Android 客户端如何与服务器时间同步

这次分享一个简单但是很实用的功能. 开发app经常会涉及获取当前时间这项功能, 大部分情况不需要特别精准, 但是遇到一些统计类的需求, 就需要精准的时间, 比如 : 需要统计消息送达时间, 服务器记录发送时间, 客户端记录接收时间, 两个时间取差值, 就可以得到消息延迟时间;

但是实际中会遇到这些问题:

客户端时间记录的五花八门, 有很多数据甚至比服务器时间还早,计算出负数的差值, 也有很多计算出天文数字的差值; 导致统计到的数据没有分析的价值;

导致问题的原因是客户端造成的, 因为手机的时间难免有些用户会自己设置, 而且即使没有设置,个别手机上的时间也可能不一致;

此时, 我们需要同步服务器的时间,让本地时间和服务器时间保持一致;

1.如何获取服务器时间

使用UDP 中的 DatagramSocket 同步ntp服务器

这里一定要使用UDP协议, 而不能使用http 或 tcp, 因为同步首要的就是要速度快, 获取服务器时间的过程消耗过长, 那时间的误差会更大; UDP的方式时间基本都在几十毫秒内;

这里封装了一个工具类:

SntpClient.java

/** Copyright (C) 2008 The Android Open Source Project** Licensed under the Apache License, Version 2.0 (the "License");* you may not use this file except in compliance with the License.* You may obtain a copy of the License at**      http://www.apache.org/licenses/LICENSE-2.0** Unless required by applicable law or agreed to in writing, software* distributed under the License is distributed on an "AS IS" BASIS,* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.* See the License for the specific language governing permissions and* limitations under the License.*/
package com.yidui.core.common.utils.timeasync;import android.os.SystemClock;
import android.util.Log;import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.Arrays;/***  系统同步ntp服务工具类* {@hide}** Simple SNTP client class for retrieving network time.** Sample usage:* <pre>SntpClient client = new SntpClient();* if (client.requestTime("time.foo.com")) {*     long now = client.getNtpTime() + SystemClock.elapsedRealtime() - client.getNtpTimeReference();* }* </pre>* create by DragonForest on 2020/6/8*/
public class SntpClient {private static final String TAG = "SntpClient";private static final boolean DBG = true;private static final int REFERENCE_TIME_OFFSET = 16;private static final int ORIGINATE_TIME_OFFSET = 24;private static final int RECEIVE_TIME_OFFSET = 32;private static final int TRANSMIT_TIME_OFFSET = 40;private static final int NTP_PACKET_SIZE = 48;private static final int NTP_PORT = 123;private static final int NTP_MODE_CLIENT = 3;private static final int NTP_MODE_SERVER = 4;private static final int NTP_MODE_BROADCAST = 5;private static final int NTP_VERSION = 3;private static final int NTP_LEAP_NOSYNC = 3;private static final int NTP_STRATUM_DEATH = 0;private static final int NTP_STRATUM_MAX = 15;// Number of seconds between Jan 1, 1900 and Jan 1, 1970// 70 years plus 17 leap daysprivate static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L;// system time computed from NTP server responseprivate long mNtpTime;// value of SystemClock.elapsedRealtime() corresponding to mNtpTimeprivate long mNtpTimeReference;// round trip time in millisecondsprivate long mRoundTripTime;private static class InvalidServerReplyException extends Exception {public InvalidServerReplyException(String message) {super(message);}}public boolean requestTime(String host, int timeout) {DatagramSocket socket = null;InetAddress address = null;try {address = InetAddress.getByName(host);socket = new DatagramSocket();socket.setSoTimeout(timeout);byte[] buffer = new byte[NTP_PACKET_SIZE];DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT);// set mode = 3 (client) and version = 3// mode is in low 3 bits of first byte// version is in bits 3-5 of first bytebuffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);// get current time and write it to the request packetfinal long requestTime = System.currentTimeMillis();final long requestTicks = SystemClock.elapsedRealtime();writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime);socket.send(request);// read the responseDatagramPacket response = new DatagramPacket(buffer, buffer.length);socket.receive(response);final long responseTicks = SystemClock.elapsedRealtime();final long responseTime = requestTime + (responseTicks - requestTicks);// extract the resultsfinal byte leap = (byte) ((buffer[0] >> 6) & 0x3);final byte mode = (byte) (buffer[0] & 0x7);final int stratum = (int) (buffer[1] & 0xff);final long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET);final long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET);final long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET);/* do sanity check according to RFC */// TODO: validate originateTime == requestTime.checkValidServerReply(leap, mode, stratum, transmitTime);long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime);// receiveTime = originateTime + transit + skew// responseTime = transmitTime + transit - skewlong clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2;//             = ((originateTime + transit + skew - originateTime) +//                (transmitTime - (transmitTime + transit - skew)))/2//             = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2//             = (transit + skew - transit + skew)/2//             = (2 * skew)/2 = skewLog.d(TAG, "Request time form ntp server success, " + address.toString() + " ,roundTripTime: " + roundTripTime);if (DBG) {Log.d(TAG, "round trip: " + roundTripTime + "ms, " +"clock offset: " + clockOffset + "ms");}// save our results - use the times on this side of the network latency// (response rather than request time)mNtpTime = responseTime + clockOffset;mNtpTimeReference = responseTicks;mRoundTripTime = roundTripTime;} catch (Exception e) {
//            if (DBG) {
//                Log.e(TAG, "Error address: " + address.toString());
//            }Log.e(TAG, "Request time from ntp server failed ,msg: " + e.getMessage());return false;} finally {if (socket != null) {socket.close();}}return true;}/*** Returns the time computed from the NTP transaction.** @return time value computed from NTP server response.*/public long getNtpTime() {return mNtpTime;}/*** Returns the reference clock value (value of SystemClock.elapsedRealtime())* corresponding to the NTP time.** @return reference clock corresponding to the NTP time.*/public long getNtpTimeReference() {return mNtpTimeReference;}/*** Returns the round trip time of the NTP transaction** @return round trip time in milliseconds.*/public long getRoundTripTime() {return mRoundTripTime;}private static void checkValidServerReply(byte leap, byte mode, int stratum, long transmitTime)throws InvalidServerReplyException {if (leap == NTP_LEAP_NOSYNC) {throw new InvalidServerReplyException("unsynchronized server");}if ((mode != NTP_MODE_SERVER) && (mode != NTP_MODE_BROADCAST)) {throw new InvalidServerReplyException("untrusted mode: " + mode);}if ((stratum == NTP_STRATUM_DEATH) || (stratum > NTP_STRATUM_MAX)) {throw new InvalidServerReplyException("untrusted stratum: " + stratum);}if (transmitTime == 0) {throw new InvalidServerReplyException("zero transmitTime");}}/*** Reads an unsigned 32 bit big endian number from the given offset in the buffer.*/private long read32(byte[] buffer, int offset) {byte b0 = buffer[offset];byte b1 = buffer[offset+1];byte b2 = buffer[offset+2];byte b3 = buffer[offset+3];// convert signed bytes to unsigned valuesint i0 = ((b0 & 0x80) == 0x80 ? (b0 & 0x7F) + 0x80 : b0);int i1 = ((b1 & 0x80) == 0x80 ? (b1 & 0x7F) + 0x80 : b1);int i2 = ((b2 & 0x80) == 0x80 ? (b2 & 0x7F) + 0x80 : b2);int i3 = ((b3 & 0x80) == 0x80 ? (b3 & 0x7F) + 0x80 : b3);return ((long)i0 << 24) + ((long)i1 << 16) + ((long)i2 << 8) + (long)i3;}/*** Reads the NTP time stamp at the given offset in the buffer and returns* it as a system time (milliseconds since January 1, 1970).*/private long readTimeStamp(byte[] buffer, int offset) {long seconds = read32(buffer, offset);long fraction = read32(buffer, offset + 4);// Special case: zero means zero.if (seconds == 0 && fraction == 0) {return 0;}return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L);}/*** Writes system time (milliseconds since January 1, 1970) as an NTP time stamp* at the given offset in the buffer.*/private void writeTimeStamp(byte[] buffer, int offset, long time) {// Special case: zero means zero.if (time == 0) {Arrays.fill(buffer, offset, offset + 8, (byte) 0x00);return;}long seconds = time / 1000L;long milliseconds = time - seconds * 1000L;seconds += OFFSET_1900_TO_1970;// write seconds in big endian formatbuffer[offset++] = (byte)(seconds >> 24);buffer[offset++] = (byte)(seconds >> 16);buffer[offset++] = (byte)(seconds >> 8);buffer[offset++] = (byte)(seconds >> 0);long fraction = milliseconds * 0x100000000L / 1000L;// write fraction in big endian formatbuffer[offset++] = (byte)(fraction >> 24);buffer[offset++] = (byte)(fraction >> 16);buffer[offset++] = (byte)(fraction >> 8);// low order bits should be random databuffer[offset++] = (byte)(Math.random() * 255.0);}
}

2.如何将服务器时间同步到本地

当然, 直接修改系统时间肯定是不行的, 但是可以通过转化,将当前时间转化成服务器的当前时间;

这里介绍一个获取系统时间的方法:

SystemClock.elapsedRealtime()  //获取系统开机到现在的时间间隔

因为开机时间是固定的, 所以这个时间是用户无法修改的; 下面我们做个公式:

获取ntp服务器的时间, 记为 serverTime

当获取ntp服务器时间后, 计算一个时间差值 timeDiff = serverTime - SystemClock.elapsedRealtime()

那么之后的系统当前时间, 我们不通过System.currentTimeMillis(), 而是通过 timeDiff+SystemClock.elapsedRealtime()获得

提供工具类:

TimeAsyncManager.kt

package com.yidui.core.common.utils.timeasyncimport android.os.SystemClock
import com.yidui.core.common.logger
import java.text.SimpleDateFormat
import java.util.*/**** 时间同步器*  create by DragonForest on 2020/11/6*/
object TimeAsyncManager {private var TAG = this.javaClass.simpleName@Volatileprivate var elapsedRealtimeDiff: Long? = null@Volatileprivate var currentTimeMillisDiff: Long? = null@Volatileprivate var currentNtpServer: String? = nullprivate var sdf = SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS")/** ntp服务器列表 */private var ntpServerList = arrayOf("ntp.aliyun.com","ntp1.aliyun.com","ntp2.aliyun.com","ntp3.aliyun.com","ntp4.aliyun.com","ntp5.aliyun.com","ntp6.aliyun.com","ntp7.aliyun.com")private fun asyncTime(ntpUrl: String): Boolean {try {currentNtpServer = ntpUrlvar sntpClient = SntpClient()if (sntpClient.requestTime(ntpUrl, 3000)) {var serverTimeStamp = sntpClient.ntpTimeif (serverTimeStamp == 0L) {return false}elapsedRealtimeDiff = serverTimeStamp - SystemClock.elapsedRealtime()currentTimeMillisDiff = serverTimeStamp - System.currentTimeMillis()logger.e(TAG, "同步服务器时间成功：server:$ntpUrl, elapsedRealtimeDiff：$elapsedRealtimeDiff ,currentTimeMillisDiff->$currentTimeMillisDiff")//用来监控用户同步服务器情况(观察)，涉及用户隐私，暂时去掉
//                var json = JSONObject()
//                json.put("ntp_server",ntpUrl)
//                json.put("ntp_timestamp",serverTimeStamp.toString())
//                json.put("elapsed_realtime_diff",elapsedRealtimeDiff.toString())
//                json.put("current_time_millis_diff",currentTimeMillisDiff.toString())
//                ApmStatUtils.trackAction("ntp_async_event", json)return true}} catch (e: Exception) {e.printStackTrace()logger.e(TAG, "同步服务器时间失败：server:$ntpUrl,error:${e.message}")}return false}/*** 当前连接的ntp服务器*/@JvmStaticfun getCurrentNtpServer(): String? {return currentNtpServer}/*** 开机时间与服务器时间误差*/@JvmStaticfun getElapsedRealtimeDiff(): Long? {return elapsedRealtimeDiff}/*** 本地时间与服务器时间误差*/@JvmStaticfun getCurrentTimeMillisDiff(): Long? {return currentTimeMillisDiff}@JvmStaticfun startAsync() {Thread(Runnable out@{ntpServerList.forEach {if (asyncTime(it)) {return@out}}}).start()}/*** 获取当前时间戳*/@JvmStaticfun getCurrentTimeStamp(): Long {var df = elapsedRealtimeDiff ?: return System.currentTimeMillis()return SystemClock.elapsedRealtime() + df}/*** 获取日期字符串*/@JvmStaticfun getCurrentDate(): String {var timeStamp = getCurrentTimeStamp()return sdf.format(Date(timeStamp))}}

后面的使用也很简单, 在Application初始化时调用:

TimeAsyncManager.startAsync()

当需要获取系统时间时,使用:

TimeAsyncManager.getCurrentTimeStamp()