用Java生成顺序事务ID

Question

I want to generate Sequential Trnsaction ID's for employees to store in the Database ... I wrote the below jsp code(This ain't the code to insert Tx ID in the database) ..The code works fine .. When i use order by clause to see the latest transaction ..i'm not getting the expected transation ID...How can i make the transaction ID's unique , Sequential so that i can retrieve them in a particular sequence by using Order By clause?

  <%
         String url = "jdbc:mysql://localhost:3306/";

        String dbName = "ystem";
        String driver = "com.mysql.jdbc.Driver";
        String userName = "root";
        String password = "";
        try {
            Class.forName(driver).newInstance();
            Connection conn = DriverManager.getConnection(url+dbName,userName,password);
            Statement stat=conn.createStatement();
            ResultSet rs=stat.executeQuery("select count(*) from `employee`");
            int x=0;
            UUID idOne = UUID.randomUUID();
            while(rs.next()) {                    
                x=Integer.parseInt(rs.getString(1))+1;
            }
            out.println(idOne.toString().toUpperCase()+"-"+String.valueOf(x));
                        stat.close(); conn.close();
        }
        catch(Exception x) {
            out.println(x.getMessage());
        }
    %>

Answer 1

Leaving aside the wisdom of this approach, appending the count to the UUID isn't going to give you something that will be meaningfully orderable by an order by clause because you are creating something in the form RANDOMSTUFF-2 . That can't be ordered in any sequence because order by will sort a string column lexically starting with the first character.

So instead, put the counter at the beginning of your string. Then you'll have something order by can meaningfully sort:

String s = String.valueOf(x) + "-" + idOne.toString().toUpperCase();

(Though you'll probably want/need to zero-pad the output of String.valueOf(x) because otherwise "1000" will be sorted to be before "2" . So you need "2" to output as "0002" , for example.)

(and of course you should use StringBuilder.append() (or StringBuffer if your version of Java is old enough) rather than string concatenation).