SocketChannel.java

/*
 * Copyright (c) 2000, 2017, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.nio.channels;

import java.io.IOException;
import java.net.Socket;
import java.net.SocketAddress;
import java.net.SocketOption;
import java.nio.ByteBuffer;
import java.nio.channels.spi.AbstractSelectableChannel;
import java.nio.channels.spi.SelectorProvider;

/**
 * A selectable channel for stream-oriented connecting sockets.
 *
 * <p> 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.
 *
 * <p> Socket channels support <i>non-blocking connection:</i>&nbsp;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.
 *
 * <p> Socket channels support <i>asynchronous shutdown,</i> 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 {@code -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}.
 *
 * <p> Socket options are configured using the {@link #setOption(SocketOption,Object)
 * setOption} method. Socket channels support the following options:
 * <blockquote>
 * <table class="striped">
 * <caption style="display:none">Socket options</caption>
 * <thead>
 *   <tr>
 *     <th scope="col">Option Name</th>
 *     <th scope="col">Description</th>
 *   </tr>
 * </thead>
 * <tbody>
 *   <tr>
 *     <th scope="row"> {@link java.net.StandardSocketOptions#SO_SNDBUF SO_SNDBUF} </th>
 *     <td> The size of the socket send buffer </td>
 *   </tr>
 *   <tr>
 *     <th scope="row"> {@link java.net.StandardSocketOptions#SO_RCVBUF SO_RCVBUF} </th>
 *     <td> The size of the socket receive buffer </td>
 *   </tr>
 *   <tr>
 *     <th scope="row"> {@link java.net.StandardSocketOptions#SO_KEEPALIVE SO_KEEPALIVE} </th>
 *     <td> Keep connection alive </td>
 *   </tr>
 *   <tr>
 *     <th scope="row"> {@link java.net.StandardSocketOptions#SO_REUSEADDR SO_REUSEADDR} </th>
 *     <td> Re-use address </td>
 *   </tr>
 *   <tr>
 *     <th scope="row"> {@link java.net.StandardSocketOptions#SO_LINGER SO_LINGER} </th>
 *     <td> Linger on close if data is present (when configured in blocking mode
 *          only) </td>
 *   </tr>
 *   <tr>
 *     <th scope="row"> {@link java.net.StandardSocketOptions#TCP_NODELAY TCP_NODELAY} </th>
 *     <td> Disable the Nagle algorithm </td>
 *   </tr>
 * </tbody>
 * </table>
 * </blockquote>
 * Additional (implementation specific) options may also be supported.
 *
 * <p> 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.  </p>
 *
 * @author Mark Reinhold
 * @author JSR-51 Expert Group
 * @since 1.4
 */
// Socket通道，支持在非阻塞模式下运行
public abstract class SocketChannel extends AbstractSelectableChannel implements ByteChannel, ScatteringByteChannel, GatheringByteChannel, NetworkChannel {
    
    /*▼ 构造器 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Initializes a new instance of this class.
     *
     * @param provider The provider that created this channel
     */
    protected SocketChannel(SelectorProvider provider) {
        super(provider);
    }
    
    /*▲ 构造器 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Opens a socket channel.
     *
     * <p> 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.  </p>
     *
     * @return A new socket channel
     *
     * @throws IOException If an I/O error occurs
     */
    // 构造一个未绑定的[客户端Socket]，内部初始化了该Socket的文件描述符
    public static SocketChannel open() throws IOException {
        return SelectorProvider.provider().openSocketChannel();
    }
    
    /**
     * Opens a socket channel and connects it to a remote address.
     *
     * <p> 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 {@code remote}, and then
     * returning that channel.  </p>
     *
     * @param remote The remote address to which the new channel is to be connected
     *
     * @return A new, and connected, socket channel
     *
     * @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
     */
    // 构造一个[客户端Socket]，并将其连接到远端
    public static SocketChannel open(SocketAddress remote) throws IOException {
        // 构造一个未绑定的[客户端Socket]，内部初始化了该Socket的文件描述符
        SocketChannel socketChannel = open();
        
        try {
            // 对[客户端Socket]执行【connect】操作，以便连接到远端Socket
            socketChannel.connect(remote);
        } catch(Throwable e) {
            try {
                socketChannel.close();
            } catch(Throwable suppressed) {
                e.addSuppressed(suppressed);
            }
            
            throw e;
        }
        
        assert socketChannel.isConnected();
        
        // 建立连接后，会为其自动绑定本地地址
        return socketChannel;
    }
    
    /*▲ 工厂方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 适配 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Retrieves a socket associated with this channel.
     *
     * <p> The returned object will not declare any public methods that are not
     * declared in the {@link java.net.Socket} class.  </p>
     *
     * @return A socket associated with this channel
     */
    // 返回由当前Socket通道适配而成的Socket
    public abstract Socket socket();
    
    /*▲ 适配 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ socket操作 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * @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}
     * @throws SecurityException               If a security manager has been installed and its
     *                                         {@link SecurityManager#checkListen checkListen} method denies
     *                                         the operation
     * @since 1.7
     */
    // 对[客户端Socket]执行【bind】操作；local为null时，使用通配IP和随机端口
    @Override
    public abstract SocketChannel bind(SocketAddress local) throws IOException;
    
    /**
     * Connects this channel's socket.
     *
     * <p> 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 {@code true}.  Otherwise this method returns
     * {@code false} and the connection operation must later be completed by
     * invoking the {@link #finishConnect finishConnect} method.
     *
     * <p> 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.
     *
     * <p> 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.
     *
     * <p> 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.  </p>
     *
     * @param remote The remote address to which this channel is to be connected
     *
     * @return {@code true} if a connection was established,
     * {@code 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
     */
    /*
     * 对[客户端Socket]执行【connect】操作，以便连接到远端Socket
     *
     * 在非阻塞模式下：如果成功建立了连接，则返回true；如果连接失败，则返回false，并且需要后续调用finishConnect()来完成连接操作。
     * 在阻塞模式下，则连接操作会陷入阻塞，直到成功建立连接，或者发生IO异常。
     *
     * 在连接过程中发起的读写操作将被阻塞，直到连接完成
     */
    public abstract boolean connect(SocketAddress remote) throws IOException;
    
    /**
     * Finishes the process of connecting a socket channel.
     *
     * <p> 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.
     *
     * <p> If this channel is already connected then this method will not block
     * and will immediately return {@code true}.  If this channel is in
     * non-blocking mode then this method will return {@code 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 {@code true} or throw a checked
     * exception describing the failure.
     *
     * <p> 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.  </p>
     *
     * @return {@code 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
     */
    /*
     * 促进当前Socket完成与远程的连接
     *
     * 在非阻塞模式下：如果成功建立了连接，则返回true；如果连接失败，则返回false
     * 在阻塞模式下 ：不断检查连接是否成功，直到连接成功建立，或发生IO异常。
     */
    public abstract boolean finishConnect() throws IOException;
    
    /*▲ socket操作 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 关闭 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Shutdown the connection for reading without closing the channel.
     *
     * <p> 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.
     *
     * @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
     */
    // 关闭从当前通道读取数据的功能
    public abstract SocketChannel shutdownInput() throws IOException;
    
    /**
     * Shutdown the connection for writing without closing the channel.
     *
     * <p> 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.
     *
     * @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
     */
    // 关闭向当前通道写入数据的功能
    public abstract SocketChannel shutdownOutput() throws IOException;
    
    /*▲ 关闭 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 读/写操作 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * @throws NotYetConnectedException If this channel is not yet connected
     */
    // 从当前通道中读取数据，读到的内容写入dst
    public abstract int read(ByteBuffer dst) throws IOException;
    
    /**
     * @throws NotYetConnectedException If this channel is not yet connected
     */
    //【散射】从当前通道中读取数据，读到的内容依次写入dsts中offset处起的length个缓冲区
    public abstract long read(ByteBuffer[] dsts, int offset, int length) throws IOException;
    
    /**
     * @throws NotYetConnectedException If this channel is not yet connected
     */
    //【散射】从当前通道中读取数据，读到的内容依次写入dsts中的各个缓冲区
    public final long read(ByteBuffer[] dsts) throws IOException {
        return read(dsts, 0, dsts.length);
    }
    
    
    /**
     * @throws NotYetConnectedException If this channel is not yet connected
     */
    // 从src中读取数据，读到的内容向当前通道中写入
    public abstract int write(ByteBuffer src) throws IOException;
    
    /**
     * @throws NotYetConnectedException If this channel is not yet connected
     */
    //【聚集】从srcs中offset处起的length个缓冲区读取数据，读到的内容向当前通道中写入
    public abstract long write(ByteBuffer[] srcs, int offset, int length) throws IOException;
    
    /**
     * @throws NotYetConnectedException If this channel is not yet connected
     */
    //【聚集】从srcs中各个缓冲区读取数据，读到的内容向当前通道中写入
    public final long write(ByteBuffer[] srcs) throws IOException {
        return write(srcs, 0, srcs.length);
    }
    
    /*▲ 读/写操作 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ Socket操作参数 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Returns an operation set identifying this channel's supported
     * operations.
     *
     * <p> Socket channels support connecting, reading, and writing, so this
     * method returns {@code (}{@link SelectionKey#OP_CONNECT}
     * {@code |}&nbsp;{@link SelectionKey#OP_READ} {@code |}&nbsp;{@link
     * SelectionKey#OP_WRITE}{@code )}.
     *
     * @return The valid-operation set
     */
    // 返回当前通道允许的有效操作参数集
    public final int validOps() {
        return (SelectionKey.OP_READ | SelectionKey.OP_WRITE | SelectionKey.OP_CONNECT);
    }
    
    /*▲ Socket操作参数 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ Socket配置参数 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws ClosedChannelException        {@inheritDoc}
     * @throws IOException                   {@inheritDoc}
     * @since 1.7
     */
    // 设置指定名称的Socket配置参数
    @Override
    public abstract <T> SocketChannel setOption(SocketOption<T> name, T value) throws IOException;
    
    /*▲ Socket配置参数 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 状态 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Tells whether or not a connection operation is in progress on this
     * channel.
     *
     * @return {@code 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
     */
    // 判断当前通道是否正在尝试连接，但还未连接上
    public abstract boolean isConnectionPending();
    
    /**
     * Tells whether or not this channel's network socket is connected.
     *
     * @return {@code true} if, and only if, this channel's network socket
     * is {@link #isOpen open} and connected
     */
    // 判断当前通道是否已建立连接
    public abstract boolean isConnected();
    
    /*▲ 状态 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 地址信息 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * {@inheritDoc}
     * <p>
     * If there is a security manager set, its {@code checkConnect} method is
     * called with the local address and {@code -1} as its arguments to see
     * if the operation is allowed. If the operation is not allowed,
     * a {@code SocketAddress} representing the
     * {@link java.net.InetAddress#getLoopbackAddress loopback} address and the
     * local port of the channel's socket is returned.
     *
     * @return The {@code SocketAddress} that the socket is bound to, or the
     * {@code SocketAddress} representing the loopback address if
     * denied by the security manager, or {@code null} if the
     * channel's socket is not bound
     *
     * @throws ClosedChannelException {@inheritDoc}
     * @throws IOException            {@inheritDoc}
     */
    // 获取绑定的本地地址
    @Override
    public abstract SocketAddress getLocalAddress() throws IOException;
    
    /**
     * Returns the remote address to which this channel's socket is connected.
     *
     * <p> 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}.
     *
     * @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
     */
    // 获取连接到的远程地址
    public abstract SocketAddress getRemoteAddress() throws IOException;
    
    /*▲ 地址信息 ████████████████████████████████████████████████████████████████████████████████┛ */
    
}