Java servlet 在服务器重启后跳过一个数据库读取

Question

所以我刚开始学习java EE，我做了一个简单的servlet，用数据库统计访问者。最好的选择是每次访问者打开网站时添加条目，并简单地读取最后一个条目的自动增量字段(主键)，这样我还可以保存访问日期和时间等。但是由于我正在学习，所以我选择做一些愚蠢的事情来进行实验，并且我有这个数据库:

id = 1，访问次数 = 21。

对于每个访问者，servlet 将访问变量更新为新数字。只要我不重启服务器，它就可以工作。当我执行 servlet 时跳过读取并开始计算 visits 设置的值变量的值(数据库本身不会重置)。
问题是我有旧笔记本电脑，所以我不想制作本地服务器进行测试，这需要几个小时，我使用的是 openshift tomcat7，但我不知道如何检查那里的错误日志。

它只是一个简单的 httpServlet，这是我的 get 方法中的逻辑:

String sqlRead = "SELECT *my_cell* FROM *my_table* WHERE id = 1";
String sqlUpdate = "UPDATE *my_table* SET *my_cell*=? WHERE id = 1";
int visits = 0;


try {
    Class.forName("com.mysql.jdbc.Driver");
    ResultSet qResult = DriverManager.getConnection
            ("jdbc:mysql://IPI.P.IPI.P:PORT/dbname", "user", "pass")
            .createStatement().executeQuery(sqlRead);

        //i know this reader starts at index-1
        qResult.next();
        this.visits = qResult.getInt("visits");
} catch (Exception e) {
    System.out.print(e);
}

visits++;

try {
    Class.forName("com.mysql.jdbc.Driver");
    Connection writeConnection = DriverManager.getConnection
            ("jdbc:mysql://IPI.P.IPI.P:PORT/dbname", "user", "pass");

        PreparedStatement preparedStmt = writeConnection.prepareStatement(sqlUpdate);
        preparedStmt.setInt(1, this.visits);
        preparedStmt.executeUpdate();

    writeConnection.close();
} catch (Exception ex) {System.out.println(ex);
}
String visitsString = ""+visits;
response.setContentType("application/json");
response.getWriter().write(visitsString);

Answer 1

好的，所以我得到了一个工作版本，我把它拆掉了，这样数据库处理就可以进入新的类(class)了。我没有读完有关并发的内容，但到目前为止，我了解到 servlet 是由 tomcat 处理的，所以我只是同步了函数，希望我做对了。请更正任何进一步的错误。这是新版本:

private int localVisits = -1;

String sqlRead = "SELECT visits FROM visitsTable WHERE id = 1;";
String sqlUpdate = "UPDATE visitsTable SET visits=visits+1 WHERE id = 1;";
Connection con;

protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    try {
        Class.forName("com.mysql.jdbc.Driver");
        con = DriverManager.getConnection
        ("jdbc:mysql://XXX.XXX.XXX.XXX:PPPP/dbName", "user", "pass");
    } catch (Exception e) {}

//calling write new visitor to db function
    writeDBCounter();

//reading from db to send to frontend
    String report = "-2";
    try {
        report = "" + readDBCounter();
    } catch (SQLException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }
    response.setContentType("application/json");
    response.getWriter().write(report);
//closing connection
try {
    con.close();
} catch (SQLException e) {
    e.printStackTrace();
}

}

// read
private synchronized int readDBCounter() throws SQLException {
    int newVisits = -1;
    ResultSet rs = null;
    try {
        java.sql.Statement s =con.createStatement();
        s.executeQuery(sqlRead);
         rs = s.getResultSet();
        rs.next();
        newVisits = rs.getInt("visits");


    } catch (Exception ex) {
        System.out.print(ex);
    }
    rs.close();
    return newVisits;
}

// write
private synchronized void writeDBCounter() {
    try {
        PreparedStatement preparedStmt = con.prepareStatement(sqlUpdate);
        preparedStmt.executeUpdate();
    } catch (Exception ex) {
    }
}

protected void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    // TODO Auto-generated method stub
    doGet(request, response);
}