Our protagonist Socket is an interface layer below the application layer and above the transport layer. It is the system interface provided by the operating system for users to access the network. We can use the Socket interface layer to communicate with the transport layer and the Internet. layer and network interface layer to implement our different application layer protocols. To give a few examples, for example, HTTP is implemented based on TCP, ping and tracerouter are implemented based on ICMP, and libpcap (using wireshare to capture network packets) is possible. (more familiar) directly reads the data of the network interface layer, but their implementation is completed with the help of Socket. It can be seen that for the application layer, if we want to implement network functions, in the final analysis, they must be implemented through Sockets. Otherwise, we will not be able to access the transport layer, Internet layer and network interface layer of the operating system. Considering this article and other articles after this article, we will only focus on the process of Socket accessing the transport layer for the time being. For other aspects, you can refer to related books. Here we recommend "TCP/IP Detailed Explanation Volume 1" and "UNIX Network Programming" (UNP). Although It is a book about Linux C language development, but it explains network programming very thoroughly. Here we also pay tribute to this great man. Although he died young, he left us with immeasurable wealth.
Let’s take a closer look at TCP and UDP. The biggest difference between the two is that TCP is reliable. That is to say, when we send data through TCP, the network protocol stack will ensure that the data is reliably transmitted to the opposite end. UDP is unreliable. If packet loss occurs, the protocol stack will not do anything, and the reliability guarantee is left to the application layer. Therefore, TCP's performance will be lower than UDP, but its reliability will be much better than UDP. In addition, there are also formal differences between the two when transmitting data. TCP's data is a stream, which you can compare to a file stream, while UDP is based on data packets, which means that the data will be packaged and sent. You may have questions, is there any difference? Of course, one of the biggest problems is that TCP has no data boundaries. Each time the data is received, the unit is bytes. If you want to distinguish the data sent twice, unless you add delimiting characters (such as http's \r\n\r) in the data \n), otherwise, TCP cannot distinguish data boundaries, and each data sent by UDP is typed as an independent data packet, so the data boundaries sent several times are very clear, and each time we receive it, it is also based on data packets. take over. For different programming languages or platforms, although the Socket interface may be different, they all provide an interface for sending data based on TCP and an interface for sending data based on UDP.
In addition, there is a concept that a Socket is uniquely identified by the client's IP and port number<=>the server's IP and port number.
Since the following articles are basically based on TCP, here in order to let everyone know what Socket is, we will first take UDP as an example, and then learn more about TCP-based Socket programming. We use Java Socket, based on UDP protocol, to realize uppercase conversion of letters. The whole process is that the client sends a string to the server. After the server converts the string to all uppercase, it sends it back to the client, and the client displays it.
For Java Socket programming, there are two concepts, one is ServerSocket and the other is Socket. A connection is established between the server and the client through Socket, and then they can communicate. First, ServerSocket will listen to a certain port on the server side. When it finds that the client has a Socket and tries to connect to it, it will accept the connection request of the Socket and establish a corresponding Socket on the server side to communicate with it. In this way, there are two Sockets, one for the client and one for the server.
The communication between Sockets is actually very simple. The server writes things into the Socket output stream, and the client can read the corresponding content through the Socket input stream. There is a two-way connection between Socket and Socket, so the client can also write things into the corresponding Socket output stream, and then the corresponding Socket input stream on the server can read the corresponding content. Let’s look at some examples of communication between the server and the client:
Server code
public class Server {
public static void main(String args[]) throws IOException {
//为了简单起见，所有的异常信息都往外抛
int port = 8899;
//定义一个ServerSocket监听在端口8899上
ServerSocket server = new ServerSocket(port);
//server尝试接收其他Socket的连接请求，server的accept方法是阻塞式的
Socket socket = server.accept();
//跟客户端建立好连接之后，我们就可以获取socket的InputStream，并从中读取客户端发过来的信息了。
Reader reader = new InputStreamReader(socket.getInputStream());
char chars[] = new char[64];
int len;
StringBuilder sb = new StringBuilder();
while ((len=reader.read(chars)) != -1) {
sb.append(new String(chars, 0, len));
}
System.out.println("from client: " + sb);
reader.close();
socket.close();
server.close();
}
}Service The operation of the client reading data from the Socket's InputStream is also blocking. If no data is read from the input stream, the program will remain there until the client writes data to the Socket's output stream or closes it. The output stream of the Socket. Of course, the same is true for the client's Socket. After the operation is completed, remember to close the corresponding resources before the entire program ends, that is, close the corresponding IO stream and Socket.
Client code
public class Client {
public static void main(String args[]) throws Exception {
//为了简单起见，所有的异常都直接往外抛
String host = "127.0.0.1"; //要连接的服务端IP地址
int port = 8899; //要连接的服务端对应的监听端口
//与服务端建立连接
Socket client = new Socket(host, port);
//建立连接后就可以往服务端写数据了
Writer writer = new OutputStreamWriter(client.getOutputStream());
writer.write("Hello Server.");
writer.flush();//写完后要记得flush
writer.close();
client.close();
}
}Be careful when the client writes data to the output stream of the Socket and passes it to the server. If the program does not correspond to the output after the writing operation Close the stream and perform other blocking operations (such as reading data from the input stream). Remember to flush. Only in this way can the server receive the data sent by the client. Otherwise, it may cause both sides to wait for each other indefinitely. . This issue will be discussed later when the client and server read and write at the same time.
前面已经说了Socket之间是双向通信的，它既可以接收数据，同时也可以发送数据。
服务端代码
public class Server {
public static void main(String args[]) throws IOException {
//为了简单起见，所有的异常信息都往外抛
int port = 8899;
//定义一个ServerSocket监听在端口8899上
ServerSocket server = new ServerSocket(port);
//server尝试接收其他Socket的连接请求，server的accept方法是阻塞式的
Socket socket = server.accept();
//跟客户端建立好连接之后，我们就可以获取socket的InputStream，并从中读取客户端发过来的信息了。
Reader reader = new InputStreamReader(socket.getInputStream());
char chars[] = new char[64];
int len;
StringBuilder sb = new StringBuilder();
while ((len=reader.read(chars)) != -1) {
sb.append(new String(chars, 0, len));
}
System.out.println("from client: " + sb);
//读完后写一句
Writer writer = new OutputStreamWriter(socket.getOutputStream());
writer.write("Hello Client.");
writer.flush();
writer.close();
reader.close();
socket.close();
server.close();
}
}在上述代码中首先我们从输入流中读取客户端发送过来的数据，接下来我们再往输出流里面写入数据给客户端，接下来关闭对应的资源文件。而实际上上述代码可能并不会按照我们预先设想的方式运行，因为从输入流中读取数据是一个阻塞式操作，在上述的while循环中当读到数据的时候就会执行循环体，否则就会阻塞，这样后面的写操作就永远都执行不了了。除非客户端对应的Socket关闭了阻塞才会停止，while循环也会跳出。针对这种可能永远无法执行下去的情况的解决方法是while循环需要在里面有条件的跳出来，纵观上述代码，在不断变化的也只有取到的长度len和读到的数据了，len已经是不能用的了，唯一能用的就是读到的数据了。针对这种情况，通常我们都会约定一个结束标记，当客户端发送过来的数据包含某个结束标记时就说明当前的数据已经发送完毕了，这个时候我们就可以进行循环的跳出了。那么改进后的代码会是这个样子：
public class Server {
public static void main(String args[]) throws IOException {
//为了简单起见，所有的异常信息都往外抛
int port = 8899;
//定义一个ServerSocket监听在端口8899上
ServerSocket server = new ServerSocket(port);
//server尝试接收其他Socket的连接请求，server的accept方法是阻塞式的
Socket socket = server.accept();
//跟客户端建立好连接之后，我们就可以获取socket的InputStream，并从中读取客户端发过来的信息了。
Reader reader = new InputStreamReader(socket.getInputStream());
char chars[] = new char[64];
int len;
StringBuilder sb = new StringBuilder();
String temp;
int index;
while ((len=reader.read(chars)) != -1) {
temp = new String(chars, 0, len);
if ((index = temp.indexOf("eof")) != -1) {//遇到eof时就结束接收
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
System.out.println("from client: " + sb);
//读完后写一句
Writer writer = new OutputStreamWriter(socket.getOutputStream());
writer.write("Hello Client.");
writer.flush();
writer.close();
reader.close();
socket.close();
server.close();
}
}在上述代码中，当服务端读取到客户端发送的结束标记，即“eof”时就会结束数据的接收，终止循环，这样后续的代码又可以继续进行了。
客户端代码
public class Client {
public static void main(String args[]) throws Exception {
//为了简单起见，所有的异常都直接往外抛
String host = "127.0.0.1"; //要连接的服务端IP地址
int port = 8899; //要连接的服务端对应的监听端口
//与服务端建立连接
Socket client = new Socket(host, port);
//建立连接后就可以往服务端写数据了
Writer writer = new OutputStreamWriter(client.getOutputStream());
writer.write("Hello Server.");
writer.flush();
//写完以后进行读操作
Reader reader = new InputStreamReader(client.getInputStream());
char chars[] = new char[64];
int len;
StringBuffer sb = new StringBuffer();
while ((len=reader.read(chars)) != -1) {
sb.append(new String(chars, 0, len));
}
System.out.println("from server: " + sb);
writer.close();
reader.close();
client.close();
}
}在上述代码中我们先是给服务端发送了一段数据，之后读取服务端返回来的数据，跟之前的服务端一样在读的过程中有可能导致程序一直挂在那里，永远跳不出while循环。这段代码配合服务端的第一段代码就正好让我们分析服务端永远在那里接收数据，永远跳不出while循环，也就没有之后的服务端返回数据给客户端，客户端也就不可能接收到服务端返回的数据。解决方法如服务端第二段代码所示，在客户端发送数据完毕后，往输出流里面写入结束标记告诉服务端数据已经发送完毕了，同样服务端返回数据完毕后也发一个标记告诉客户端。那么修改后的客户端代码就应该是这个样子：
public class Client {
public static void main(String args[]) throws Exception {
//为了简单起见，所有的异常都直接往外抛
String host = "127.0.0.1"; //要连接的服务端IP地址
int port = 8899; //要连接的服务端对应的监听端口
//与服务端建立连接
Socket client = new Socket(host, port);
//建立连接后就可以往服务端写数据了
Writer writer = new OutputStreamWriter(client.getOutputStream());
writer.write("Hello Server.");
writer.write("eof");
writer.flush();
//写完以后进行读操作
Reader reader = new InputStreamReader(client.getInputStream());
char chars[] = new char[64];
int len;
StringBuffer sb = new StringBuffer();
String temp;
int index;
while ((len=reader.read(chars)) != -1) {
temp = new String(chars, 0, len);
if ((index = temp.indexOf("eof")) != -1) {
sb.append(temp.substring(0, index));
break;
}
sb.append(new String(chars, 0, len));
}
System.out.println("from server: " + sb);
writer.close();
reader.close();
client.close();
}
}我们日常使用的比较多的都是这种客户端发送数据给服务端，服务端接收数据后再返回相应的结果给客户端这种形式。只是客户端和服务端之间不再是这种一对一的关系，而是下面要讲到的多个客户端对应同一个服务端的情况。
像前面讲的两个例子都是服务端接收一个客户端的请求之后就结束了，不能再接收其他客户端的请求了，这往往是不能满足我们的要求的。通常我们会这样做：
public class Server {
public static void main(String args[]) throws IOException {
//为了简单起见，所有的异常信息都往外抛
int port = 8899;
//定义一个ServerSocket监听在端口8899上
ServerSocket server = new ServerSocket(port);
while (true) {
//server尝试接收其他Socket的连接请求，server的accept方法是阻塞式的
Socket socket = server.accept();
//跟客户端建立好连接之后，我们就可以获取socket的InputStream，并从中读取客户端发过来的信息了。
Reader reader = new InputStreamReader(socket.getInputStream());
char chars[] = new char[64];
int len;
StringBuilder sb = new StringBuilder();
String temp;
int index;
while ((len=reader.read(chars)) != -1) {
temp = new String(chars, 0, len);
if ((index = temp.indexOf("eof")) != -1) {//遇到eof时就结束接收
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
System.out.println("from client: " + sb);
//读完后写一句
Writer writer = new OutputStreamWriter(socket.getOutputStream());
writer.write("Hello Client.");
writer.flush();
writer.close();
reader.close();
socket.close();
}
}
}在上面代码中我们用了一个死循环，在循环体里面ServerSocket调用其accept方法试图接收来自客户端的连接请求。当没有接收到请求的时候，程序会在这里阻塞直到接收到来自客户端的连接请求，之后会跟当前建立好连接的客户端进行通信，完了后会接着执行循环体再次尝试接收新的连接请求。这样我们的ServerSocket就能接收来自所有客户端的连接请求了，并且与它们进行通信了。这就实现了一个简单的一个服务端与多个客户端进行通信的模式。
上述例子中虽然实现了一个服务端跟多个客户端进行通信，但是还存在一个问题。在上述例子中，我们的服务端处理客户端的连接请求是同步进行的，每次接收到来自客户端的连接请求后，都要先跟当前的客户端通信完之后才能再处理下一个连接请求。这在并发比较多的情况下会严重影响程序的性能，为此，我们可以把它改为如下这种异步处理与客户端通信的方式：
public class Server {
public static void main(String args[]) throws IOException {
//为了简单起见，所有的异常信息都往外抛
int port = 8899;
//定义一个ServerSocket监听在端口8899上
ServerSocket server = new ServerSocket(port);
while (true) {
//server尝试接收其他Socket的连接请求，server的accept方法是阻塞式的
Socket socket = server.accept();
//每接收到一个Socket就建立一个新的线程来处理它
new Thread(new Task(socket)).start();
}
}
/**
* 用来处理Socket请求的
*/
static class Task implements Runnable {
private Socket socket;
public Task(Socket socket) {
this.socket = socket;
}
public void run() {
try {
handleSocket();
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* 跟客户端Socket进行通信
* @throws Exception
*/
private void handleSocket() throws Exception {
Reader reader = new InputStreamReader(socket.getInputStream());
char chars[] = new char[64];
int len;
StringBuilder sb = new StringBuilder();
String temp;
int index;
while ((len=reader.read(chars)) != -1) {
temp = new String(chars, 0, len);
if ((index = temp.indexOf("eof")) != -1) {//遇到eof时就结束接收
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
System.out.println("from client: " + sb);
//读完后写一句
Writer writer = new OutputStreamWriter(socket.getOutputStream());
writer.write("Hello Client.");
writer.flush();
writer.close();
reader.close();
socket.close();
}
}
}在上面代码中，每次ServerSocket接收到一个新的Socket连接请求后都会新起一个线程来跟当前Socket进行通信，这样就达到了异步处理与客户端Socket进行通信的情况。
在从Socket的InputStream中接收数据时，像上面那样一点点的读就太复杂了，有时候我们就会换成使用BufferedReader来一次读一行，如：
public class Server {
public static void main(String args[]) throws IOException {
//为了简单起见，所有的异常信息都往外抛
int port = 8899;
//定义一个ServerSocket监听在端口8899上
ServerSocket server = new ServerSocket(port);
while (true) {
//server尝试接收其他Socket的连接请求，server的accept方法是阻塞式的
Socket socket = server.accept();
//每接收到一个Socket就建立一个新的线程来处理它
new Thread(new Task(socket)).start();
}
}
/**
* 用来处理Socket请求的
*/
static class Task implements Runnable {
private Socket socket;
public Task(Socket socket) {
this.socket = socket;
}
public void run() {
try {
handleSocket();
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* 跟客户端Socket进行通信
* @throws Exception
*/
private void handleSocket() throws Exception {
BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
StringBuilder sb = new StringBuilder();
String temp;
int index;
while ((temp=br.readLine()) != null) {
System.out.println(temp);
if ((index = temp.indexOf("eof")) != -1) {//遇到eof时就结束接收
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
System.out.println("from client: " + sb);
//读完后写一句
Writer writer = new OutputStreamWriter(socket.getOutputStream());
writer.write("Hello Client.");
writer.write("eof\n");
writer.flush();
writer.close();
br.close();
socket.close();
}
}
}这个时候需要注意的是，BufferedReader的readLine方法是一次读一行的，这个方法是阻塞的，直到它读到了一行数据为止程序才会继续往下执行，那么readLine什么时候才会读到一行呢？直到程序遇到了换行符或者是对应流的结束符readLine方法才会认为读到了一行，才会结束其阻塞，让程序继续往下执行。所以我们在使用BufferedReader的readLine读取数据的时候一定要记得在对应的输出流里面一定要写入换行符（流结束之后会自动标记为结束，readLine可以识别），写入换行符之后一定记得如果输出流不是马上关闭的情况下记得flush一下，这样数据才会真正的从缓冲区里面写入。对应上面的代码我们的客户端程序应该这样写：
public class Client {
public static void main(String args[]) throws Exception {
//为了简单起见，所有的异常都直接往外抛
String host = "127.0.0.1"; //要连接的服务端IP地址
int port = 8899; //要连接的服务端对应的监听端口
//与服务端建立连接
Socket client = new Socket(host, port);
//建立连接后就可以往服务端写数据了
Writer writer = new OutputStreamWriter(client.getOutputStream());
writer.write("Hello Server.");
writer.write("eof\n");
writer.flush();
//写完以后进行读操作
BufferedReader br = new BufferedReader(new InputStreamReader(client.getInputStream()));
StringBuffer sb = new StringBuffer();
String temp;
int index;
while ((temp=br.readLine()) != null) {
if ((index = temp.indexOf("eof")) != -1) {
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
System.out.println("from server: " + sb);
writer.close();
br.close();
client.close();
}
}假设有这样一种需求，我们的客户端需要通过Socket从服务端获取到XX信息，然后给用户展示在页面上。我们知道Socket在读数据的时候是阻塞式的，如果没有读到数据程序会一直阻塞在那里。在同步请求的时候我们肯定是不能允许这样的情况发生的，这就需要我们在请求达到一定的时间后控制阻塞的中断，让程序得以继续运行。Socket为我们提供了一个setSoTimeout()方法来设置接收数据的超时时间，单位是毫秒。当设置的超时时间大于0，并且超过了这一时间Socket还没有接收到返回的数据的话，Socket就会抛出一个SocketTimeoutException。
假设我们需要控制我们的客户端在开始读取数据10秒后还没有读到数据就中断阻塞的话我们可以这样做：
public class Client {
public static void main(String args[]) throws Exception {
//为了简单起见，所有的异常都直接往外抛
String host = "127.0.0.1"; //要连接的服务端IP地址
int port = 8899; //要连接的服务端对应的监听端口
//与服务端建立连接
Socket client = new Socket(host, port);
//建立连接后就可以往服务端写数据了
Writer writer = new OutputStreamWriter(client.getOutputStream());
writer.write("Hello Server.");
writer.write("eof\n");
writer.flush();
//写完以后进行读操作
BufferedReader br = new BufferedReader(new InputStreamReader(client.getInputStream()));
//设置超时间为10秒
client.setSoTimeout(10*1000);
StringBuffer sb = new StringBuffer();
String temp;
int index;
try {
while ((temp=br.readLine()) != null) {
if ((index = temp.indexOf("eof")) != -1) {
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
} catch (SocketTimeoutException e) {
System.out.println("数据读取超时。");
}
System.out.println("from server: " + sb);
writer.close();
br.close();
client.close();
}
}对于这种服务端或客户端接收中文乱码的情况通常是因为数据发送时使用的编码跟接收时候使用的编码不一致。比如有下面这样一段服务端代码：
public class Server {
public static void main(String args[]) throws IOException {
//为了简单起见，所有的异常信息都往外抛
int port = 8899;
//定义一个ServerSocket监听在端口8899上
ServerSocket server = new ServerSocket(port);
while (true) {
//server尝试接收其他Socket的连接请求，server的accept方法是阻塞式的
Socket socket = server.accept();
//每接收到一个Socket就建立一个新的线程来处理它
new Thread(new Task(socket)).start();
}
}
/**
* 用来处理Socket请求的
*/
static class Task implements Runnable {
private Socket socket;
public Task(Socket socket) {
this.socket = socket;
}
public void run() {
try {
handleSocket();
} catch (Exception e) {
e.printStackTrace();
}
}
/**
* 跟客户端Socket进行通信
* @throws Exception
*/
private void handleSocket() throws Exception {
BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream(), "GBK"));
StringBuilder sb = new StringBuilder();
String temp;
int index;
while ((temp=br.readLine()) != null) {
System.out.println(temp);
if ((index = temp.indexOf("eof")) != -1) {//遇到eof时就结束接收
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
System.out.println("客户端: " + sb);
//读完后写一句
Writer writer = new OutputStreamWriter(socket.getOutputStream(), "UTF-8");
writer.write("你好，客户端。");
writer.write("eof\n");
writer.flush();
writer.close();
br.close();
socket.close();
}
}
}这里用来测试我就弄的混乱了一点。在上面服务端代码中我们在定义输入流的时候明确定义了使用GBK编码来读取数据，而在定义输出流的时候明确指定了将使用UTF-8编码来发送数据。如果客户端上送数据的时候不以GBK编码来发送的话服务端接收的数据就很有可能会乱码；同样如果客户端接收数据的时候不以服务端发送数据的编码，即UTF-8编码来接收数据的话也极有可能会出现数据乱码的情况。所以，对于上述服务端代码，为使我们的程序能够读取对方发送过来的数据，而不出现乱码情况，我们的客户端应该是这样的：
public class Client {
public static void main(String args[]) throws Exception {
//为了简单起见，所有的异常都直接往外抛
String host = "127.0.0.1"; //要连接的服务端IP地址
int port = 8899; //要连接的服务端对应的监听端口
//与服务端建立连接
Socket client = new Socket(host, port);
//建立连接后就可以往服务端写数据了
Writer writer = new OutputStreamWriter(client.getOutputStream(), "GBK");
writer.write("你好，服务端。");
writer.write("eof\n");
writer.flush();
//写完以后进行读操作
BufferedReader br = new BufferedReader(new InputStreamReader(client.getInputStream(), "UTF-8"));
//设置超时间为10秒
client.setSoTimeout(10*1000);
StringBuffer sb = new StringBuffer();
String temp;
int index;
try {
while ((temp=br.readLine()) != null) {
if ((index = temp.indexOf("eof")) != -1) {
sb.append(temp.substring(0, index));
break;
}
sb.append(temp);
}
} catch (SocketTimeoutException e) {
System.out.println("数据读取超时。");
}
System.out.println("服务端: " + sb);
writer.close();
br.close();
client.close();
}
}The above is the detailed content of Socket programming principles and code tutorial sharing in Java. For more information, please follow other related articles on the PHP Chinese website!
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