从servlet访问数据 [英] Accessing data from servlet

问题描述

我要求只能从localhost访问mysql数据库。我必须实现一个访问数据库的servlet，允许该系统中的其他服务器访问数据（servlet将作为代理工作）。但是，该系统由一个远程服务器组成，该服务器下载执行语句的大部分数据，如：

I have got a requirement that mysql database can only be accessed from localhost. I have to implement a servlet that would access the database allowing other servers in this system to access data (servlet would work as a proxy). However, this system consists of a remote server which downloads large portions of data executing a statement like:

select * from database limit 100;

有人可以建议我如何编写一个能够有效地传输这些数据的servlet（我是新的数据库）？

Can somebody suggest me how to write a servlet that would stream such data in a efficient way (I am new to databases)?

推荐答案

首先，我不建议为此使用servlet。请参阅aioobe和mdma的答案以获得正确的方法。但如果真的没有其他选择，那么继续阅读：

First of all, I don't recommend to use a servlet for this. See the answers of aioobe and mdma for the right approach. But if there is really no other option, then continue reading:

只需立即将数据写入响应 随着数据的进入。不要将所有内容存储在Java的内存中。所以基本上： writer.write（resultSet.getString（col））。此外，默认情况下，MySQL JDBC驱动程序会在向 ResultSet＃next（）提供任何内容之前缓存Java内存中的所有内容。你想让它通过设置 语句#setFetchSize（） 根据 MySQL JDBC驱动程序文档。

Just write the data to the response immediately as the data comes in. Don't store everything in Java's memory. So basically: writer.write(resultSet.getString("col")). Further, the MySQL JDBC driver will by default cache everything in Java's memory before giving anything to ResultSet#next(). You'd like to let it give the data immediately row-by-row by setting the Statement#setFetchSize() as per the MySQL JDBC driver documentation.

这是一个启动示例，假设您想以CSV格式输出数据：

Here's a kickoff example, assuming you'd like to output the data in CSV format:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/csv");

    Connection connection = null;
    Statement statement = null;
    ResultSet resultSet = null;
    PrintWriter writer = response.getWriter();

    try {
        connection = database.getConnection();
        statement = connection.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
        statement.setFetchSize(Integer.MIN_VALUE);
        resultSet = statement.executeQuery("SELECT col1, col2, col3 FROM tbl");

        while (resultSet.next()) {
            writer.append(resultSet.getString("col1")).append(',');
            writer.append(resultSet.getString("col2")).append(',');
            writer.append(resultSet.getString("col3")).println();
            // PS: don't forget to sanitize quotes/commas as per RFC4130.
        }
    } catch (SQLException e) {
        throw new ServletException("Query failed!", e);
    } finally { 
        if (resultSet != null) try { resultSet.close; } catch (SQLException logOrIgnore) {}
        if (statement != null) try { statement.close; } catch (SQLException logOrIgnore) {}
        if (connection != null) try { connection.close; } catch (SQLException logOrIgnore) {}
    }
}

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