《Thinking in Enterprise Java》中第一章描述了用Socket和Channel的网络编程，核心即为Socket和Channel，本文简单讲述Socket的应用。
Socket可以认为是两个互联机器终端应用软件的抽象，即对于一个网络连接，两端都有一个Socket，应用可以通过套接字进行交互通信。
在Java中，创建Socket连接另一台机器，可以从Socket中获取InputStream和OutputStream，将其作为输入输出流，使应用程序与操作本地文件IO类似。存在2个基于流的Socket类：ServerSocket和Socket。
- ServerSocket用于服务器端，监听客户端连接
- Socket用于客户端与服务端交互
- 服务段accept()方法处于阻塞状态，直到有客户端连接，创建一个服务端Socket，与客户端交互
另外，当创建ServerSocket时，只需要提供一个端口号，IP信息为本机默认信息；创建Socket时，必须提供IP和端口号；由ServerSocket.accept( )创建的不需要，其已包含所有信息。
1. 简单客户端和服务端
服务器端：
import java.io.BufferedReader; import java.io.BufferedWriter; import java.io.IOException; import java.io.InputStreamReader; import java.io.OutputStreamWriter; import java.io.PrintWriter; import java.net.ServerSocket; import java.net.Socket; public class JabberServer { public static final int PORT = 8080; public static void main(String[] args) throws IOException{ ServerSocket server = new ServerSocket(PORT); System.out.println("开始： " + server); try { Socket socket = server.accept(); System.out.println("Connection socket: " + socket); try { BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream())); //Output is automatically flushed by PrintWrite PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())),true); while(true) { String str = in.readLine(); if("END".equals(str)) break; System.out.println("Echoing: " + str); out.println(str); } } finally { System.out.println("CLosing...."); socket.close(); } } finally { server.close(); } } }
输出：
开始： ServerSocket[addr=0.0.0.0/0.0.0.0,localport=8080]
大致步骤：
- 创建ServerSocket，绑定端口8080
- 调accept()方法监听连接，并返回套接字Socket
- 获取输入流，并通过InputStreamReader转为字符，缓存在BufferdReader中
- 获取输出流，通过OutputStreamWriter将BufferedWriter中的字符转换为字节，并通过PrintWriter格式化输出，同时自动flush
- 根据输入流读取的字符，如果是END则结束会话
- 关闭套接字和ServerSocket
客户端：
import java.io.BufferedReader; import java.io.BufferedWriter; import java.io.InputStreamReader; import java.io.OutputStreamWriter; import java.io.PrintWriter; import java.net.InetAddress; import java.net.Socket; /** * 根据服务器ip和端口/服务器地址等，创建Socket * Socket可以获取输入和输出流，默认是使用AbstractPlainSocketImpl类中的SocketInputStream和SocketOutputStream * */ public class JabberClient { public static void main(String[] args) throws Exception{ //服务器端信息，address和8080；后台连接服务器，还会绑定客户端 InetAddress address = InetAddress.getByName(null); System.out.println("address = " + address); Socket socket = new Socket(address, 8080); try { System.out.println("Socket = " +socket); BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream())); PrintWriter out = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())),true); for(int i = 0; i < 10; i++) { out.println("hello " + i); String str = in.readLine(); System.out.println(str); } out.println("END"); } finally { System.out.println("Closing socket..."); socket.close(); } } }
大致步骤与服务端相似，不同之处在于创建客户端套接字，需要指定服务端地址和端口号。
服务器端输出：
Connection socket: Socket[addr=/127.0.0.1,port=35702,localport=8080] Echoing: hello 0 Echoing: hello 1 Echoing: hello 2 Echoing: hello 3 Echoing: hello 4 Echoing: hello 5 Echoing: hello 6 Echoing: hello 7 Echoing: hello 8 Echoing: hello 9 CLosing....
客户端输出：
address = localhost/127.0.0.1 Socket = Socket[addr=localhost/127.0.0.1,port=8080,localport=35702] hello 0 hello 1 hello 2 hello 3 hello 4 hello 5 hello 6 hello 7 hello 8 hello 9 Closing socket...
2. 源码分析
无论是创建ServerSocket还是Socket，都需要与底层实现SocketImpl关联，以实现具体网络交互的逻辑。
（1）服务端创建ServerSocket
ServerSocket server = new ServerSocket(PORT);具体构造过程如下，构造参数为服务端监听端口：
/** * Creates a server socket, bound to the specified port. A port number * of {@code 0} means that the port number is automatically * allocated, typically from an ephemeral port range. This port * number can then be retrieved by calling {@link #getLocalPort getLocalPort}. * <p> * The maximum queue length for incoming connection indications (a * request to connect) is set to {@code 50}. If a connection * indication arrives when the queue is full, the connection is refused. * <p> * If the application has specified a server socket factory, that * factory's {@code createSocketImpl} method is called to create * the actual socket implementation. Otherwise a "plain" socket is created. * <p> * If there is a security manager, * its {@code checkListen} method is called * with the {@code port} argument * as its argument to ensure the operation is allowed. * This could result in a SecurityException. * * * @param port the port number, or {@code 0} to use a port * number that is automatically allocated. * * @exception IOException if an I/O error occurs when opening the socket. * @exception SecurityException * if a security manager exists and its {@code checkListen} * method doesn't allow the operation. * @exception IllegalArgumentException if the port parameter is outside * the specified range of valid port values, which is between * 0 and 65535, inclusive. * * @see java.net.SocketImpl * @see java.net.SocketImplFactory#createSocketImpl() * @see java.net.ServerSocket#setSocketFactory(java.net.SocketImplFactory) * @see SecurityManager#checkListen */ public ServerSocket(int port) throws IOException { this(port, 50, null); } /** * Create a server with the specified port, listen backlog, and * local IP address to bind to. The <i>bindAddr</i> argument * can be used on a multi-homed host for a ServerSocket that * will only accept connect requests to one of its addresses. * If <i>bindAddr</i> is null, it will default accepting * connections on any/all local addresses. * The port must be between 0 and 65535, inclusive. * A port number of {@code 0} means that the port number is * automatically allocated, typically from an ephemeral port range. * This port number can then be retrieved by calling * {@link #getLocalPort getLocalPort}. * * <P>If there is a security manager, this method * calls its {@code checkListen} method * with the {@code port} argument * as its argument to ensure the operation is allowed. * This could result in a SecurityException. * * The {@code backlog} argument is the requested maximum number of * pending connections on the socket. Its exact semantics are implementation * specific. In particular, an implementation may impose a maximum length * or may choose to ignore the parameter altogther. The value provided * should be greater than {@code 0}. If it is less than or equal to * {@code 0}, then an implementation specific default will be used. * <P> * @param port the port number, or {@code 0} to use a port * number that is automatically allocated. * @param backlog requested maximum length of the queue of incoming * connections. * @param bindAddr the local InetAddress the server will bind to * * @throws SecurityException if a security manager exists and * its {@code checkListen} method doesn't allow the operation. * * @throws IOException if an I/O error occurs when opening the socket. * @exception IllegalArgumentException if the port parameter is outside * the specified range of valid port values, which is between * 0 and 65535, inclusive. * * @see SocketOptions * @see SocketImpl * @see SecurityManager#checkListen * @since JDK1.1 */ public ServerSocket(int port, int backlog, InetAddress bindAddr) throws IOException { setImpl(); if (port < 0 || port > 0xFFFF) throw new IllegalArgumentException( "Port value out of range: " + port); if (backlog < 1) backlog = 50; try { bind(new InetSocketAddress(bindAddr, port), backlog); } catch(SecurityException e) { close(); throw e; } catch(IOException e) { close(); throw e; } }
实际调用的构造方法为ServerSocket(int port, int backlog, InetAddress bindAddr)，port表示端口号，backlog表示服务端请求连接队列最大数（默认为50），bindAddr表示服务器要绑定的本地地址（默认为null）。
- setImpl()，设置系统默认类型SocketImpl，其是服务端和客户端套接字创建、连接、交互等操作的核心
- bind(new InetSocketAddress(bindAddr, port), backlog)，绑定对应的地址和端口，并设置最大连接数（超过连接数，服务器拒绝连接）
（2）服务端accept
Socket socket = server.accept();
服务端会阻塞等待客户端连接，直到有客户端连接，并创建一个服务端Socket，与客户端交互。
/** * 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; }
① 指定SocketImpl，创建一个非连接的Socket构造方法，如下：
/** * Creates an unconnected Socket with a user-specified * SocketImpl. * <P> * @param impl an instance of a <B>SocketImpl</B> * the subclass wishes to use on the Socket. * * @exception SocketException if there is an error in the underlying protocol, * such as a TCP error. * @since JDK1.1 */ protected Socket(SocketImpl impl) throws SocketException { this.impl = impl; if (impl != null) { checkOldImpl(); this.impl.setSocket(this); } }
② implAccept(s)方法
上一步创建Socket的参数SocketImpl为null，该方法为Socket创建具体的SocketImpl，绑定地址和文件描述符，具体可见源码。
（3）客户端创建套接字
Socket socket = new Socket(address, 8080);具体创建过程如下，构造参数为InetAddress和port：
/** * Creates a stream socket and connects it to the specified port * number at the specified IP address. * <p> * If the application has specified a socket factory, that factory's * {@code createSocketImpl} method is called to create the * actual socket implementation. Otherwise a "plain" socket is created. * <p> * If there is a security manager, its * {@code checkConnect} method is called * with the host address and {@code port} * as its arguments. This could result in a SecurityException. * * @param address the IP address. * @param port the port number. * @exception IOException if an I/O error occurs when creating the socket. * @exception SecurityException if a security manager exists and its * {@code checkConnect} method doesn't allow the operation. * @exception IllegalArgumentException if the port parameter is outside * the specified range of valid port values, which is between * 0 and 65535, inclusive. * @exception NullPointerException if {@code address} is null. * @see java.net.Socket#setSocketImplFactory(java.net.SocketImplFactory) * @see java.net.SocketImpl * @see java.net.SocketImplFactory#createSocketImpl() * @see SecurityManager#checkConnect */ public Socket(InetAddress address, int port) throws IOException { this(address != null ? new InetSocketAddress(address, port) : null, (SocketAddress) null, true); } private Socket(SocketAddress address, SocketAddress localAddr, boolean stream) throws IOException { setImpl(); // backward compatibility if (address == null) throw new NullPointerException(); try { createImpl(stream); if (localAddr != null) bind(localAddr); connect(address); } catch (IOException | IllegalArgumentException | SecurityException e) { try { close(); } catch (IOException ce) { e.addSuppressed(ce); } throw e; } }
- 首先，setImpl()，与服务端相似，设置系统默认类型SocketImpl，其是服务端和客户端套接字创建、连接、交互等操作的核心
- 其次，createImpl(stream)，根据stream布尔值创建socket实现，true时创建基于流的socket（或者面向连接），false时创建无连接UDP套接字
- 最后，connect(address)，连接服务器，连接一直处于阻塞状态，直到连接成功，或者超时或报错等
（4）SocketImpl类
SocketImpl类是服务器和客户端连接的核心，源码如下，包含Socket、ServerSocket、文件描述符、IP地址、端口号和套接字连接的本地端口号：
/** * The abstract class {@code SocketImpl} is a common superclass * of all classes that actually implement sockets. It is used to * create both client and server sockets. * <p> * A "plain" socket implements these methods exactly as * described, without attempting to go through a firewall or proxy. * * @author unascribed * @since JDK1.0 */ public abstract class SocketImpl implements SocketOptions { /** * The actual Socket object. */ Socket socket = null; ServerSocket serverSocket = null; /** * The file descriptor object for this socket. */ protected FileDescriptor fd; /** * The IP address of the remote end of this socket. */ protected InetAddress address; /** * The port number on the remote host to which this socket is connected. */ protected int port; /** * The local port number to which this socket is connected. */ protected int localport;
类结构如下：
（5）套接字输入输出流
SocketImpl默认子类是AbstractPlainSocketImpl，大部分套接字的创建、连接等操作都通过该类进行。套接字通过获取输入输出流，使应用可以像操作本地I/O流一样，操作网络数据。
Socket获取输入输出流的方法是getInputStream()和getOutputStream()，底层调AbstractPlainSocketImpl的方法获取，实际流对象为SocketInputStream和SocketOutputStream，具体细节此处不阐述。
socket.getInputStream()//获取输入流 socket.getOutputStream()//获取输出流
AbstractPlainSocketImpl类中包含2个套接字输入输出流属性，如下：
private SocketInputStream socketInputStream = null; private SocketOutputStream socketOutputStream = null;
他们分别继承自FileInputStream和FileOutputStream，以表示输入输出源。并且他们都不是public类型的，一般不会直接使用。
class SocketInputStream extends FileInputStream class SocketOutputStream extends FileOutputStream
3. 总结
1. 网络连接的核心是套接字Socket，服务端ServerSocket监听连接，创建套接字；客户端创建套接字，绑定服务端ip和端口
2. SocketImpl类和子类AbstractPlainSocketImpl是服务端和客户端套接字创建、连接、交互的核心
3. 通过套接字获取输入输出流，应用程序可以与本地I/O流操作一样