import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.text.DecimalFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.concurrent.TimeUnit;

/**
   * JAVA版本的自動生成有規則的訂單號(或編號)
   * 生成的格式是: 200908010001 前面幾位為當前的日期,后面五位為系統自增長類型的編號
   * 原理:
   *      1.獲取當前日期格式化值;
   *      2.讀取文件,上次編號的值+1最為當前此次編號的值
   *      (新的一天會重新從1開始編號)
   */

public class Test01 {
   
    public static void main(String[] args) throws InterruptedException {
        SerialNumber serial = new FileEveryDaySerialNumber(5, "EveryDaySerialNumber.dat");
        while(true) {
            System.out.println(serial.getSerialNumber());
            TimeUnit.SECONDS.sleep(2);
        }
    }
}

abstract class SerialNumber {

    public synchronized String getSerialNumber() {
        return process();
    }
    protected abstract String process();
}

abstract class EveryDaySerialNumber extends SerialNumber {
   
    protected final static SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
    protected DecimalFormat df = null;
   
    public EveryDaySerialNumber(int width) {
        if(width < 1) {
            throw new IllegalArgumentException("Parameter length must be great than 1!");
        }
        char[] chs = new char[width];
        for(int i = 0; i < width; i++) {
            chs[i] = '0';
        }
        df = new DecimalFormat(new String(chs));
    }
   
    protected String process() {
        Date date = new Date();
        int n = getOrUpdateNumber(date, 1);
        return format(date) + format(n);
    }
   
    protected String format(Date date) {
        return sdf.format(date);
    }
    protected String format(int num) {
        return df.format(num);
    }
   
    /**
     * 獲得序列號，同時更新持久化存儲中的序列
     * @param current 當前的日期
     * @param start   初始化的序號
     * @return 所獲得新的序列號
     */
    protected abstract int getOrUpdateNumber(Date current, int start);
}

class FileEveryDaySerialNumber extends EveryDaySerialNumber {

    /**
     * 持久化存儲的文件
     */   
    private File file = null;
   
    /**
     * 存儲的分隔符
     */
    private final static String FIELD_SEPARATOR = ",";   

    public FileEveryDaySerialNumber(int width, String filename) {
        super(width);
        file = new File(filename);
    }

    @Override
    protected int getOrUpdateNumber(Date current, int start) {
        String date = format(current);
        int num = start;
        if(file.exists()) {
            List<String> list = FileUtil.readList(file);       
            String[] data = list.get(0).split(FIELD_SEPARATOR);
            if(date.equals(data[0])) {
                num = Integer.parseInt(data[1]);
            }
        }
        FileUtil.rewrite(file, date + FIELD_SEPARATOR + (num + 1));
        return num;
    }       
}

class FileUtil {

    public static void rewrite(File file, String data) {
        BufferedWriter bw = null;
        try {
            bw = new BufferedWriter(new FileWriter(file));
            bw.write(data);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {       
            if(bw != null) {
               try {
                   bw.close();
               } catch(IOException e) {
                   e.printStackTrace();
               }
            }           
        }
    }
   
    public static List<String> readList(File file) {
        BufferedReader br = null;
        List<String> data = new ArrayList<String>();
        try {
            br = new BufferedReader(new FileReader(file));
            for(String str = null; (str = br.readLine()) != null; ) {
                data.add(str);
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if(br != null) {
               try {
                   br.close();
               } catch(IOException e) {
                   e.printStackTrace();
               }
            }
        }
        return data;
    }
}

存儲自動編號值的文件如下(文件名: EveryDaySerialNumber.dat)