1.　　转账操作 (不添加事务)

public static void transfer(String username1,String username2,int amount) {

        Connection con = null;
        PreparedStatement stmt1 = null;
        PreparedStatement stmt2 = null;
        
        try {
            con = JDBCUtils.getConnection();
            
            String sql = "update user set balance = balance - ? where username = ?";
            stmt1 = con.prepareStatement(sql);
            stmt1.setInt(1, amount);
            stmt1.setString(2,username1);    
            stmt1.executeUpdate();
    
            // 如果在这里出现异常, 则转账人会损失金钱   
            
            sql = "update user set balance = balance + ? where username = ?";
            stmt2 = con.prepareStatement(sql);
            stmt2.setInt(1, amount);
            stmt2.setString(2,username2);
            stmt2.executeUpdate();
            
    
            
        } catch (Exception e) {        
            e.printStackTrace();
        } finally {
            JDBCUtils.close(stmt2, stmt1, con);
        }
    }                          

2.　　转账操作 (添加事务)

public static void transfer(String username1,String username2,int amount) {

        Connection con = null;
        PreparedStatement stmt1 = null;
        PreparedStatement stmt2 = null;
        
        try {
            con = JDBCUtils.getConnection();
            
            con.setAutoCommit(false); // 开启事务
            
            String sql = "update user set balance = balance -? where username =?";
            stmt1 = con.prepareStatement(sql);
            stmt1.setInt(1, amount);
            stmt1.setString(2,username1);    
            stmt1.executeUpdate();
            
　　　　　　　 // 在"事务"中, 如果有代码执行不成功,＂事务＂就不会执行　


            sql = "update user set balance = balance + ? where username =?";
            stmt2 = con.prepareStatement(sql);
            stmt2.setInt(1, amount);
            stmt2.setString(2,username2);
            stmt2.executeUpdate();
            
            con.commit(); // 提交事务
    
            
        } catch (Exception e) {        
            e.printStackTrace();
        } finally {
            JDBCUtils.close(stmt2, stmt1, con);
        }
    }