源码解读：socket编程(SocketImpl)

简介

“一切皆Socket！”

话虽些许夸张，但是事实也是，现在的网络编程几乎都是用的socket。

网络中的进程是通过socket来通信的，那什么是socket呢？
> socket起源于Unix，而Unix/Linux基本哲学之一就是“一切皆文件”，都可以用“打开open –> 读写write/read –> 关闭close”模式来操作。
> 我的理解就是Socket就是该模式的一个实现，socket即是一种特殊的文件，一些socket函数就是对其进行的操作（读/写IO、打开、关闭），这些函数我们在后面进行介绍。

类图

Socketoptions接口

该接口定义了socket编程里的基本方法，该接口被SocketImpl和DataGramSocketImpl所实现。

声明成员

修饰符	类型	成员名	作用
@Native public static final	int	TCP_NODELAY	0x0001
@Native public static final	int	SO_BINDADDR	0x000F
@Native public static final	int	SO_REUSEADDR	0x04
@Native public static final	int	SO_REUSEPORT	0x0E
@Native public static final	int	SO_BROADCAST	0x0020
@Native public static final	int	IP_MULTICAST_IF	0x10
@Native public static final	int	IP_MULTICAST_IF2	0x1f
@Native public static final	int	IP_MULTICAST_LOOP	0x12
@Native public static final	int	IP_TOS	0x3
@Native public static final	int	SO_LINGER	0x0080
@Native public static final	int	SO_TIMEOUT	0x1006
@Native public static final	int	SO_SNDBUF	0x1001
@Native public static final	int	SO_RCVBUF	0x1002
@Native public static final	int	SO_KEEPALIVE	0x0008
@Native public static final	int	SO_OOBINLINE	0x1003
private static final	Set<SocketOption<?>>	socketOptions
private static final	Set<ServerSocketOption<?>>	ServerSocketOptions

提供了两个方法来设置和获取这些键值

public void setOption(int optID, Object value) throws SocketException;

public Object getOption(int optID) throws SocketException;

成员变量及方法

修饰符	类型	变量名	默认值	说明
default	Socket	socket	null	实际socket对象
default	ServerSocket	serverSocket	null	实际serverSocket对象
protected	FileDesceriptor	fd		文件描述器
protected	InetAddress	address		远端主机地址
protected	int	port		远端主机端口
protected	int	localPort		本机地址

方法

//根据是否是stream创建Socket或DataGramSocket
protected abstract void create(boolean stream) throws IOException;

//根据远端主机地址和端口进行连接
protected abstract void connect(String host, int port) throws IOException;

protected abstract void connect(InetAddress address, int port) throws IOException;

//指定套接字地址和超时时间进行连接远端主机
protected abstract void connect(SocketAddress address, int timeout) throws IOException;

//监听本地指定地址和端口
protected abstract void bind(InetAddress host, int port) throws IOException;

//设定最大连接数
protected abstract void listen(int backlog) throws IOException;

//接受即将到来的连接请求，调用该方法时进入线程阻塞状态
protected abstract void accept(SocketImpl s) throws IOException;

//获取远端发送的字节流
protected abstract InputStream getInputStream() throws IOException;

//获取用于向远端发送数据的字节流
protected abstract OutputStream getOutputStream() throws IOException;

//判断该socket是否可用
protected abstract int available() throws IOException;

//关闭跟远端建立的连接
protected abstract void close() throws IOException;

//停掉当前连接，但不断开当前连接，调用该方法后任何发送到该socket的数据都不会被处理。
protected void shutdownInput() throws IOException {
  throw new IOException("Method not implemented!");
}

/**
 * @see java.net.Socket#shutdownOutput()
 * 与上面的方法类似，任何输出输出数据都不会成功，并抛出异常
 */
protected void shutdownOutput() throws IOException {
  throw new IOException("Method not implemented!");
}

protected FileDescriptor getFileDescriptor() {
    return fd;
}

protected InetAddress getInetAddress() {
    return address;
}

protected int getPort() {
    return port;
}

//判断是否支持发送UrgentData,语义暂时不明
protected boolean supportsUrgentData () {
    return false; // must be overridden in sub-class
}

protected abstract void sendUrgentData (int data) throws IOException;

protected int getLocalPort() {
    return localport;
}

public String toString() {
    return "Socket[addr=" + getInetAddress() +
        ",port=" + getPort() + ",localport=" + getLocalPort()  + "]";
}

socketOptions和ServerSocketServer初始化时机

 static {
     socketOptions = Set.of(StandardSocketOptions.SO_KEEPALIVE,
                            StandardSocketOptions.SO_SNDBUF,
                            StandardSocketOptions.SO_RCVBUF,
                            StandardSocketOptions.SO_REUSEADDR,
                            StandardSocketOptions.SO_LINGER,
                            StandardSocketOptions.IP_TOS,
                            StandardSocketOptions.TCP_NODELAY);

     serverSocketOptions = Set.of(StandardSocketOptions.SO_RCVBUF,
                                  StandardSocketOptions.SO_REUSEADDR,
                                  StandardSocketOptions.IP_TOS);
 }
 
protected Set<SocketOption<?>> supportedOptions() {
     if (getSocket() != null) {
         return socketOptions;
     } else {
         return serverSocketOptions;
     }
 }