java - 如何在java中的where子句中传递变量的值？

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 4 年前。

我正在尝试执行一个包含where条件的sql查询。但每当我执行查询时，它都会给我 java.lang.NullPointerException 。请帮助我!

我正在使用 mssql 和 java

dbURL = "jdbc:sqlserver://ip:portnumber;databaseName=abc";
        username = "abc";
        password = "abc";

        //Load MS SQL JDBC Driver
        Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
        con = DriverManager.getConnection(dbURL,username,password);

        //Creating statement object
        st = con.createStatement();
        selectquery = "select * from abc where TradeType = '"+tradepair+"'";
        rs = st.executeQuery(selectquery);

        while (rs.next()) {

            getpair = rs.getString("TradeType");
            dbTradePair.add(getpair);

        }
        System.out.println("Pairs :-"+dbTradePair);

当执行“selectquery”时，我得到“java.lang.NullPointerException”。我希望我能以正确的格式执行 sql 查询。有人可以帮我解决这个问题吗？

Answer 1

像这样(编辑 - 之后正确关闭资源):

//Load MS SQL JDBC Driver
    try {

        Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");

    } catch (ClassNotFoundException e) {

        System.out.println(e.getMessage());

    }

    final String dbURL = "jdbc:sqlserver://ip:portnumber;databaseName=abc";
    final String username = "abc";
    final String password = "abc";

    Connection  con = null;
    PreparedStatement preparedStatement = null;
    final String selectquery = "select * from abc where TradeType = ?";
    try {
        con = DriverManager.getConnection(dbURL,username,password);
        preparedStatement = con.prepareStatement(selectquery);
        preparedStatement.setString(1, tradepair);

        // execute select SQL stetement
        ResultSet rs = preparedStatement.executeQuery();

        while (rs.next()) {

            String tradeType = rs.getString("TradeType");

            System.out.println("tradeType : " + tradeType);
        }

    } catch (SQLException e) {

        System.out.println(e.getMessage());

    } finally {

        if (preparedStatement != null) {
            preparedStatement.close();
        }

        if (con != null) {
            con.close();
        }

    }