java.nio.channels.Selector.select() 立即返回 0

Question

以下代码打开 UDP 套接字，设置多播，发送消息并启动读取循环。它使用 Selector.select(( 进行超时读取。

int TIMEOUT = 10000;
String id = "8154@Think420";
try {
NetworkInterface iface = NetworkInterface.getByInetAddress(InetAddress.getByName(address));
try (DatagramChannel channel = DatagramChannel.open(StandardProtocolFamily.INET).setOption(StandardSocketOptions.SO_REUSEADDR, true)) {
channel.socket().bind(new InetSocketAddress(PORT));
channel.setOption(StandardSocketOptions.IP_MULTICAST_IF, iface);
channel.configureBlocking(false);
InetAddress group = InetAddress.getByName("225.4.5.6");
MembershipKey key = channel.join(group, iface);
InetSocketAddress mcast = new InetSocketAddress(key.group(), PORT);
channel.send(ByteBuffer.wrap(id.getBytes()), mcast);
Selector selector = Selector.open();
channel.register(selector, SelectionKey.OP_READ);
ByteBuffer buffer = ByteBuffer.allocate(4096);
while (key.isValid()) {
if (selector.select(TIMEOUT) == 0) {
System.err.println("timeout");
continue;
}
buffer.clear();
InetSocketAddress address = (InetSocketAddress) channel.receive(buffer);
buffer.flip();
String message = Charset.forName("ASCII").decode(buffer).toString();
System.err.format("From %s received: %sn", address.getHostString(), message);
}
} catch(IOException e) {
e.printStackTrace();
}
} catch(UnknownHostException | SocketException e) {
throw new IllegalArgumentException(e);
}

当我运行应用程序的第一个实例时，它工作正常：从自身接收一条消息，然后在循环中打印"超时"。

下面是一个示例输出：

From 127.0.0.1 received: 8154@Think420
timeout
timeout
timeout

当我运行另一个实例时出现问题。这个可以正常工作，但第一个实例立即开始报告超时，淹没其输出。此处的预期行为是，当第二个实例启动时，第一个和第二个实例都会收到一条消息，然后每十秒报告一次超时。我做错了什么？

Answer 1

selector.select()返回后，必须清除选定的键，因此修改后的循环体变为

if (selector.select(TIMEOUT) == 0) {
System.err.println("timeout");
continue;
}
buffer.clear();
InetSocketAddress address = (InetSocketAddress) channel.receive(buffer);
buffer.flip();
String message = Charset.forName("ASCII").decode(buffer).toString();
System.err.format("From %s received: %sn", address.getHostString(), message);
selector.selectedKeys().clear()

有关更多信息，请参阅为什么应该在 java nio 的 'selector.selectedKeys((.iterator((' 中删除密钥？