在我的sql one servlet中查询和更新两个单独的表

Question

目前我正在做一个应用程序，这是一个租车的应用程序。该应用程序可以租车，还车等。对于还车，我输入还车者的用户名，试图查询数据库并提取出他们正在租哪辆车，然后更新用户不再有租车，并更新汽车细节以变得可用。然而，我注意到，当我查询数据库时，我得到了一个空返回。我看到我的表有这个用户，所以也许我的语法是错误的，请帮助。

protected void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
    String carid = "";
    String uname = req.getParameter("User_Name");
    String defaulted = "User has no car";

    int rs;
    Connection conn = null;
    java.sql.PreparedStatement st = null;
    String nativeSQL = "";
    ResultSet result  =null;

    try {
        Context ctx = new InitialContext();
        Context env = (Context) ctx.lookup("java:comp/env");
        DataSource ds = (DataSource) env.lookup("jdbc/carRentalSystem");
        conn = ds.getConnection();

        conn.setAutoCommit(false);
        result = st.executeQuery("SELECT * FROM userdetails where User_Name='"+ uname+"'");
        if(result.next()) {
            carid= result.getString("Car_Rental");
        }

        st = conn.prepareStatement("update userdetails SET Car_Rental ='" + defaulted+ "' where User_Name='" + uname+ "' ");
        st.clearParameters();
        rs = st.executeUpdate();

        st = conn.prepareStatement("update cardetails SET Availability = 'Available' where id='" + carid+ "'");
        st.clearParameters();
        rs = st.executeUpdate();
        if (rs != 0) {
            res.sendRedirect("carRental.jsp");
            return;
        } else {

        } 
        conn.commit();


    } catch (Exception e) {
        try{conn.rollback();}catch(Exception e1){}
        e.printStackTrace();
    } finally {
        try {
            if (st != null)
                st.close();
        } catch (java.sql.SQLException e) {
        }
        try {
            if (conn != null)
                conn.close();
        } catch (java.sql.SQLException e) {
        }

    }
}

INFO: Server startup in 7620 ms

java.lang.nullpointerException位于com.rental.servlet.adminReturnCarServlet.doPost(adminReturnCarServlet.java:63)，位于javax.servlet.http.httpservlet.service(httpservlet.java:648)，位于javax.servlet.http.httpservlet.service(httpservlet.java:729)，位于org.apache.catalina.core.applicationFilterChain.InternaldoFilter(applicationFilterChain.java:292)，位于doFilter(wsfilter.java:52)，org.apache.catalina.core.applicationFilterChain.internaldoFilter(applicationFilterChain.java:240)，org.apache.catalina.core.applicationFilterChain.doFilter(applicationFilterChain.java:207)，org.apache.catalina.core.standardWrapperValve.invoke(StandardWrapperValve.invoke(StandardWrapperValve.invoke(StandardContIna.Valves.Abst在org.apache.catalina.core.standardHostValve.invoke（standardHostValve.java:141)在org.apache.catalina.valves.errorReportValve.invoke（errorReportValve.invoke（errorReportValve.java:79)在org.apache.catalina.valves.abractAccessLogValve.invoke（abractAccessLogValve.java:620)在）在org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1132)在org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:684)在org.apache.tomcat.util.net.nioEndpoint$socketProcessor.dorun(NioEndpoint.java:1539)在ent.threadpoolexecutor$worker.run（threadpoolexecutor.java:624)在org.apache.tomcat.util.threads.taskthread$wrappingrunnable.run(taskthread.java:61)在java.lang.thread.run(thread.java:748)

Answer 1

您的错误是因为java.sql.PreparedStatement st=null；在行处执行之前没有初始化：

st.executeQuery("SELECT * FROM userdetails where User_Name='"+ uname+"'");

希望能有所帮助。