我正在尝试用 Java 实现一个简单的客户端-服务器应用程序。
代码如下:
客户端.java
public class Client implements Runnable {
private String hostName;
private int portNumber;
private String message;
private Socket socket;
private PrintWriter writer;
private BufferedReader reader;
public Client(String hostName, int portNumber, String message) {
this.hostName = hostName;
this.portNumber = portNumber;
this.message = message;
}
public void connect() {
try {
socket = new Socket(hostName, portNumber);
writer = new PrintWriter(socket.getOutputStream(), true);
reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
writer.println(message);
} catch (UnknownHostException e) {
System.err.println("Could not resolve the host name '" + hostName + "'.");
} catch (IOException e) {
System.err.println("Could not get the I/O for the connection to '" + hostName + "'.");
}
}
private void listenForMessages() {
while (true) {
try {
System.out.println("In loop!");
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
} catch (IOException e) {
System.err.println(e.getMessage());
}
}
}
public void run() {
connect();
listenForMessages();
}
}
服务器.java
public class Server implements Runnable {
private int portNumber;
private String message;
private ServerSocket serverSocket;
private Socket clientSocket;
private PrintWriter writer;
private BufferedReader reader;
public Server(int portNumber, String message) {
this.portNumber = portNumber;
this.message = message;
}
private void listen() {
try {
serverSocket = new ServerSocket(portNumber);
} catch (IOException e) {
System.err.println(e.getMessage());
}
while (true) {
try {
clientSocket = serverSocket.accept();
writer = new PrintWriter(clientSocket.getOutputStream(), true);
reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
String line;
while ((line = reader.readLine()) != null) {
System.out.println(line);
}
writer.println(message);
} catch (IOException e) {
System.err.println(e.getMessage());
break;
}
}
}
public void run() {
listen();
}
}
这是主类:
public class Test {
public static void main(String[] args) {
Client client = new Client("localhost", 4444, "Hello from client!");
Server server = new Server(4444, "Hello from server!");
Thread serverThread = new Thread(server);
serverThread.start();
Thread clientThread = new Thread(client);
clientThread.start();
}
}
代码的逻辑很简单:客户端和服务器都在 while(true)
循环中等待消息。
服务器listen
方法中的while
循环执行得很好。但是,在 listenForMessages
方法内部,循环似乎只执行一次。我只在屏幕上看到一个“In loop”。
你能找出问题所在吗?
提前致谢!
最佳答案
However, inside the
listenForMessages
method, the loop seems to be executed only once. I only see one "In loop" printed on the screen.
其实不是因为循环只执行一次,而是因为 reader.readLine()
会让当前线程等待,直到它接收到一整行,这里如果你检查代码Server
,它首先读取并无限循环读取,因为 reader.readLine()
只会在流的末尾返回 null
所以在这种情况下套接字何时关闭。
如果你想在客户端和服务器之间实现某种乒乓球,只需在一侧读取然后写入,然后在另一侧写入和读取,如下所示:
客户端
代码:
public void connect() {
try {
socket = new Socket(hostName, portNumber);
writer = new PrintWriter(socket.getOutputStream(), true);
reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
} catch (UnknownHostException e) {
System.err.println("Could not resolve the host name '" + hostName + "'.");
} catch (IOException e) {
System.err.println(
"Could not get the I/O for the connection to '" + hostName + "'."
);
}
}
private void listenForMessages() {
while (true) {
try {
System.out.println("In loop!");
// Write the message for the server
writer.println(message);
// Read the message from the server
System.out.println(reader.readLine());
} catch (IOException e) {
System.err.println(e.getMessage());
}
}
}
服务器
代码:
while (true) {
try {
clientSocket = serverSocket.accept();
writer = new PrintWriter(clientSocket.getOutputStream(), true);
reader = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
while (true) {
// Read the message from the client
System.out.println(reader.readLine());
// Write the message for the client
writer.println(message);
}
} catch (IOException e) {
System.err.println(e.getMessage());
break;
}
}
输出:
In loop!
Hello from client!
Hello from server!
In loop!
Hello from client!
Hello from server!
...
关于Java while(true) 循环在线程内只执行一次,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/40396692/