java程序如何获得 ntp时间

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import java.io.IOException;
import java.io.InterruptedIOException;
import java.net.ConnectException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.net.NoRouteToHostException;
import java.net.UnknownHostException;


public class TestNtp {

public static void main(String[] args){
int retry = 2;
int port = 123;
int timeout = 3000;

// get the address and NTP address request
//
InetAddress ipv4Addr = null;
try {
ipv4Addr = InetAddress.getByName("203.117.180.36");//更多NTP时间服务器参考附注
} catch (UnknownHostException e1) {
e1.printStackTrace();
}

int serviceStatus = -1;
DatagramSocket socket = null;
long responseTime = -1;
try {
socket = new DatagramSocket();
socket.setSoTimeout(timeout); // will force the
// InterruptedIOException

for (int attempts = 0; attempts <= retry && serviceStatus != 1; attempts++) {
try {
// Send NTP request
//
byte[] data = new NtpMessage().toByteArray();
DatagramPacket outgoing = new DatagramPacket(data, data.length, ipv4Addr, port);
long sentTime = System.currentTimeMillis();
socket.send(outgoing);

// Get NTP Response
//
// byte[] buffer = new byte[512];
DatagramPacket incoming = new DatagramPacket(data, data.length);
socket.receive(incoming);
responseTime = System.currentTimeMillis() - sentTime;
double destinationTimestamp = (System.currentTimeMillis() / 1000.0) + 2208988800.0;
//这里要加2208988800，是因为获得到的时间是格林尼治时间，所以要变成东八区的时间，否则会与与北京时间有8小时的时差

// Validate NTP Response
// IOException thrown if packet does not decode as expected.
NtpMessage msg = new NtpMessage(incoming.getData());
double localClockOffset = ((msg.receiveTimestamp - msg.originateTimestamp) + (msg.transmitTimestamp - destinationTimestamp)) / 2;

// System.out.print("===" + localClockOffset + ", responseTime= " + responseTime + "ms");
System.out.println("poll: NTP message : " + msg.toString());
serviceStatus = 1;
} catch (InterruptedIOException ex) {
// Ignore, no response received.
}
}
} catch (NoRouteToHostException e) {
System.out.println("No route to host exception for address: " + ipv4Addr);
} catch (ConnectException e) {
// Connection refused. Continue to retry.
e.fillInStackTrace();
System.out.println("Connection exception for address: " + ipv4Addr);
} catch (IOException ex) {
ex.fillInStackTrace();
System.out.println("IOException while polling address: " + ipv4Addr);
} finally {
if (socket != null)
socket.close();
}

// Store response time if available
//
if (serviceStatus == 1) {
System.out.println("responsetime=="+responseTime);
}


}
}