java修改系统时间

import java.io.IOException;
public class SetTime {
 public static void main(String args[]){
  String osName = System.getProperty("os.name");  
  String cmd = "";  
  try {  
      if (osName.matches("^(?i)Windows.*$")) {// Window 系统  
      // 格式 HH:mm:ss  
      cmd = "  cmd /c time 22:35:00";  
      Runtime.getRuntime().exec(cmd);  
      // 格式：yyyy-MM-dd  
      cmd = " cmd /c date 2009-03-26";  
      Runtime.getRuntime().exec(cmd);  
  } else {// Linux 系统  
     // 格式：yyyyMMdd  
     cmd = "  date -s 20090326";  
     Runtime.getRuntime().exec(cmd);  
     // 格式 HH:mm:ss  
     cmd = "  date -s 22:35:00";  
     Runtime.getRuntime().exec(cmd);  
  }  
  } catch (IOException e) {  
      e.printStackTrace();  
  }  
 }
}


---------------获取互联网时间-------------------
 public static void main(String[] args) throws Exception {
		  // TODO Auto-generated method stub
		       URL url=new URL("http://www.bjtime.cn");//取得资源对象
		       URLConnection uc=url.openConnection();//生成连接对象
		       uc.connect(); //发出连接
		       long ld=uc.getDate(); //取得网站日期时间
		       Date date=new Date(ld); //转换为标准时间对象
		       System.out.println(date);
		      
		 }

------通过ntp 获取系统准确时间---------
本程序是java通过网络访问NTP时间服务器，而获得精准的时间。不是获得本地时间

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.println("poll: valid NTP request received the local clock offset is " + 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);  
        }  
  
          
    }  
}  

import java.text.DecimalFormat;  
import java.text.SimpleDateFormat;  
import java.util.Date;  
  
public class NtpMessage {  
    /** *//** 
     * This is a two-bit code warning of an impending leap second to be 
     * inserted/deleted in the last minute of the current day. It''s values may 
     * be as follows: 
     *  
     * Value Meaning ----- ------- 0 no warning 1 last minute has 61 seconds 2 
     * last minute has 59 seconds) 3 alarm condition (clock not synchronized) 
     */  
    public byte leapIndicator = 0;  
  
    /** *//** 
     * This value indicates the NTP/SNTP version number. The version number is 3 
     * for Version 3 (IPv4 only) and 4 for Version 4 (IPv4, IPv6 and OSI). If 
     * necessary to distinguish between IPv4, IPv6 and OSI, the encapsulating 
     * context must be inspected. 
     */  
    public byte version = 3;  
  
    /** *//** 
     * This value indicates the mode, with values defined as follows: 
     *  
     * Mode Meaning ---- ------- 0 reserved 1 symmetric active 2 symmetric 
     * passive 3 client 4 server 5 broadcast 6 reserved for NTP control message 
     * 7 reserved for private use 
     *  
     * In unicast and anycast modes, the client sets this field to 3 (client) in 
     * the request and the server sets it to 4 (server) in the reply. In 
     * multicast mode, the server sets this field to 5 (broadcast). 
     */  
    public byte mode = 0;  
  
    /** *//** 
     * This value indicates the stratum level of the local clock, with values 
     * defined as follows: 
     *  
     * Stratum Meaning ---------------------------------------------- 0 
     * unspecified or unavailable 1 primary reference (e.g., radio clock) 2-15 
     * secondary reference (via NTP or SNTP) 16-255 reserved 
     */  
    public short stratum = 0;  
  
    /** *//** 
     * This value indicates the maximum interval between successive messages, in 
     * seconds to the nearest power of two. The values that can appear in this 
     * field presently range from 4 (16 s) to 14 (16284 s); however, most 
     * applications use only the sub-range 6 (64 s) to 10 (1024 s). 
     */  
    public byte pollInterval = 0;  
  
    /** *//** 
     * This value indicates the precision of the local clock, in seconds to the 
     * nearest power of two. The values that normally appear in this field 
     * range from -6 for mains-frequency clocks to -20 for microsecond clocks 
     * found in some workstations. 
     */  
    public byte precision = 0;  
  
    /** *//** 
     * This value indicates the total roundtrip delay to the primary reference 
     * source, in seconds. Note that this variable can take on both positive and 
     * negative values, depending on the relative time and frequency offsets. 
     * The values that normally appear in this field range from negative values 
     * of a few milliseconds to positive values of several hundred milliseconds. 
     */  
    public double rootDelay = 0;  
  
    /** *//** 
     * This value indicates the nominal error relative to the primary reference 
     * source, in seconds. The values that normally appear in this field range 
     * from 0 to several hundred milliseconds. 
     */  
    public double rootDispersion = 0;  
  
    /** *//** 
     * This is a 4-byte array identifying the particular reference source. In 
     * the case of NTP Version 3 or Version 4 stratum-0 (unspecified) or 
     * stratum-1 (primary) servers, this is a four-character ASCII string, left 
     * justified and zero padded to 32 bits. In NTP Version 3 secondary servers, 
     * this is the 32-bit IPv4 address of the reference source. In NTP Version 4 
     * secondary servers, this is the low order 32 bits of the latest transmit 
     * timestamp of the reference source. NTP primary (stratum 1) servers should 
     * set this field to a code identifying the external reference source 
     * according to the following list. If the external reference is one of 
     * those listed, the associated code should be used. Codes for sources not 
     * listed can be contrived as appropriate. 
     *  
     * Code External Reference Source ---- ------------------------- LOCL 
     * uncalibrated local clock used as a primary reference for a subnet without 
     * external means of synchronization PPS atomic clock or other 
     * pulse-per-second source individually calibrated to national standards 
     * ACTS NIST dialup modem service USNO USNO modem service PTB PTB (Germany) 
     * modem service TDF Allouis (France) Radio 164 kHz DCF Mainflingen 
     * (Germany) Radio 77.5 kHz MSF Rugby (UK) Radio 60 kHz WWV Ft. Collins (US) 
     * Radio 2.5, 5, 10, 15, 20 MHz WWVB Boulder (US) Radio 60 kHz WWVH Kaui 
     * Hawaii (US) Radio 2.5, 5, 10, 15 MHz CHU Ottawa (Canada) Radio 3330, 
     * 7335, 14670 kHz LORC LORAN-C radionavigation system OMEG OMEGA 
     * radionavigation system GPS Global Positioning Service GOES Geostationary 
     * Orbit Environment Satellite 
     */  
    public byte[] referenceIdentifier = { 0, 0, 0, 0 };  
  
    /** *//** 
     * This is the time at which the local clock was last set or corrected, in 
     * seconds since 00:00 1-Jan-1900. 
     */  
    public double referenceTimestamp = 0;  
  
    /** *//** 
     * This is the time at which the request departed the client for the server, 
     * in seconds since 00:00 1-Jan-1900. 
     */  
    public double originateTimestamp = 0;  
  
    /** *//** 
     * This is the time at which the request arrived at the server, in seconds 
     * since 00:00 1-Jan-1900. 
     */  
    public double receiveTimestamp = 0;  
  
    /** *//** 
     * This is the time at which the reply departed the server for the client, 
     * in seconds since 00:00 1-Jan-1900. 
     */  
    public double transmitTimestamp = 0;  
  
    /** *//** 
     * Constructs a new NtpMessage from an array of bytes. 
     */  
    public NtpMessage(byte[] array) {  
        // See the packet format diagram in RFC 2030 for details  
        leapIndicator = (byte) ((array[0] >> 6) & 0x3);  
        version = (byte) ((array[0] >> 3) & 0x7);  
        mode = (byte) (array[0] & 0x7);  
        stratum = unsignedByteToShort(array[1]);  
        pollInterval = array[2];  
        precision = array[3];  
  
        rootDelay = (array[4] * 256.0) + unsignedByteToShort(array[5]) + (unsignedByteToShort(array[6]) / 256.0) + (unsignedByteToShort(array[7]) / 65536.0);  
  
        rootDispersion = (unsignedByteToShort(array[8]) * 256.0) + unsignedByteToShort(array[9]) + (unsignedByteToShort(array[10]) / 256.0) + (unsignedByteToShort(array[11]) / 65536.0);  
  
        referenceIdentifier[0] = array[12];  
        referenceIdentifier[1] = array[13];  
        referenceIdentifier[2] = array[14];  
        referenceIdentifier[3] = array[15];  
  
        referenceTimestamp = decodeTimestamp(array, 16);  
        originateTimestamp = decodeTimestamp(array, 24);  
        receiveTimestamp = decodeTimestamp(array, 32);  
        transmitTimestamp = decodeTimestamp(array, 40);  
    }  
  
    /** *//** 
     * Constructs a new NtpMessage 
     */  
    public NtpMessage(byte leapIndicator, byte version, byte mode, short stratum, byte pollInterval, byte precision, double rootDelay, double rootDispersion, byte[] referenceIdentifier, double referenceTimestamp, double originateTimestamp, double receiveTimestamp, double transmitTimestamp) {  
        // ToDo: Validity checking  
        this.leapIndicator = leapIndicator;  
        this.version = version;  
        this.mode = mode;  
        this.stratum = stratum;  
        this.pollInterval = pollInterval;  
        this.precision = precision;  
        this.rootDelay = rootDelay;  
        this.rootDispersion = rootDispersion;  
        this.referenceIdentifier = referenceIdentifier;  
        this.referenceTimestamp = referenceTimestamp;  
        this.originateTimestamp = originateTimestamp;  
        this.receiveTimestamp = receiveTimestamp;  
        this.transmitTimestamp = transmitTimestamp;  
    }  
  
    /** *//** 
     * Constructs a new NtpMessage in client -> server mode, and sets the 
     * transmit timestamp to the current time. 
     */  
    public NtpMessage() {  
        // Note that all the other member variables are already set with  
        // appropriate default values.  
        this.mode = 3;  
        this.transmitTimestamp = (System.currentTimeMillis() / 1000.0) + 2208988800.0;  
    }  
  
    /** *//** 
     * This method constructs the data bytes of a raw NTP packet. 
     */  
    public byte[] toByteArray() {  
        // All bytes are automatically set to 0  
        byte[] p = new byte[48];  
  
        p[0] = (byte) (leapIndicator << 6 | version << 3 | mode);  
        p[1] = (byte) stratum;  
        p[2] = (byte) pollInterval;  
        p[3] = (byte) precision;  
  
        // root delay is a signed 16.16-bit FP, in Java an int is 32-bits  
        int l = (int) (rootDelay * 65536.0);  
        p[4] = (byte) ((l >> 24) & 0xFF);  
        p[5] = (byte) ((l >> 16) & 0xFF);  
        p[6] = (byte) ((l >> 8) & 0xFF);  
        p[7] = (byte) (l & 0xFF);  
  
        // root dispersion is an unsigned 16.16-bit FP, in Java there are no  
        // unsigned primitive types, so we use a long which is 64-bits  
        long ul = (long) (rootDispersion * 65536.0);  
        p[8] = (byte) ((ul >> 24) & 0xFF);  
        p[9] = (byte) ((ul >> 16) & 0xFF);  
        p[10] = (byte) ((ul >> 8) & 0xFF);  
        p[11] = (byte) (ul & 0xFF);  
  
        p[12] = referenceIdentifier[0];  
        p[13] = referenceIdentifier[1];  
        p[14] = referenceIdentifier[2];  
        p[15] = referenceIdentifier[3];  
  
        encodeTimestamp(p, 16, referenceTimestamp);  
        encodeTimestamp(p, 24, originateTimestamp);  
        encodeTimestamp(p, 32, receiveTimestamp);  
        encodeTimestamp(p, 40, transmitTimestamp);  
  
        return p;  
    }  
  
    /** *//** 
     * Returns a string representation of a NtpMessage 
     */  
    public String toString() {  
        String precisionStr = new DecimalFormat("0.#E0").format(Math.pow(2, precision));  
        return "Leap indicator: " + leapIndicator + " " + "Version: " + version + " " + "Mode: " + mode + " " + "Stratum: " + stratum + " " + "Poll: " + pollInterval + " " + "Precision: " + precision + " (" + precisionStr + " seconds) " + "Root delay: " + new DecimalFormat("0.00").format(rootDelay * 1000) + " ms " + "Root dispersion: " + new DecimalFormat("0.00").format(rootDispersion * 1000) + " ms " + "Reference identifier: " + referenceIdentifierToString(referenceIdentifier, stratum, version) + " " + "Reference timestamp: " + timestampToString(referenceTimestamp) + " " + "Originate timestamp: " + timestampToString(originateTimestamp) + " " + "Receive timestamp:   " + timestampToString(receiveTimestamp) + " " + "Transmit timestamp: " + timestampToString(transmitTimestamp);  
    }  
  
    /** *//** 
     * Converts an unsigned byte to a short. By default, Java assumes that a 
     * byte is signed. 
     */  
    public static short unsignedByteToShort(byte b) {  
        if ((b & 0x80) == 0x80)  
            return (short) (128 + (b & 0x7f));  
        else  
            return (short) b;  
    }  
  
    /** *//** 
     * Will read 8 bytes of a message beginning at <code>pointer</code> and 
     * return it as a double, according to the NTP 64-bit timestamp format. 
     */  
    public static double decodeTimestamp(byte[] array, int pointer) {  
        double r = 0.0;  
  
        for (int i = 0; i < 8; i++) {  
            r += unsignedByteToShort(array[pointer + i]) * Math.pow(2, (3 - i) * 8);  
        }  
  
        return r;  
    }  
  
    /** *//** 
     * Encodes a timestamp in the specified position in the message 
     */  
    public static void encodeTimestamp(byte[] array, int pointer, double timestamp) {  
        // Converts a double into a 64-bit fixed point  
        for (int i = 0; i < 8; i++) {  
            // 2^24, 2^16, 2^8, .. 2^-32  
            double base = Math.pow(2, (3 - i) * 8);  
  
            // Capture byte value  
            array[pointer + i] = (byte) (timestamp / base);  
  
            // Subtract captured value from remaining total  
            timestamp = timestamp - (double) (unsignedByteToShort(array[pointer + i]) * base);  
        }  
  
        // From RFC 2030: It is advisable to fill the non-significant  
        // low order bits of the timestamp with a random, unbiased  
        // bitstring, both to avoid systematic roundoff errors and as  
        // a means of loop detection and replay detection.  
        array[7] = (byte) (Math.random() * 255.0);  
    }  
  
    /** *//** 
     * Returns a timestamp (number of seconds since 00:00 1-Jan-1900) as a 
     * formatted date/time string. 
     */  
    public static String timestampToString(double timestamp) {  
        if (timestamp == 0)  
            return "0";  
  
        // timestamp is relative to 1900, utc is used by Java and is relative  
        // to 1970  
        double utc = timestamp - (2208988800.0);  
  
        // milliseconds  
        long ms = (long) (utc * 1000.0);  
  
        // date/time  
        String date = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss").format(new Date(ms));  
  
        // fraction  
        double fraction = timestamp - ((long) timestamp);  
        String fractionSting = new DecimalFormat(".000000").format(fraction);  
  
        return date + fractionSting;  
    }  
  
    /** *//** 
     * Returns a string representation of a reference identifier according to 
     * the rules set out in RFC 2030. 
     */  
    public static String referenceIdentifierToString(byte[] ref, short stratum, byte version) {  
        // From the RFC 2030:  
        // In the case of NTP Version 3 or Version 4 stratum-0 (unspecified)  
        // or stratum-1 (primary) servers, this is a four-character ASCII  
        // string, left justified and zero padded to 32 bits.  
        if (stratum == 0 || stratum == 1) {  
            return new String(ref);  
        }  
  
        // In NTP Version 3 secondary servers, this is the 32-bit IPv4  
        // address of the reference source.  
        else if (version == 3) {  
            return unsignedByteToShort(ref[0]) + "." + unsignedByteToShort(ref[1]) + "." + unsignedByteToShort(ref[2]) + "." + unsignedByteToShort(ref[3]);  
        }  
  
        // In NTP Version 4 secondary servers, this is the low order 32 bits  
        // of the latest transmit timestamp of the reference source.  
        else if (version == 4) {  
            return "" + ((unsignedByteToShort(ref[0]) / 256.0) + (unsignedByteToShort(ref[1]) / 65536.0) + (unsignedByteToShort(ref[2]) / 16777216.0) + (unsignedByteToShort(ref[3]) / 4294967296.0));  
        }  
  
        return "";  
    }  
}  

 
参见:http://lshh83.iteye.com/blog/546405