二、Socket基础
1.地址的获得
public static void main(String[] args) {
try {
Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
while (interfaces.hasMoreElements()) {
NetworkInterface iface = interfaces.nextElement();
System.out.println("Interface: " + iface.getName());
Enumeration<InetAddress> addrList = iface.getInetAddresses();
if (!addrList.hasMoreElements())
System.out.println("No address");
while (addrList.hasMoreElements()) {
InetAddress address = addrList.nextElement();
System.out.println("Address: " + address.getHostAddress());
}
}
} catch (SocketException e) {
e.printStackTrace();
}
}2.TCP实例程序
要注意一点，虽然在Client端只用了一个write()方法发送字符串，服务器端也可能从
多个块中接受该信息。即使回馈字符串在服务器返回时存于一个块中，也可能被TCP
协议分割成多个部分。
TCPEchoClientTest.java
public static void main(String[] args) throws IOException {
String server = args[0];
byte[] data = args[1].getBytes();
int port = 7;
Socket socket = new Socket(server, port);
System.out.println("Connected to server...");
InputStream in = socket.getInputStream();
OutputStream out = socket.getOutputStream();
out.write(data);
int totalBytesRcvd = 0;
int bytesRcvd;
while (totalBytesRcvd < data.length) {
if ((bytesRcvd = in.read(data, totalBytesRcvd,
data.length - totalBytesRcvd)) == -1)
throw new SocketException("Connection closed");
totalBytesRcvd += bytesRcvd;
}
System.out.println("Received: " + new String(data));
socket.close();
}
TCPEchoServerTest.java
private static final int BUFSIZE = 32;
public static void main(String[] args) throws IOException {
ServerSocket serverSocket = new ServerSocket(7);
int recvMsgSize;
byte[] receiveBuf = new byte[BUFSIZE];
while (true) {
Socket socket = serverSocket.accept();
System.out.println("Handling client " +
" from remote " + socket.getRemoteSocketAddress() +
" at local " + socket.getLocalSocketAddress());
InputStream in = socket.getInputStream();
OutputStream out = socket.getOutputStream();
while ((recvMsgSize = in.read(receiveBuf)) != -1) {
out.write(receiveBuf, 0, recvMsgSize);
}
socket.close();
}
}注意new Socket时指定的是远端服务器监听的端口号而没有指定本地端口，因此将
采用默认地址和可用的端口号。在我的机器上Client端口是4593，连接到服务器的
端口7。
3.UDP实例程序
为什么使用UDP协议？如果应用程序只交换少量的数据，TCP连接的建立阶段就至少
要传输其两倍的信息量（还有两倍的往返时间）。
UDPEchoClientTest.java
public static void main(String[] args) throws IOException {
InetAddress serverAddress = InetAddress.getByName(args[0]);
byte[] bytesToSend = args[1].getBytes();
DatagramSocket socket = new DatagramSocket();
socket.setSoTimeout(3000);
DatagramPacket sendPacket = new DatagramPacket(
bytesToSend, bytesToSend.length, serverAddress, 7);
DatagramPacket receivePacket = new DatagramPacket(
new byte[bytesToSend.length], bytesToSend.length);
// Packets may be lost, so we have to keep trying
int tries = 0;
boolean receivedResponse = false;
do {
socket.send(sendPacket);
try {
socket.receive(receivePacket);
if (!receivePacket.getAddress().equals(serverAddress))
throw new IOException("Receive from unknown source");
receivedResponse = true;
}
catch (IOException e) {
tries++;
System.out.println("Timeout, try again");
}
} while (!receivedResponse && tries < 5);
if (receivedResponse)
System.out.println("Received: " + new String(receivePacket.getData()));
else
System.out.println("No response");
socket.close();
}UDPEchoServerTest.java
private static final int ECHOMAX = 255;
public static void main(String[] args) throws IOException {
DatagramSocket socket = new DatagramSocket(7);
DatagramPacket packet = new DatagramPacket(new byte[ECHOMAX], ECHOMAX);
while (true) {
socket.receive(packet);
System.out.println("Handling client at " + packet.getAddress());
socket.send(packet);
packet.setLength(ECHOMAX);
}
}通过这个例子与之前TCP的实例进行比较，有如下区别：
A.DatagramSocket在创建时不需要指定目的地址，因为UDP不需要建立连接，每个
数据报文都可以发送或接收于不同的目的地址。
B.如果像TCP一样在read()上阻塞等待，将可能永远阻塞在那里，因为UDP协议只是
简单地扩展了IP协议，UDP报文可能丢失掉。所以一定要设置阻塞等待的超时时间。
C.UDP协议保留了消息的边界信息，每次receive()调用最多只能接收一次send()方法
调用所发送的数据。
D.一个UDP报文DatagramPacket能传输的最大数据是65507字节，超出部分的字节将
自动被丢弃，而且对接收程序也没有任何的提示。因此缓存数组可以设置成65000字节
左右是安全的。
E.如果反复使用同一个DatagramPacket实例调用receive()方法，每次调用前都必须显式
地将消息的内部长度重置为缓存区的实际长度。