连接数据库java.lang.nullPointerException的错误怎么解决?

[Manlandyes]

代码如下：



package test;

import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DBServlet extends HttpServlet {

/**
*
*/
private static final long serialVersionUID = 1L;
static private Connection con = null;
static private Statement st = null;
static private ResultSet rs = null;



public static Connection getConnection(){
try{
Class.forName("com.mysql.jdbc.Driver");
String url ="jdbc:mysql://localhost:3306/wing?useUnicode=true&characterEncoding=gb2312";
String username="123456";
String password="123456";

Connection con= DriverManager.getConnection(url,username,password);
if(con!=null){System.out.println("连接成功!!");}
else{System.out.println("失败!!");}
} catch (ClassNotFoundException e) {e.printStackTrace();}
catch (SQLException e) {e.printStackTrace();}
return con;
}


public static ResultSet getResultSet(String sql){
con = getConnection();
try {
st = con.createStatement();
rs = st.executeQuery(sql);
} catch (SQLException e) {e.printStackTrace();}
return rs;
}





public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=GB2312");//设置响应的MIME类型。
request.setCharacterEncoding("gb2312");


PrintWriter out = response.getWriter();
String sql = "select * from wings";
rs = getResultSet(sql);
try {
out.println("<table border='1' align='center'>");
out.println("<tr>");
out.println("<td>"+"学号"+"</td>");
out.println("<td>"+"姓名"+"</td>");
out.println("<td>"+"性别"+"</td>");
out.println("<td>"+"年龄"+"</td>");
out.println("</tr>");
while(rs.next()){
out.println("<tr>");
out.println("<td>"+rs.getInt("ID")+"</td>");
out.println("<td>"+rs.getString("name")+"</td>");
out.println("<td>"+rs.getString("sex")+"</td>");
out.println("<td>"+rs.getInt("age")+"</td>");
out.println("</tr>");
}
out.println("</table>");
} catch (SQLException e) {e.printStackTrace();}
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}




}



出现错误如下：




java.sql.SQLException: Access denied for user '123456'@'localhost' (using password: YES)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:946)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2870)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:812)
at com.mysql.jdbc.MysqlIO.secureAuth411(MysqlIO.java:3269)
at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1182)
at com.mysql.jdbc.Connection.createNewIO(Connection.java:2670)
at com.mysql.jdbc.Connection.<init>(Connection.java:1531)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:266)
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at test.DBServlet.getConnection(DBServlet.java:26)
at test.DBServlet.getResultSet(DBServlet.java:36)
at test.DBServlet.doPost(DBServlet.java:57)
at test.DBServlet.doGet(DBServlet.java:80)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Unknown Source)
2010-8-27 17:20:09 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet DBServlet threw exception
java.lang.NullPointerException
at test.DBServlet.getResultSet(DBServlet.java:38)
at test.DBServlet.doPost(DBServlet.java:57)
at test.DBServlet.doGet(DBServlet.java:80)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:857)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Unknown Source)


这种错误怎么解决 谢谢!

[jsdragon]

11111111111

[jsdragon]

11111111111

[Manlandyes]

[Quote=引用 13 楼 crazylaa 的回复:]
引用 11 楼 manlandyes 的回复:

引用 9 楼 crazylaa 的回复:
at test.DBServlet.getResultSet(DBServlet.java:38)
这里是哪句话？
实在不行，你debug跟踪一下。
你的代码写的不好，哪有在servlet里面定义static变量来获取连接的。。。


38：st = con.createStatement……
[/Quote]


代码变成这样了：


package test;

import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DBServlet extends HttpServlet {

/**
*
*/
private static final long serialVersionUID = 1L;
private Connection con = null;
private Statement st = null;
private ResultSet rs = null;



public Connection getConnection(){
try{
Class.forName("com.mysql.jdbc.Driver");
String url ="jdbc:mysql://localhost:3306/wing?useUnicode=true&characterEncoding=gb2312";
String username="root";
String password="123456";

Connection Connection= DriverManager.getConnection(url,username,password);
if(Connection!=null){System.out.println("连接成功!!");}
else{System.out.println("失败!!");}
} catch (ClassNotFoundException e) {e.printStackTrace();}
catch (SQLException e) {e.printStackTrace();}
return con;
}


public ResultSet getResultSet(String sql){
con = getConnection();
try {
st = con.createStatement();
rs = st.executeQuery(sql);
} catch (SQLException e) {e.printStackTrace();}
return rs;
}





public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=GB2312");//设置响应的MIME类型。
request.setCharacterEncoding("gb2312");


PrintWriter out = response.getWriter();
String sql = "select * from wings";
rs = getResultSet(sql);
try {
out.println("<table border='1' align='center'>");
out.println("<tr>");
out.println("<td>"+"学号"+"</td>");
out.println("<td>"+"姓名"+"</td>");
out.println("<td>"+"性别"+"</td>");
out.println("<td>"+"年龄"+"</td>");
out.println("</tr>");
while(rs.next()){
out.println("<tr>");
out.println("<td>"+rs.getInt("ID")+"</td>");
out.println("<td>"+rs.getString("name")+"</td>");
out.println("<td>"+rs.getString("sex")+"</td>");
out.println("<td>"+rs.getInt("age")+"</td>");
out.println("</tr>");
}
out.println("</table>");
} catch (SQLException e) {e.printStackTrace();}
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}




}


还是：
2010-8-27 21:17:28 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet DBServlet threw exception
java.lang.NullPointerException

[crazylaa]

public Connection getConnection(){
try{
Class.forName("com.mysql.jdbc.Driver");
String url ="jdbc:mysql://localhost:3306/wing?useUnicode=true&characterEncoding=gb2312";
String username="root";
String password="123456";

Connection Connection= DriverManager.getConnection(url,username,password);
if(Connection!=null){System.out.println("连接成功!!");}
else{System.out.println("失败!!");}
} catch (ClassNotFoundException e) {e.printStackTrace();}
catch (SQLException e) {e.printStackTrace();}
return con;
}
把红色的两处改为 con1
蓝色的改为 con1
变量作用域问题。

[Manlandyes]

代码如下了：
package test;

import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DBServlet extends HttpServlet {

/**
*
*/
private static final long serialVersionUID = 1L;
private Connection con1 = null;
private Statement st = null;
private ResultSet rs = null;



public Connection getConnection(){
try{
Class.forName("com.mysql.jdbc.Driver");
String url ="jdbc:mysql://localhost:3306/wing?useUnicode=true&characterEncoding=gb2312";
String username="root";
String password="123456";

Connection con1= DriverManager.getConnection(url,username,password);
if(con1!=null){System.out.println("连接成功!!");}
else{System.out.println("失败!!");}
} catch (ClassNotFoundException e) {e.printStackTrace();}
catch (SQLException e) {e.printStackTrace();}
return con1;
}


public ResultSet getResultSet(String sql){
con1 = getConnection();
try {
st = con1.createStatement();
rs = st.executeQuery(sql);
} catch (SQLException e) {e.printStackTrace();}
return rs;
}





public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=GB2312");//设置响应的MIME类型。
request.setCharacterEncoding("gb2312");


PrintWriter out = response.getWriter();
String sql = "select * from wings";
rs = getResultSet(sql);
try {
out.println("<table border='1' align='center'>");
out.println("<tr>");
out.println("<td>"+"学号"+"</td>");
out.println("<td>"+"姓名"+"</td>");
out.println("<td>"+"性别"+"</td>");
out.println("<td>"+"年龄"+"</td>");
out.println("</tr>");
while(rs.next()){
out.println("<tr>");
out.println("<td>"+rs.getInt("ID")+"</td>");
out.println("<td>"+rs.getString("name")+"</td>");
out.println("<td>"+rs.getString("sex")+"</td>");
out.println("<td>"+rs.getInt("age")+"</td>");
out.println("</tr>");
}
out.println("</table>");
} catch (SQLException e) {e.printStackTrace();}
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}




}
还是：
2010-8-27 22:14:00 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet DBServlet threw exception
java.lang.NullPointerException

[crazylaa]

package test;

import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class DBServlet extends HttpServlet {

private static final long serialVersionUID = 1L;

private Connection con1 = null;

private Statement st = null;

private ResultSet rs = null;

private Connection getConnection() {
try {
Class.forName("com.mysql.jdbc.Driver");
String url = "jdbc:mysql://localhost:3306/wing?useUnicode=true&characterEncoding=gb2312";
String username = "root";
String password = "123456";

con1 = DriverManager.getConnection(url, username,
password);
if (con1 != null) {
System.out.println("连接成功!!");
} else {
System.out.println("失败!!");
}
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SQLException e) {
e.printStackTrace();
}
return con1;
}

public ResultSet getResultSet(String sql) {
con1 = getConnection();
try {
st = con1.createStatement();
rs = st.executeQuery(sql);
} catch (SQLException e) {
e.printStackTrace();
}
return rs;
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=GB2312");// 设置响应的MIME类型。
request.setCharacterEncoding("gb2312");

PrintWriter out = response.getWriter();
String sql = "select * from wings";
rs = getResultSet(sql);
try {
out.println("<table border='1' align='center'>");
out.println("<tr>");
out.println("<td>" + "学号" + "</td>");
out.println("<td>" + "姓名" + "</td>");
out.println("<td>" + "性别" + "</td>");
out.println("<td>" + "年龄" + "</td>");
out.println("</tr>");
while (rs.next()) {
out.println("<tr>");
out.println("<td>" + rs.getInt("ID") + "</td>");
out.println("<td>" + rs.getString("name") + "</td>");
out.println("<td>" + rs.getString("sex") + "</td>");
out.println("<td>" + rs.getInt("age") + "</td>");
out.println("</tr>");
}
out.println("</table>");
} catch (SQLException e) {
e.printStackTrace();
}
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}
}

[magong]

连接成功了？恭喜。

19楼中你的代码是有问题的，无论是否连接成功getConnection方法都返回null。

关键在于不要混用同名的局部变量和成员变量，像20楼那样，只用成员变量，应该不会有这个问题了。试试他的代码吧。

或者你也可以只用局部变量。

[fuxi922721]

一步一步测

[Manlandyes]

正确代码：

package test;

import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class DBServlet extends HttpServlet {

/**
*
*/
private static final long serialVersionUID = 1L;
static private Connection con = null;
static private Statement st = null;
static private ResultSet rs = null;



public static Connection getConnection(){
try{
Class.forName("com.mysql.jdbc.Driver");
con=DriverManager.getConnection("jdbc:MySQL://localhost:3306/wing?user=root&password=123456&characterEncoding=gb2312");
if(con!=null){System.out.println("连接成功!!");}
else{System.out.println("失败!!");}
} catch (ClassNotFoundException e) {e.printStackTrace();}
catch (SQLException e) {e.printStackTrace();}
return con;
}


public static ResultSet getResultSet(String sql){
con = getConnection();
try {
st = con.createStatement();
rs = st.executeQuery(sql);
} catch (SQLException e) {e.printStackTrace();}
return rs;
}





public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=GB2312");//设置响应的MIME类型。
request.setCharacterEncoding("gb2312");


PrintWriter out = response.getWriter();
String sql = "select * from wings";
rs = getResultSet(sql);
try {
out.println("<table border='1' align='center'>");
out.println("<tr>");
out.println("<td>"+"学号"+"</td>");
out.println("<td>"+"姓名"+"</td>");
out.println("<td>"+"性别"+"</td>");
out.println("<td>"+"年龄"+"</td>");
out.println("</tr>");
while(rs.next()){
out.println("<tr>");
out.println("<td>"+rs.getInt("ID")+"</td>");
out.println("<td>"+rs.getString("name")+"</td>");
out.println("<td>"+rs.getString("sex")+"</td>");
out.println("<td>"+rs.getInt("age")+"</td>");
out.println("</tr>");
}
out.println("</table>");
} catch (SQLException e) {e.printStackTrace();}
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}




}


总结：试过数十种方法，左改右改，每次都运行不同，刚才，我拿回原来的代码，把用户名的123456改为root（即变成了上面的代码），运行，成功了！心情激动不已，但还是感谢回帖的各位，你们使我学到很多东西，可惜鄙人分数太小，不能每个人都给，想给不能给，想多给只能小给，多多不便敬请原谅！

[Manlandyes]

[Quote=引用 21 楼 magong 的回复:]
引用楼主 manlandyes 的回复:
26： Connection con= DriverManager.getConnection(url,username,password);
at test.DBServlet.getConnection(DBServlet.java:26)

不知道搞出来没有。
第一稿中，看异常（如上）就很明显是数据库没连上，后面每稿，只盯着空指针异常看肯定……
[/Quote]

但它那里显示“连接成功!!”了

[kaida_7]

看到这个错误的第一反应是：你数据库密码没写对！

[Manlandyes]

[Quote=引用 23 楼 kaida_7 的回复:]
看到这个错误的第一反应是：你数据库密码没写对！
[/Quote]

不是，第一条红色错误已经改过了，能显示“连接成功!!”了，就是第二条红色错误没消失~

[magong]

[Quote=引用楼主 manlandyes 的回复:]
26： Connection con= DriverManager.getConnection(url,username,password);
at test.DBServlet.getConnection(DBServlet.java:26)
[/Quote]
不知道搞出来没有。
第一稿中，看异常（如上）就很明显是数据库没连上，后面每稿，只盯着空指针异常看肯定会忽略主要问题。
就是数据库连不上嘛。
先把别的代码删掉，到能连上了再回填其他代码。

[cengshilieren]

看了帖子很受用，也感谢楼主把问题给大家分享

[juhuacha159]

mark up!

[whlcy]

mark!!!!

[wang1986614]

打个断点！很简单的

[chengjing123456]

用户名和密码输入错误。。。。。。。。。。

[guyuewei]

其实是声明了2个Connection返回的是那个没连接的con！基础有点薄弱啊~！

[yuzhonglian2004]

con为null了，连接还是没有建立起来