Java订单号（时间加流水号）

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;

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);
    }

    protected abstract int getOrUpdateNumber(Date current, int start);
}

class FileEveryDaySerialNumber extends EveryDaySerialNumber {

    private File file = null;

    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;
    }
}

注意：代码中有个叫EveryDaySerialNumber.dat，可以自行创建这样的文件，然后放到项目的根目录下就可以。此文件主要是用于记录上一次执行程序的节点。