershipkeyimpl)
                    continue;
                iterator.remove();
                break;
            } while(true);
	    //多播关系key集合为空，则从注册器中移除对应的多播组地址映射
            if(list.isEmpty())
                groups.remove(inetaddress);
        }
    }
    //使注册器中的所有多播组中的多播关系key无效
      void invalidateAll()
    {
        if(groups != null)
        {
	    //遍历多播组
            for(Iterator iterator = groups.keySet().iterator(); iterator.hasNext();)
            {
                InetAddress inetaddress = (InetAddress)iterator.next();
                Iterator iterator1 = ((List)groups.get(inetaddress)).iterator();
		//遍历多播关系key集合
                while(iterator1.hasNext()) 
                {
                    MembershipKeyImpl membershipkeyimpl = (MembershipKeyImpl)iterator1.next();
		    //使多播关系key无效
                    membershipkeyimpl.invalidate();
                }
            }

        }
    }
    //检查多播关系注册器中是否有，多播组地址为inetaddress，源地址为inetaddress1，网络接口为networkinterface的MembershipKey
     MembershipKey checkMembership(InetAddress inetaddress, NetworkInterface networkinterface, InetAddress inetaddress1)
    {
label0:
        {
            if(groups == null)
                break label0;
            //获取多播组对应的多播关系集合
            List list = (List)groups.get(inetaddress);
            if(list == null)
                break label0;
            Iterator iterator = list.iterator();
            MembershipKeyImpl membershipkeyimpl;
	    //遍历多播关系集合中源地址为inetaddress1的多播关系key
            do
            {
	        //遍历源地址为inetaddress1的多播关系key，找到网络接口为networkinterface的MembershipKeyImpl
                do
                {
                    if(!iterator.hasNext())
                        break label0;
                    membershipkeyimpl = (MembershipKeyImpl)iterator.next();
                } while(!membershipkeyimpl.networkInterface().equals(networkinterface));
                if(inetaddress1 == null)
                    if(membershipkeyimpl.sourceAddress() == null)
                        return membershipkeyimpl;
                    else
                        throw new IllegalStateException("Already a member to receive all packets");
                if(membershipkeyimpl.sourceAddress() == null)
                    throw new IllegalStateException("Already have source-specific membership");
            } while(!inetaddress1.equals(membershipkeyimpl.sourceAddress()));
            return membershipkeyimpl;
        }
        return null;
    }
}

从上可以看出多播关系注册器MembershipRegistry主要是通过一个Map-HashMap<InetAddress,LinkedList<MembershipKeyImpl>>来管理多播组和多播组成员关系key的映射（关系）。
再来看构造

//根据选择器提供者创建报文通道
public DatagramChannelImpl(SelectorProvider selectorprovider)
        throws IOException
    {
        super(selectorprovider);
        readerThread = 0L;
        writerThread = 0L;
        readLock = new Object();
        writeLock = new Object();
        stateLock = new Object();
        state = -1;
	//更新socket计数器，自增1
        ResourceManager.beforeUdpCreate();
        try
        {
	    //确定网络协议
            family = Net.isIPv6Available() ? ((ProtocolFamily) (StandardProtocolFamily.INET6)) : ((ProtocolFamily) (StandardProtocolFamily.INET));
            //获取文件描述
	    fd = Net.socket(family, false);
	    //获取文件描述的id
            fdVal = IOUtil.fdVal(fd);
            state = 0;
        }
        catch(IOException ioexception)
        {
	    //更新socket计数器，自减1
            ResourceManager.afterUdpClose();
            throw ioexception;
        }
    }

这个构造我们需要关注的是

//更新socket计数器，自增1
ResourceManager.beforeUdpCreate();
//更新socket计数器，自减1
ResourceManager.afterUdpClose();

我们来看一下ResourceManager
//ResourceManager

package sun.net;

import java.net.SocketException;
import java.security.AccessController;
import java.util.concurrent.atomic.AtomicInteger;
import sun.security.action.GetPropertyAction;

public class ResourceManager
{
    private static final int DEFAULT_MAX_SOCKETS = 25;//默认最大socket数量
    private static final int maxSockets;//最大socket数量
    private static final AtomicInteger numSockets = new AtomicInteger(0);//当前socket数量

    static 
    {
        //获取虚拟机最大报文socket配置
        String s = (String)AccessController.doPrivileged(new GetPropertyAction("sun.net.maxDatagramSockets"));
        int i = 25;
        try
        {
            if(s != null)
                i = Integer.parseInt(s);
        }
        catch(NumberFormatException numberformatexception) { }
        maxSockets = i;
    } 
    public ResourceManager()
    {
    }

    public static void beforeUdpCreate()
        throws SocketException
    {
        //如果系统安全检查器不为空，同时当前socket数量自增后大于最大Socket数量，则抛出SocketException
        if(System.getSecurityManager() != null && numSockets.incrementAndGet() > maxSockets)
        {
            numSockets.decrementAndGet();
            throw new SocketException("maximum number of DatagramSockets reached");
        } else
        {
            return;
        }
    }
    //更新当前报文socket计数器，自减1
    public static void afterUdpClose()
    {
        if(System.getSecurityManager() != null)
            numSockets.decrementAndGet();
    }
}

我们再看其他两个构造方法：
  

 public DatagramChannelImpl(SelectorProvider selectorprovider, ProtocolFamily protocolfamily)
        throws IOException
    {
        super(selectorprovider);
        readerThread = 0L;
        writerThread = 0L;
        readLock = new Object();
        writeLock = new Object();
        stateLock = new Object();
        state = -1;
        if(protocolfamily != StandardProtocolFamily.INET && protocolfamily != StandardProtocolFamily.INET6)
            if(protocolfamily == null)
                throw new NullPointerException("'family' is null");
            else
                throw new UnsupportedOperationException("Protocol family not supported");
        if(protocolfamily == StandardProtocolFamily.INET6 && !Net.isIPv6Available())
        {
            throw new UnsupportedOperationException("IPv6 not available");
        } else
        {
            family = protocolfamily;
            fd = Net.socket(protocolfamily, false);
            fdVal = IOUtil.fdVal(fd);
            state = 0;
            return;
        }
    }

    public DatagramChannelImpl(SelectorProvider selectorprovider, FileDescriptor filedescriptor)
        throws IOException
    {
        super(selectorprovider);
        readerThread = 0L;
        writerThread = 0L;
        readLock = new Object();
        writeLock = new Object();
        stateLock = new Object();
        state = -1;
        family = Net.isIPv6Available() ? ((ProtocolFamily) (StandardProtocolFamily.INET6)) : ((ProtocolFamily) (StandardProtocolFamily.INET));
        fd = filedescriptor;
        fdVal = IOUtil.fdVal(filedescriptor);
        state = 0;
	//初始化报文socket本地地址
        localAddress = Net.localAddress(filedescriptor);
    }

从上面三个构造方法可以看出，主要是初始化读写线程，及读写锁和状态锁，初始化网络协议family，及报文通道描述符和文件描述id。
总结：
DatagramChannelImpl主要成员有报文socket分发器，这个与SocketChannleImpl中的socket分发器原理基本相同，报文socket分发器可以理解为报文通道的静态代理；网络协议family表示当前报文通道的网络协议family；多播关系注册器MembershipRegistry，
主要是通过一个Map-HashMap<InetAddress,LinkedList<MembershipKeyImpl>>来管理多播组和多播组成员关系key的映射（关系）；通道本地读写线程记录器，及读写锁控制通道读写，一个状态锁，当通道状态改变时，需要获取状态锁。DatagramChannelImpl构造方法，主要是初始化读写线程，及读写锁和状态锁，初始化网络协议family，及报文通道描述符和文件描述id。DatagramChannelImpl(SelectorProvider selectorprovider)与其他两个不同的是构造时更新当前报文socket的数量。
DatagramChannelImpl 解析二（报文发送与接收）：http://donald-draper.iteye.com/blog/2373281