如何实现服务端主动向客户端发送消息？

要实现服务端主动向客户端发送消息,主要有以下方法:

1. 服务端使用Socket的getOutputStream()获取输出流,写入消息:

// 服务端
ServerSocket server = new ServerSocket(8000);
Socket socket = server.accept();

OutputStream out = socket.getOutputStream();
out.write("Hello".getBytes());

2. 服务端定期向客户端发送消息,使用线程实现:

// 服务端
ServerSocket server = new ServerSocket(8000);
Socket socket = server.accept();

new Thread() {
    public void run() {
        OutputStream out = socket.getOutputStream();
        while (true) {
            out.write("Hello".getBytes());
            try {
                Thread.sleep(1000);   // 1s发送一次
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}.start();

3. 客户端读取服务端发送的消息:

// 客户端
Socket socket = new Socket("127.0.0.1", 8000);
InputStream in = socket.getInputStream();
byte[] buffer = new byte[1024];
while (true) {
    int len = in.read(buffer);
    if (len != -1) {
        String msg = new String(buffer, 0, len);
        System.out.println(msg);   // 打印服务端发送的消息
    }
}

4. 服务端使用DatagramSocket发送UDP消息:

// 服务端
DatagramSocket socket = new DatagramSocket();

while (true) {
    byte[] data = "Hello".getBytes();
    InetSocketAddress address = new InetSocketAddress("127.0.0.1", 8000);
    DatagramPacket packet = new DatagramPacket(data, data.length, address);
    socket.send(packet);   // 发送UDP消息
} 

5. 客户端使用DatagramSocket接收UDP消息:

// 客户端  
DatagramSocket socket = new DatagramSocket(8000);
byte[] buffer = new byte[1024];
while (true) {
    DatagramPacket packet = new DatagramPacket(buffer, buffer.length);
    socket.receive(packet);   // 接收UDP消息 
    String msg = new String(packet.getData(), 0, packet.getLength());
    System.out.println(msg); 
}