Socket的I/O调用可能会因为多种原因阻塞，数据输入方法read和receive方法在没有数据可读时会阻塞。
TCP套接字的write方法在没有足够的空间缓存传输的数据时可能阻塞，ServerSocket的accept方法和Socket的构造函数都会阻塞等待，直到连接建立。同时，长的信息往返时间，高错误率的连接和慢速的（或已发生故障的）服务器，都可能导致需要很长的时间来建立连接。所有这些情况，只有在连接请求得到满足后这些方法才会返回。
accept、read、receive方法，可以使用Socket类、ServerSocket类和DatagramSocket类的setSoTimeout方法，设置其阻塞的最长时间（以毫秒为单位）。如果在指定时间内这些方法没有返回，则将抛出一个InterruptedIOException异常。
对于Socket实例，在调用read方法前，还可以使用InputStream的available方法来检测是否可读的数据。
连接和写数据
Socket类的构造函数会尝试根据参数中指定的主机和端口来建立连接，并阻塞等待，直到连接成功建立或发生了系统定义的超时。不过，系统定义的超时时间很长，而Java又没有提供任何缩短它的方法，要改变这个情况，可以使用Socket类的无参数构造函数，它返回一个没有建立连接的Socket实例，需要建立连接时，调用该实例的connect方法，并制定一个远程终端和超时时间（毫秒）。
write方法调用也会阻塞等待，直到最后一个字节成功写入到TCP实现的本地缓存中。如果可用的缓存空间比要写入的数据小，在write方法调用返回前，必须把一些数据成功传输到连接的另一端。因此，write方法阻塞总时间最终还是取决于接收端的应用程序。Java现在还没有提供任何方法使write超时或由其他方法打断的方法。一个可以在Socket实例上发送大量数据的协议可能会无限期地阻塞下去。
public class TimelimitEchoProtocol implements Runnable {
private static final int BUFSIZE = 32; // Size (bytes) buffer
private static final String TIMELIMIT = "10000"; // Default limit (ms)
private static final String TIMELIMITPROP = "Timelimit"; // Thread property
private static int timelimit;
private Socket clntSock;
private Logger logger;
public TimelimitEchoProtocol(Socket clntSock, Logger logger) {
this.clntSock = clntSock;
this.logger = logger;
// Get the time limit from the System properties or take the default
timelimit = Integer.parseInt(System.getProperty(TIMELIMITPROP,TIMELIMIT));
}
public static void handleEchoClient(Socket clntSock, Logger logger) {
try {
// Get the input and output I/O streams from socket
InputStream in = clntSock.getInputStream();
OutputStream out = clntSock.getOutputStream();
int recvMsgSize; // Size of received message
int totalBytesEchoed = 0; // Bytes received from client
byte[] echoBuffer = new byte[BUFSIZE]; // Receive buffer
long endTime = System.currentTimeMillis() + timelimit;
int timeBoundMillis = timelimit;
clntSock.setSoTimeout(timeBoundMillis);
// Receive until client closes connection, indicated by -1
while ((timeBoundMillis > 0) && // catch zero values
((recvMsgSize = in.read(echoBuffer)) != -1)) {
out.write(echoBuffer, 0, recvMsgSize);
totalBytesEchoed += recvMsgSize;
timeBoundMillis = (int) (endTime - System.currentTimeMillis()) ;
clntSock.setSoTimeout(timeBoundMillis);
}
logger.info("Client " + clntSock.getRemoteSocketAddress() +
", echoed " + totalBytesEchoed + " bytes.");
} catch (IOException ex) {
logger.log(Level.WARNING, "Exception in echo protocol", ex);
}
}
public void run() {
handleEchoClient(this.clntSock, this.logger);
}
}试图将总时间限制在10秒内，每次read调用结束后将重新计算剩余的timeout。
关闭套接字
网络协议通常会明确指定了由谁来发起关闭连接。
对于HTTP协议，由于客户端不知道文件大小，因此是由服务器端发起关闭套接字来指示文件的结束。
调用Socket的close方法将同时终止两个方向的数据流，一旦一个终端（客户端或服务端）关闭了套接字，它将无法再发送或接收数据。这就是意味着close方法只能够在调用者完成通信之后用来给另一端发送信号。
Socket类的shutdownInput和shutdownOutput方法能够将输入输出流相互独立地关闭，关闭之后，任何没有发送的数据都将毫无提示地被丢弃，任何想从套接字的输入流读取数据的操作都将返回-1。同理，关闭之后，任何尝试向输出流写数据的操作都将抛出一个IOException异常。
在客户端调用了shutdownOutput之后，它还要从服务器读取剩余的已经压缩的字节
public class CompressClient {
public static final int BUFSIZE = 256; // Size of read buffer
public static void main(String[] args) throws IOException {
if (args.length != 3) { // Test for correct # of args
throw new IllegalArgumentException("Parameter(s): <Server> <Port> <File>");
}
String server = args[0]; // Server name or IP address
int port = Integer.parseInt(args[1]); // Server port
String filename = args[2]; // File to read data from
// Open input and output file (named input.gz)
FileInputStream fileIn = new FileInputStream(filename);
FileOutputStream fileOut = new FileOutputStream(filename + ".gz");
// Create socket connected to server on specified port
Socket sock = new Socket(server, port);
// Send uncompressed byte stream to server
sendBytes(sock, fileIn);
// Receive compressed byte stream from server
InputStream sockIn = sock.getInputStream();
int bytesRead; // Number of bytes read
byte[] buffer = new byte[BUFSIZE]; // Byte buffer
while ((bytesRead = sockIn.read(buffer)) != -1) {
fileOut.write(buffer, 0, bytesRead);
System.out.print("R"); // Reading progress indicator
}
System.out.println(); // End progress indicator line
sock.close(); // Close the socket and its streams
fileIn.close(); // Close file streams
fileOut.close();
}
private static void sendBytes(Socket sock, InputStream fileIn)
throws IOException {
OutputStream sockOut = sock.getOutputStream();
int bytesRead; // Number of bytes read
byte[] buffer = new byte[BUFSIZE]; // Byte buffer
while ((bytesRead = fileIn.read(buffer)) != -1) {
sockOut.write(buffer, 0, bytesRead);
System.out.print("W"); // Writing progress indicator
}
sock.shutdownOutput(); // Finished sending
}
}基于GZIP压缩算法的压缩协议
public class CompressProtocol implements Runnable {
public static final int BUFSIZE = 1024; // Size of receive buffer
private Socket clntSock;
private Logger logger;
public CompressProtocol(Socket clntSock, Logger logger) {
this.clntSock = clntSock;
this.logger = logger;
}
public static void handleCompressClient(Socket clntSock, Logger logger) {
try {
// Get the input and output streams from socket
InputStream in = clntSock.getInputStream();
GZIPOutputStream out = new GZIPOutputStream(clntSock.getOutputStream());
byte[] buffer = new byte[BUFSIZE]; // Allocate read/write buffer
int bytesRead; // Number of bytes read
// Receive until client closes connection, indicated by -1 return
while ((bytesRead = in.read(buffer)) != -1)
out.write(buffer, 0, bytesRead);
out.finish(); // Flush bytes from GZIPOutputStream
logger.info("Client " + clntSock.getRemoteSocketAddress() + " finished");
} catch (IOException ex) {
logger.log(Level.WARNING, "Exception in echo protocol", ex);
}
try { // Close socket
clntSock.close();
} catch (IOException e) {
logger.info("Exception = " + e.getMessage());
}
}
public void run() {
handleCompressClient(this.clntSock, this.logger);
}
}