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
ServerSocketChannel定义： http://donald-draper.iteye.com/blog/2369836
ServerSocketChannelImpl解析： http://donald-draper.iteye.com/blog/2370912
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
ByteChannel，分散聚集通道接口的定义（SocketChannel）:
http://donald-draper.iteye.com/blog/2371065
前面一篇文章我们看了一个socket通道继承的接口 ByteChannel， ByteChannel主要是继承了
可读（ReadableByteChannel）可写（WritableByteChannel）通道接口和分散（ScatteringByteChannel）聚集（ScatteringByteChannel）通道接口；可读通道接口，可以从通道读取字节序列写到缓存区；可写通道接口，可以从缓存区读取字节序列写到通道；分散通道可以从通道读取字节序列，写到一组缓存区中，聚集通道可以从一组缓存区读取字节序列，写到通道。今天我们就来看下SocketChannel接口的定义。
package java.nio.channels;
import java.io.IOException;
import java.net.Socket;
import java.net.SocketOption;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.spi.AbstractSelectableChannel;
import java.nio.channels.spi.SelectorProvider;
/**
* A selectable channel for stream-oriented connecting sockets.
*SocketChannel是一个面向流的连接socket可选通道。
* A socket channel is created by invoking one of the {@link #open open}
* methods of this class. It is not possible to create a channel for an arbitrary,
* pre-existing socket. A newly-created socket channel is open but not yet
* connected. An attempt to invoke an I/O operation upon an unconnected
* channel will cause a {@link NotYetConnectedException} to be thrown. A
* socket channel can be connected by invoking its {@link #connect connect}
* method; once connected, a socket channel remains connected until it is
* closed. Whether or not a socket channel is connected may be determined by
* invoking its {@link #isConnected isConnected} method.
*socket通道调用#open方法创建一个socket通道。如果socket已经存在，则不可能强制创建一个通道。
一个新创建的socket通道处于打开状态，但还没有连接。在一个没有连接的通道，调用IO操作，将会引起
一个NotYetConnectedException抛出。socket通道可以调用#connect方法连接一个socket地址；
一旦建立连接，在连接关闭之前，通道都是可连接的。我们可以通道#isConnected方法，判断一个socket通道
是否建立连接。
* A socket
* channel may be created and the process of establishing the link to the
* remote socket may be initiated via the {@link #connect connect} method for
* later completion by the {@link #finishConnect finishConnect} method.
* Whether or not a connection operation is in progress may be determined by
* invoking the {@link #isConnectionPending isConnectionPending} method.
*socket通道支持非阻塞模式连接；一个socket通道已经建立的情况下，调用#connect方法
初始化建立远端socket过程，之后通过#finishConnect完成连接。一个连接操作是否正在进行，
可以通过#isConnectionPending方法判断。
* Socket channels support asynchronous shutdown, which is similar
* to the asynchronous close operation specified in the {@link Channel} class.
* If the input side of a socket is shut down by one thread while another
* thread is blocked in a read operation on the socket's channel, then the read
* operation in the blocked thread will complete without reading any bytes and
* will return -1. If the output side of a socket is shut down by one
* thread while another thread is blocked in a write operation on the socket's
* channel, then the blocked thread will receive an {@link
* AsynchronousCloseException}.
*socket通道支持异步关闭，这个与Channel的关闭操作相似。如果其他线程阻塞在通道的读操作上，
一个线程关闭了socket的输入流，阻塞的读操作线程将会返回-1，即没有读取任何字节。
如果其他线程阻塞在通道的写操作上， 一个线程关闭了socket的输出流，
阻塞的写操作线程将会将会接受到一个AsynchronousCloseException。
*
Socket options are configured using the {@link #setOption(SocketOption,Object)
* setOption} method. Socket channels support the following options:
*
*
* Socket的选项配置通过setOption(SocketOption,Object)方法，Socket channels
支持一下选项配置。
* Option Name
* Description
*
* 发送缓存区大小
* {@link java.net.StandardSocketOptions#SO_SNDBUF SO_SNDBUF}
* The size of the socket send buffer
*
* 接受缓存区大小
* {@link java.net.StandardSocketOptions#SO_RCVBUF SO_RCVBUF}
* The size of the socket receive buffer
*
* 是否保活，心跳
* {@link java.net.StandardSocketOptions#SO_KEEPALIVE SO_KEEPALIVE}
* Keep connection alive
*
* 地址是否可以重用
* {@link java.net.StandardSocketOptions#SO_REUSEADDR SO_REUSEADDR}
* Re-use address
*
* 在通道阻塞模式下，再通道关闭时，如果socket输出流还有数据没发送，则延时等待
数据发送。
* {@link java.net.StandardSocketOptions#SO_LINGER SO_LINGER}
* Linger on close if data is present (when configured in blocking mode
* only)
*
* TCP延时，Disable the Nagle algorithm.具体可以在StandardSocketOptions中查看
* {@link java.net.StandardSocketOptions#TCP_NODELAY TCP_NODELAY}
* Disable the Nagle algorithm
*
*
*
* Additional (implementation specific) options may also be supported.
*
* Socket channels are safe for use by multiple concurrent threads. They
* support concurrent reading and writing, though at most one thread may be
* reading and at most one thread may be writing at any given time. The {@link
* #connect connect} and {@link #finishConnect finishConnect} methods are
* mutually synchronized against each other, and an attempt to initiate a read
* or write operation while an invocation of one of these methods is in
* progress will block until that invocation is complete.
*多线程可以安全地并发访问时socket通道。尽管至多只有一个线程进行读写操作，socket通道
支持并发的读写。#connect和#finishConnect是手动synchronized，在#connect和#finishConnect方法
调用的过程中，尝试读写操作，将会阻塞至#connect和#finishConnect方法结束。
* @author Mark Reinhold
* @author JSR-51 Expert Group
* @since 1.4
*/
public abstract class SocketChannel
extends AbstractSelectableChannel
implements ByteChannel, ScatteringByteChannel, GatheringByteChannel, NetworkChannel
{
/**
* Initializes a new instance of this class.
*/
protected SocketChannel(SelectorProvider provider) {
super(provider);
}
/**
* Opens a socket channel.
*打开一个通道
* The new channel is created by invoking the {@link
* java.nio.channels.spi.SelectorProvider#openSocketChannel
* openSocketChannel} method of the system-wide default {@link
* java.nio.channels.spi.SelectorProvider} object.
*一个通道通过默认的SelectorProvider的openSocketChannel创建
* @return A new socket channel
*
* @throws IOException
* If an I/O error occurs
*/
public static SocketChannel open() throws IOException {
return SelectorProvider.provider().openSocketChannel();
}
/**
* Opens a socket channel and connects it to a remote address.
*打开一个通道，并连接一个远程地址
* This convenience method works as if by invoking the {@link #open()}
* method, invoking the {@link #connect(SocketAddress) connect} method upon
* the resulting socket channel, passing it remote, and then
* returning that channel.
*此方法与open方法基本相同，只是在创建SocketChannel后，调用connect(SocketAddress)
连接远程地址
* @param remote
* The remote address to which the new channel is to be connected
*
* @throws AsynchronousCloseException
* If another thread closes this channel
* while the connect operation is in progress
*
* @throws ClosedByInterruptException
* If another thread interrupts the current thread
* while the connect operation is in progress, thereby
* closing the channel and setting the current thread's
* interrupt status
*
* @throws UnresolvedAddressException
* If the given remote address is not fully resolved
*
* @throws UnsupportedAddressTypeException
* If the type of the given remote address is not supported
*
* @throws SecurityException
* If a security manager has been installed
* and it does not permit access to the given remote endpoint
*
* @throws IOException
* If some other I/O error occurs
*/
public static SocketChannel open(SocketAddress remote)
throws IOException
{
SocketChannel sc = open();
try {
//连接远程地址
sc.connect(remote);
} catch (Throwable x) {
try {
sc.close();
} catch (Throwable suppressed) {
//当一个异常被抛出的时候，可能有其他异常因为该异常而被抑制住，
//从而无法正常抛出。这时可以通过Java 7中为Throwable类增加的addSuppressed方法把这些被抑制的方法记录下来。
//被抑制的异常会出现在抛出的异常的堆栈信息中，
//也可以通过getSuppressed方法来获取这些异常。这样做的好处是不会丢失任何异常，方便开发人员进行调试。
x.addSuppressed(suppressed);
}
throw x;
}
//断言是否连接
assert sc.isConnected();
return sc;
}
/**
* Returns an operation set identifying this channel's supported
* operations.
*返回通道支持的操作事件集，通道支持读写连接操作事件
* Socket channels support connecting, reading, and writing, so this
* method returns ({@link SelectionKey#OP_CONNECT}
* | {@link SelectionKey#OP_READ} | {@link
* SelectionKey#OP_WRITE}).
*
* @return The valid-operation set
*/
public final int validOps() {
return (SelectionKey.OP_READ
| SelectionKey.OP_WRITE
| SelectionKey.OP_CONNECT);
}
// -- Socket-specific operations --
/**
* @throws ConnectionPendingException
* If a non-blocking connect operation is already in progress on
* this channel
* @throws AlreadyBoundException {@inheritDoc}
* @throws UnsupportedAddressTypeException {@inheritDoc}
* @throws ClosedChannelException {@inheritDoc}
* @throws IOException {@inheritDoc}
*地址绑定
* @since 1.7
*/
@Override
public abstract SocketChannel bind(SocketAddress local)
throws IOException;
/**
* @throws UnsupportedOperationException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws ClosedChannelException {@inheritDoc}
* @throws IOException {@inheritDoc}
*数字socket通道选项配置
* @since 1.7
*/
@Override
public abstract SocketChannel setOption(SocketOption name, T value)
throws IOException;
/**
* Shutdown the connection for reading without closing the channel.
*在没有关闭通道的情况下，关闭读操作连接，即不能从通道读取数据，写到缓冲区。
* Once shutdown for reading then further reads on the channel will
* return {@code -1}, the end-of-stream indication. If the input side of the
* connection is already shutdown then invoking this method has no effect.
*读连接关闭，进一步的读操作将会返回-1，表示读到流的末端。如果连接的输入流已经
关闭，调用此方法无效。
* @return The channel
*
* @throws NotYetConnectedException
* If this channel is not yet connected
* @throws ClosedChannelException
* If this channel is closed
* @throws IOException
* If some other I/O error occurs
*
* @since 1.7
*/
public abstract SocketChannel shutdownInput() throws IOException;
/**
* Shutdown the connection for writing without closing the channel.
*在没有关闭通道的情况下，关闭到通道的写操作连接
* Once shutdown for writing then further attempts to write to the
* channel will throw {@link ClosedChannelException}. If the output side of
* the connection is already shutdown then invoking this method has no
* effect.
*在写连接关闭时，进一步尝试从缓冲区读数据，写到通道，将会抛出ClosedChannelException。
如果连接的输出流关闭，则调用此方法无效。
* @return The channel
*
* @throws NotYetConnectedException
* If this channel is not yet connected
* @throws ClosedChannelException
* If this channel is closed
* @throws IOException
* If some other I/O error occurs
*
* @since 1.7
*/
public abstract SocketChannel shutdownOutput() throws IOException;
/**
* Retrieves a socket associated with this channel.
*获取与通道关联的socket
*
The returned object will not declare any public methods that are not
* declared in the {@link java.net.Socket} class.
*返回的socket不会声明任何在socket中没有声明的public方法
* @return A socket associated with this channel
*/
public abstract Socket socket();
/**
* Tells whether or not this channel's network socket is connected.
*判断通道的网络socket是否连接
* @return true if, and only if, this channel's network socket
* is {@link #isOpen open} and connected
当且仅当通道的网络socket打开，或者连接，则返回true
*/
public abstract boolean isConnected();
/**
* Tells whether or not a connection operation is in progress on this
* channel.
*判断一个通道是不是正在进行连接操作。
* @return true if, and only if, a connection operation has been
* initiated on this channel but not yet completed by invoking the
* {@link #finishConnect finishConnect} method
当且仅当一个连接操作正在进行，还没有完成，调用#finishConnect方法，完成连接。
*/
public abstract boolean isConnectionPending();
/**
* Connects this channel's socket.
*连接通道的socket
* If this channel is in non-blocking mode then an invocation of this
* method initiates a non-blocking connection operation. If the connection
* is established immediately, as can happen with a local connection, then
* this method returns true. Otherwise this method returns
* false and the connection operation must later be completed by
* invoking the {@link #finishConnect finishConnect} method.
*如果通道是非阻塞模式，调用此方法将会初始化一个非阻塞的连接操作。如果连接立即
建立，则伴随着一个本地的连接，方法将会返回true。如果连接操作通过调用#finishConnect
方法完成，则方法返回false。
*
If this channel is in blocking mode then an invocation of this
* method will block until the connection is established or an I/O error
* occurs.
*如果通道是阻塞模式的，方法将会阻塞到连接建立或者一个IO操作异常。
*
This method performs exactly the same security checks as the {@link
* java.net.Socket} class. That is, if a security manager has been
* installed then this method verifies that its {@link
* java.lang.SecurityManager#checkConnect checkConnect} method permits
* connecting to the address and port number of the given remote endpoint.
*此方法将进行与java.net.Socket相同的安全检查。如果如果安全管理器配置，这个方法将会
通过java.lang.SecurityManager#checkConnect检查是否允许连接到远程终端的地址和port。
*
This method may be invoked at any time. If a read or write
* operation upon this channel is invoked while an invocation of this
* method is in progress then that operation will first block until this
* invocation is complete. If a connection attempt is initiated but fails,
* that is, if an invocation of this method throws a checked exception,
* then the channel will be closed.
*此方法可以在任何时候调用。如果当前线程调用读写操作时，有其他线程正在进程
读写操作，则当前线程阻塞到其他线程完成读写操作后，方可读写通道。如果连接已经初始化，
单连接失败调用此方法将会抛出一个检查异常，然后关闭通道。
* @param remote
* The remote address to which this channel is to be connected
*
* @return true if a connection was established,
* false if this channel is in non-blocking mode
* and the connection operation is in progress
*
* @throws AlreadyConnectedException
* If this channel is already connected
*
* @throws ConnectionPendingException
* If a non-blocking connection operation is already in progress
* on this channel
*
* @throws ClosedChannelException
* If this channel is closed
*
* @throws AsynchronousCloseException
* If another thread closes this channel
* while the connect operation is in progress
*
* @throws ClosedByInterruptException
* If another thread interrupts the current thread
* while the connect operation is in progress, thereby
* closing the channel and setting the current thread's
* interrupt status
*
* @throws UnresolvedAddressException
* If the given remote address is not fully resolved
*
* @throws UnsupportedAddressTypeException
* If the type of the given remote address is not supported
*
* @throws SecurityException
* If a security manager has been installed
* and it does not permit access to the given remote endpoint
*
* @throws IOException
* If some other I/O error occurs
*/
public abstract boolean connect(SocketAddress remote) throws IOException;
/**
* Finishes the process of connecting a socket channel.
*完成到socket通道的连接任务。
* A non-blocking connection operation is initiated by placing a socket
* channel in non-blocking mode and then invoking its {@link #connect
* connect} method. Once the connection is established, or the attempt has
* failed, the socket channel will become connectable and this method may
* be invoked to complete the connection sequence. If the connection
* operation failed then invoking this method will cause an appropriate
* {@link java.io.IOException} to be thrown.
*在设置socket通道非阻塞模式时，一个非阻塞的连接操作初始化，然后调用#connect方法建立
到通道连接。如果连接已经建立或尝试连接失败，在finishConnect方法调用完成连接工作后，
socket通道将会变得connectable。如果一个连接操作失败，调用此方将会抛出IO异常。
*
If this channel is already connected then this method will not block
* and will immediately return true. If this channel is in
* non-blocking mode then this method will return false if the
* connection process is not yet complete. If this channel is in blocking
* mode then this method will block until the connection either completes
* or fails, and will always either return true or throw a checked
* exception describing the failure.
*如果通道已经连接，则此方法不会阻塞，将会立即返回true。如果通道是非阻塞模式，在连接还没有
建立时，此方法将会返回false。如果通道是阻塞模式，调用finishConnect将会阻塞到连接操作完成
或者失败，返回true或者抛出一个不检查的异常描述失败。
*
This method may be invoked at any time. If a read or write
* operation upon this channel is invoked while an invocation of this
* method is in progress then that operation will first block until this
* invocation is complete. If a connection attempt fails, that is, if an
* invocation of this method throws a checked exception, then the channel
* will be closed.
*此方法可以在任何时候调用。如果当前线程调用读写操作时，有其他线程正在进程
读写操作，则当前线程阻塞到其他线程完成读写操作后，方可读写通道。如果连接已经初始化，
单连接失败调用此方法将会抛出一个检查异常，然后关闭通道。
* @return true if, and only if, this channel's socket is now
* connected
*
* @throws NoConnectionPendingException
* If this channel is not connected and a connection operation
* has not been initiated
*
* @throws ClosedChannelException
* If this channel is closed
*
* @throws AsynchronousCloseException
* If another thread closes this channel
* while the connect operation is in progress
*
* @throws ClosedByInterruptException
* If another thread interrupts the current thread
* while the connect operation is in progress, thereby
* closing the channel and setting the current thread's
* interrupt status
*
* @throws IOException
* If some other I/O error occurs
*/
public abstract boolean finishConnect() throws IOException;
/**
* Returns the remote address to which this channel's socket is connected.
*返回通道socket连接的远端地址。
*
Where the channel is bound and connected to an Internet Protocol
* socket address then the return value from this method is of type {@link
* java.net.InetSocketAddress}.
*在通道绑定地址并连接到一个网络协议的socket地址，此方法返回的值为类型为java.net.InetSocketAddress
* @return The remote address; {@code null} if the channel's socket is not
* connected
*
* @throws ClosedChannelException
* If the channel is closed
* @throws IOException
* If an I/O error occurs
*
* @since 1.7
*/
public abstract SocketAddress getRemoteAddress() throws IOException;
// -- ByteChannel operations --
/**
从通道读取字节序列，写到缓存区
* @throws NotYetConnectedException
* If this channel is not yet connected
*/
public abstract int read(ByteBuffer dst) throws IOException;
/**
从通道读取字节序列，写到一组缓存区
* @throws NotYetConnectedException
* If this channel is not yet connected
*/
public abstract long read(ByteBuffer[] dsts, int offset, int length)
throws IOException;
/**
与read(ByteBuffer[] dsts, int offset, int length)含义相同
* @throws NotYetConnectedException
* If this channel is not yet connected
*/
public final long read(ByteBuffer[] dsts) throws IOException {
return read(dsts, 0, dsts.length);
}
/**
从缓冲区读取字节序列，写到通道
* @throws NotYetConnectedException
* If this channel is not yet connected
*/
public abstract int write(ByteBuffer src) throws IOException;
/**
从一组缓冲区读取字节序列，写到通道
* @throws NotYetConnectedException
* If this channel is not yet connected
*/
public abstract long write(ByteBuffer[] srcs, int offset, int length)
throws IOException;
/**
与方法write(ByteBuffer[] srcs, int offset, int length)无异
* @throws NotYetConnectedException
* If this channel is not yet connected
*/
public final long write(ByteBuffer[] srcs) throws IOException {
return write(srcs, 0, srcs.length);
}
}
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