由一个Servlet 看java入门常犯的几个错误

安装完java环境后，cmd-javac 报错           -------------》环境变量配错了，最后全配成系统变量，ok了

能浪费一天的时间

写一个最简单的Servlet ，tomcat报错404

看到一个

信息: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: D:\Program Files\jdk\bin;C:\Windows\Sun\Java\bin;C:\Windows\system32;C:\Windows;D:\Program Files\jdk\jre\bin;C:\Program Files (x86)\Intel\iCLS Client\;C:\Program Files\Intel\iCLS Client\;C:\Pro

以为是它的原因，其实都不重要

重要是没找到index.jsp        -----------------------》文件位置、目录错了

能浪费一天的时间

Servlet 读取远程orcale，报错   -----------------------------》java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver

jdk和驱动是匹配的，查过了

变量也加了

其实是webcontent\web-inf\lib里没有放jar包

能浪费一天的时间

java连接orcale需要安装客户端吗？----------------------------》不需要，事实上有一个驱动就ok了，驱动就是一个小客户端了

能浪费一天的时间

import javax.servlet.*;报错---------------》原因，没有build Servlet-api.jar

大纲目录

xml配置

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
  <display-name>fleam</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>

  <!-- 通过getServletContext().getInitParameter("XX")读取 -->
<context-param>
   <param-name>driver</param-name>
   <param-value>oracle.jdbc.driver.OracleDriver</param-value>
</context-param>

<context-param>
   <param-name>url</param-name>
   <param-value>jdbc:oracle:thin:@XXXXXXXXXXXXXXXXX:1521:XXX</param-value>
</context-param>

<context-param>
   <param-name>username</param-name>
   <param-value>XXX</param-value>
</context-param>

<context-param>
   <param-name>password</param-name>
   <param-value>XXXXX</param-value>
</context-param>

</web-app>

程序

package helloworld;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.json.JSONException;
import org.json.JSONObject;

import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

/**
 */
@WebServlet("/HelloWorld")
public class HelloWorld extends HttpServlet {
    private static final long serialVersionUID = 1L;

    private String diverClass;
    private String userName;
    private String password;
    private String url;

    /**
     * Default constructor.
     */
    public HelloWorld() {
        // TODO Auto-generated constructor stub

    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    @Override

    protected void doGet(HttpServletRequest request,

            HttpServletResponse response) throws ServletException, IOException {

        Connection conn = null;

        Statement stmt = null;

        ResultSet rs = null;

        JSONObject jsonObject = new JSONObject();

        response.setContentType("text/html");

        response.setCharacterEncoding("gb2312");

        PrintWriter out = response.getWriter();

        diverClass = /* getServletConfig(). */getServletContext().getInitParameter("driver");
           userName = /* getServletConfig(). */getServletContext().getInitParameter("username");
           password = /* getServletConfig(). */getServletContext().getInitParameter("password");
           url = /* getServletConfig(). */getServletContext().getInitParameter("url");

        try {

            Class.forName(diverClass);

            //conn = DriverManager.getConnection("jdbc:mysql://localhost/bbs?user=root&password=12345678");

            conn = DriverManager.getConnection(url, userName,password);

            stmt = conn.createStatement();

            rs = stmt.executeQuery("SELECT * FROM ANIMALS"); 

            while(rs.next()){

                try {

                     jsonObject.put("name",rs.getString("NAME"));
                } catch (JSONException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
                //out.println(rs.getString("NAME"));
                out.println(jsonObject);

            }

        } catch (ClassNotFoundException e) {

            e.printStackTrace();

        } catch (SQLException e) {

            e.printStackTrace();

        } finally {

            try {

                if(rs != null) {

                    rs.close();

                    rs = null;

                }

                if(stmt != null) {

                    stmt.close();

                    stmt= null;

                }

                if(conn != null) {

                    conn.close();

                    conn = null;

                }

            } catch (SQLException e) {

                e.printStackTrace();

            }

        }

    }

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

         PrintWriter out = response.getWriter();
         out.println("<div style=‘width:100px;height:100px;background-color:red‘>123123</div>");

    }

}