Socket是什么

socket起源于Unix，可以理解成一个文件，可以执行“读，写，打开，关闭”等操作，实际上是对TCP/IP协议的封装，太复杂，这篇简单看下Java层的实现。
Java的Socket默认是TCP协议，会建立一个稳定的长连接。

Socket的生命周期

Socket的构造过程像是工厂方法模式，Socket是一个抽象产品，具体产品是在SocketImpl类实现的。不同工厂可以根据需求实现不一样的具体产品，比如SocksSocketImpl，DualStackPlainSocketImpl等都是SocketImpl的子类。

    /**
     * Sets impl to the system-default type of SocketImpl.
     * @since 1.4
     */
    void setImpl() {
        if (factory != null) {
            impl = factory.createSocketImpl();
            checkOldImpl();
        } else {
            // No need to do a checkOldImpl() here, we know it's an up to date
            // SocketImpl!
            impl = new SocksSocketImpl();
        }
        if (impl != null)
            impl.setSocket(this);
    }

Java Socket通信.png

1. create()

    /**
     * Creates a socket with a boolean that specifies whether this
     * is a stream socket (true) or an unconnected UDP socket (false).
     */
    protected synchronized void create(boolean stream) throws IOException {
        this.stream = stream;
        if (!stream) {
            ResourceManager.beforeUdpCreate();
            // only create the fd after we know we will be able to create the socket
            fd = new FileDescriptor();
            try {
                socketCreate(false);
            } catch (IOException ioe) {
                ResourceManager.afterUdpClose();
                fd = null;
                throw ioe;
            }
        } else {
            fd = new FileDescriptor();
            socketCreate(true);
        }
        if (socket != null)
            socket.setCreated();
        if (serverSocket != null)
            serverSocket.setCreated();
    }

create()方法创建文件描述符（FileDescriptor），后续可以对这个socket进行读写操作。
传入的参数boolean stream用来区分协议，true标识TCP协议，false标识UDP协议。
文件描述符只能在本机标识这个socket，要进行网络通信，还需要进一步执行bind()。

2. bind()

    /**
     * Binds the socket to a local address.
     * <P>
     * If the address is {@code null}, then the system will pick up
     * an ephemeral port and a valid local address to bind the socket.
     *
     * @param   bindpoint the {@code SocketAddress} to bind to
     * @throws  IOException if the bind operation fails, or if the socket
     *                     is already bound.
     * @throws  IllegalArgumentException if bindpoint is a
     *          SocketAddress subclass not supported by this socket
     * @throws  SecurityException  if a security manager exists and its
     *          {@code checkListen} method doesn't allow the bind
     *          to the local port.
     *
     * @since   1.4
     * @see #isBound
     */
    public void bind(SocketAddress bindpoint) throws IOException {
        if (isClosed())
            throw new SocketException("Socket is closed");
        if (!oldImpl && isBound())
            throw new SocketException("Already bound");

        if (bindpoint != null && (!(bindpoint instanceof InetSocketAddress)))
            throw new IllegalArgumentException("Unsupported address type");
        InetSocketAddress epoint = (InetSocketAddress) bindpoint;
        if (epoint != null && epoint.isUnresolved())
            throw new SocketException("Unresolved address");
        if (epoint == null) {
            epoint = new InetSocketAddress(0);
        }
        InetAddress addr = epoint.getAddress();
        int port = epoint.getPort();
        checkAddress (addr, "bind");
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkListen(port);
        }
        getImpl().bind (addr, port);
        bound = true;
    }

bind()方法传入一个SocketAddress类型，给这个socket绑定一个地址，不考虑协议的情况下，需要IP地址+端口才能唯一标识一个socket。
在没有指定地址的情况下，系统会暂时分配一个空闲的端口给socket进行连接。
bind()方法是在Impl类实现的，可以看下socketBind()方法，具体实现应该是在native层。

3. connect()

如果是客户端，下一步会执行connet()方法尝试与服务端连接。

    /**
     * Connects this socket to the server with a specified timeout value.
     * A timeout of zero is interpreted as an infinite timeout. The connection
     * will then block until established or an error occurs.
     *
     * @param   endpoint the {@code SocketAddress}
     * @param   timeout  the timeout value to be used in milliseconds.
     * @throws  IOException if an error occurs during the connection
     * @throws  SocketTimeoutException if timeout expires before connecting
     * @throws  java.nio.channels.IllegalBlockingModeException
     *          if this socket has an associated channel,
     *          and the channel is in non-blocking mode
     * @throws  IllegalArgumentException if endpoint is null or is a
     *          SocketAddress subclass not supported by this socket
     * @since 1.4
     * @spec JSR-51
     */
    public void connect(SocketAddress endpoint, int timeout) throws IOException {
        if (endpoint == null)
            throw new IllegalArgumentException("connect: The address can't be null");

        if (timeout < 0)
          throw new IllegalArgumentException("connect: timeout can't be negative");

        if (isClosed())
            throw new SocketException("Socket is closed");

        if (!oldImpl && isConnected())
            throw new SocketException("already connected");

        if (!(endpoint instanceof InetSocketAddress))
            throw new IllegalArgumentException("Unsupported address type");

        InetSocketAddress epoint = (InetSocketAddress) endpoint;
        InetAddress addr = epoint.getAddress ();
        int port = epoint.getPort();
        checkAddress(addr, "connect");

        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            if (epoint.isUnresolved())
                security.checkConnect(epoint.getHostName(), port);
            else
                security.checkConnect(addr.getHostAddress(), port);
        }
        if (!created)
            createImpl(true);
        if (!oldImpl)
            impl.connect(epoint, timeout);
        else if (timeout == 0) {
            if (epoint.isUnresolved())
                impl.connect(addr.getHostName(), port);
            else
                impl.connect(addr, port);
        } else
            throw new UnsupportedOperationException("SocketImpl.connect(addr, timeout)");
        connected = true;
        /*
         * If the socket was not bound before the connect, it is now because
         * the kernel will have picked an ephemeral port & a local address
         */
        bound = true;
    }

connect()方法传入两个参数，SocketAddress代表目标地址，timeout代表超时时间，单位是毫秒，默认是0。
这个地址要跟上一个bind()方法区分，bind传入的是本地要绑定的地址，connect()传入的是要连接的服务端地址。具体实现也是在native层。

4. listen()

如果是服务端，下一步是调用listen()方法，进入被动监听，等待连接请求的过程。

    /**
     * Sets the maximum queue length for incoming connection indications
     * (a request to connect) to the {@code count} argument. If a
     * connection indication arrives when the queue is full, the
     * connection is refused.
     *
     * @param      backlog   the maximum length of the queue.
     * @exception  IOException  if an I/O error occurs when creating the queue.
     */
    protected abstract void listen(int backlog) throws IOException;

传入的参数backlog表示请求队列的最大长度。这部分也是native代码，具体可以看下socketListen()方法。
下一步需要对客户端的请求进行接收和处理。

5. accept()

accept()方法对客户端请求进行接收，具体实现在Impl类，具体可以看下socketAccept()里面的native调用。

    /**
     * Listens for a connection to be made to this socket and accepts
     * it. The method blocks until a connection is made.
     *
     * <p>A new Socket {@code s} is created and, if there
     * is a security manager,
     * the security manager's {@code checkAccept} method is called
     * with {@code s.getInetAddress().getHostAddress()} and
     * {@code s.getPort()}
     * as its arguments to ensure the operation is allowed.
     * This could result in a SecurityException.
     *
     * @exception  IOException  if an I/O error occurs when waiting for a
     *               connection.
     * @exception  SecurityException  if a security manager exists and its
     *             {@code checkAccept} method doesn't allow the operation.
     * @exception  SocketTimeoutException if a timeout was previously set with setSoTimeout and
     *             the timeout has been reached.
     * @exception  java.nio.channels.IllegalBlockingModeException
     *             if this socket has an associated channel, the channel is in
     *             non-blocking mode, and there is no connection ready to be
     *             accepted
     *
     * @return the new Socket
     * @see SecurityManager#checkAccept
     * @revised 1.4
     * @spec JSR-51
     */
    public Socket accept() throws IOException {
        if (isClosed())
            throw new SocketException("Socket is closed");
        if (!isBound())
            throw new SocketException("Socket is not bound yet");
        Socket s = new Socket((SocketImpl) null);
        implAccept(s);
        return s;
    }

在没有请求的情况下，线程会阻塞等待，有客户端请求才会唤醒。

6. close()

当发生异常或者通信结束，调用close()方法关闭socket。

protected void close() throws IOException {
        synchronized(fdLock) {
            if (fd != null) {
                if (!stream) {
                    ResourceManager.afterUdpClose();
                }
                if (fdUseCount == 0) {
                    if (closePending) {
                        return;
                    }
                    closePending = true;
                    /*
                     * We close the FileDescriptor in two-steps - first the
                     * "pre-close" which closes the socket but doesn't
                     * release the underlying file descriptor. This operation
                     * may be lengthy due to untransmitted data and a long
                     * linger interval. Once the pre-close is done we do the
                     * actual socket to release the fd.
                     */
                    try {
                        socketPreClose();
                    } finally {
                        socketClose();
                    }
                    fd = null;
                    return;
                } else {
                    /*
                     * If a thread has acquired the fd and a close
                     * isn't pending then use a deferred close.
                     * Also decrement fdUseCount to signal the last
                     * thread that releases the fd to close it.
                     */
                    if (!closePending) {
                        closePending = true;
                        fdUseCount--;
                        socketPreClose();
                    }
                }
            }
        }
    }