java - Java和JSP中的搜索查询不起作用

我正在尝试在jsp文件中获得一个输入框，以搜索特定的作者并将我的数据库结果显示为表格。

我已经使用此代码列出了数据库中的所有内容，并将其显示在jsp上。但是现在我只想显示共享相同作者姓名的每个结果。

SearchQuery.java

    package dbhelpers;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import model.Book;

public class SearchQuery {
    private Connection connection = null;
    private ResultSet results;

    public SearchQuery(String dbName, String uname, String pwd) {

        String url = "jdbc:mysql://localhost:3306/" + dbName;

        //setting up driver

        try {

            Class.forName("com.mysql.jdbc.Driver").newInstance();
            this.connection = DriverManager.getConnection(url, uname, pwd);

        } catch (InstantiationException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

}
public void doRead(){

        String query = "select title, author, pages from books where author =?";

        try {

            PreparedStatement ps = this.connection.prepareStatement(query);
            this.results = ps.executeQuery();

        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    public String getHTMLTable(){
    String table ="";

    table += "<table border =1>";

    try {
        while(this.results.next()) {
            Book book = new Book();
            book.setBookID(this.results.getInt("bookID"));
            book.setTitle(this.results.getString("title"));
            book.setAuthor(this.results.getString("author"));
            book.setPages(this.results.getInt("pages"));

            table += "<tr>";
            table += "<td>";
            table +=    book.getTitle();
            table += "</td>";

            table += "<td>";
            table +=    book.getAuthor();
            table += "</td>";

            table += "<td>";
            table +=    book.getPages();
            table += "</td>";

            table += "<td>";
                table +="<a href = update?bookID=" +  book.getBookID() +">update</a> <a href = delete?bookID="+ book.getBookID() + ">delete</a>";

            table += "</td>";

            table += "</tr>";
        }
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    table += "</table>";

    return table;



}

}

SearchServlet.java

    package controller;

import java.io.IOException;


import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import dbhelpers.SearchQuery;

/**
 * Servlet implementation class SearchServlet
 */
public class SearchServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public SearchServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        SearchQuery sq = new SearchQuery("book_lib","root", "");

        //getting html table from read query object

        sq.doRead();
        String table;

            table = sq.getHTMLTable();


        request.setAttribute("table", table);
        String url ="/search.jsp";

        RequestDispatcher dispatcher = request.getRequestDispatcher(url);
        dispatcher.forward(request,response);
    }

}

我得到错误：


  java.lang.NullPointerException
    dbhelpers.SearchQuery.getHTMLTable（SearchQuery.java:65）
    controller.SearchServlet.doPost（SearchServlet.java:46）
    javax.servlet.http.HttpServlet.service（HttpServlet.java:646）
    javax.servlet.http.HttpServlet.service（HttpServlet.java:727）
    org.apache.tomcat.websocket.server.WsFilter.doFilter（WsFilter.java:52）


有什么建议么

最佳答案

基本上NullpointerException可能是因为

 String query = "select title, author, pages from books where author =?";

需要一个参数作为占位符，但是在执行查询之前，请先替换占位符

PreparedStatement ps = this.connection.prepareStatement(query);
this.results = ps.executeQuery();

你需要做类似的事情

ps.setString(1,"yourAuthor");