求帮助，jsp使用Java类时出现空指针错误，谢谢！

haiyantai 2013-09-23 11:34:02

<%@page import="java.sql.*"%>
<%@page import="servlet.DBConnect"%>
<% DBConnect db=new DBConnect();
ResultSet result=db.executeQuery("select * from mapdata");
%>

然后，下面是DBConnect类代码
package servlet;
import java.sql.*;

public class DBConnect
{
public static final String driver="com.mysql.jdbc.Driver";
public static final String url="jdbc:mysql://localhost:3306/mydata";
public static final String user="root";
public static final String password="2224101360";
ResultSet resultset=null;
PreparedStatement statement=null;
Connection conn=null;

public Connection getConnection()
{ try{Class.forName(driver);
return DriverManager.getConnection(url,user,password);
}
catch(ClassNotFoundException e){System.out.println("Driver not found");return null;}
catch(SQLException e){System.out.println("Error:"+e.getMessage());return null;}
}

public ResultSet executeQuery(String sql) throws Exception
{
conn=this.getConnection();
statement=conn.prepareStatement(sql);
return resultset=statement.executeQuery();
}
public int executeUpdate(String sql)
{
try{
conn=this.getConnection();
statement=conn.prepareStatement(sql);
return statement.executeUpdate();

}catch(Exception e){System.out.println("Error:"+e.getMessage());return 0;}
}
public void closeAll() throws Exception
{
if(null!=resultset) resultset.close();
if(null!=statement) statement.close();
if(null!=conn) conn.close();

}
public DBConnect() {}

...全文

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haiyantai 2013-09-23

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嗯，我试了下，刚才那个空指针错误没了，不过有其他错误，可能是我其他的代码还有问题，谢谢啦！

枫醉秋 2013-09-23

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把mysql的驱动包放到\WEB-INF\lib下去试试

haiyantai 2013-09-23

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刚才那个有点错误 package servlet; import java.sql.*; public class DBConnect { public static final String driver="com.mysql.jdbc.Driver"; public static final String url="jdbc:mysql://localhost:3306/mydata"; public static final String user="root"; public static final String password="2224101360"; ResultSet resultset=null; PreparedStatement statement=null; Connection conn=null; public Connection getConnection() { try{Class.forName(driver); return DriverManager.getConnection(url,user,password); } catch(ClassNotFoundException e){System.out.println("Driver not found");return null;} catch(SQLException e){System.out.println("Error:"+e.getMessage());return null;} } public ResultSet executeQuery(String sql) throws Exception { conn=this.getConnection(); statement=conn.prepareStatement(sql); return resultset=statement.executeQuery(); } public int executeUpdate(String sql) { try{ conn=this.getConnection(); statement=conn.prepareStatement(sql); return statement.executeUpdate(); }catch(Exception e){System.out.println("Error:"+e.getMessage());return 0;} } public void closeAll() throws Exception { if(null!=resultset) resultset.close(); if(null!=statement) statement.close(); if(null!=conn) conn.close(); } public DBConnect() {} public static void main(String[] args) { try{ servlet.DBConnect db=new servlet.DBConnect(); //int i=db.executeUpdate("insert into userinfo values (2,'wangqiang','123')"); ResultSet result=db.executeQuery("select * from mapdata"); System.out.println("-----------Result-------------"); System.out.println("username:"+"\t"+"password"); while(result.next()) { System.out.println(result.getFloat("lng")+"\t"+result.getFloat("lat")); } }catch(Exception e) {System.out.println(e.getMessage());} } }

haiyantai 2013-09-23

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这样运行没有错 package servlet; import java.sql.*; public class DBConnect { public static final String driver="com.mysql.jdbc.Driver"; public static final String url="jdbc:mysql://localhost:3306/mydata"; public static final String user="root"; public static final String password="2224101360"; ResultSet resultset=null; PreparedStatement statement=null; Connection conn=null; public Connection getConnection() { try{Class.forName(driver); return DriverManager.getConnection(url,user,password); } catch(ClassNotFoundException e){System.out.println("Driver not found");return null;} catch(SQLException e){System.out.println("Error:"+e.getMessage());return null;} } public ResultSet executeQuery(String sql) throws Exception { conn=this.getConnection(); statement=conn.prepareStatement(sql); return resultset=statement.executeQuery(); } public int executeUpdate(String sql) { try{ conn=this.getConnection(); statement=conn.prepareStatement(sql); return statement.executeUpdate(); }catch(Exception e){System.out.println("Error:"+e.getMessage());return 0;} } public void closeAll() throws Exception { if(null!=resultset) resultset.close(); if(null!=statement) statement.close(); if(null!=conn) conn.close(); } public DBConnect() {} public static void main(String[] args) { try{ servlet.DBConnect db=new servlet.DBConnect(); //int i=db.executeUpdate("insert into userinfo values (2,'wangqiang','123')"); //ResultSet result=db.executeQuery("select * from mapdata"); ResultSet result=db.doGetMapData(); System.out.println("-----------Result-------------"); System.out.println("username:"+"\t"+"password"); while(result.next()) { System.out.println(result.getFloat("lng")+"\t"+result.getFloat("lat")); } }catch(Exception e) {System.out.println(e.getMessage());} } }

效林少爷 2013-09-23

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引用 2 楼 scqlmm 的回复:

这个空指针有可能是你在去得连接的时候失败了，造成conn为null了

同感

haiyantai 2013-09-23

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但是我在单独的DBConnect类测试的时候没有问题

haiyantai 2013-09-23

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好的，type Exception report message An exception occurred processing JSP page /success.jsp at line 18 description The server encountered an internal error that prevented it from fulfilling this request. exception org.apache.jasper.JasperException: An exception occurred processing JSP page /success.jsp at line 18 15: </head> 16: <body> 17: <% DBConnect db=new DBConnect(); 18: ResultSet result=db.executeQuery("select * from mapdata"); 19: %> 20: <div id="allmap"></div> 21: </body> Stacktrace: org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:568) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:470) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334) javax.servlet.http.HttpServlet.service(HttpServlet.java:728) root cause java.lang.NullPointerException servlet.DBConnect.executeQuery(DBConnect.java:26) org.apache.jsp.success_jsp._jspService(success_jsp.java:81) org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70) javax.servlet.http.HttpServlet.service(HttpServlet.java:728) org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432) org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390) org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334) javax.servlet.http.HttpServlet.service(HttpServlet.java:728) note The full stack trace of the root cause is available in the Apache Tomcat/7.0.42 logs.

枫醉秋 2013-09-23