有关java.lang.NullPointerException错误的问题

[小瑾～]

代码如下：
public static boolean validate(String username,String password)
{
boolean bool=false;

try
{
Class.forName("com.mysql.jdbc.Driver");
}
catch(java.lang.ClassNotFoundException e)
{
System.out.print("Class not found error occur.Message is:");
System.out.print(e.getMessage());
}

try
{
Statement stmt=null;
ResultSet rs;
String url="jdbc:mysql://localhost:3306/student";
String user="root";
String passwordOfDB="summer";
Connection conn=null;
try
{
conn=DriverManager.getConnection(url,user,passwordOfDB);
}
catch(Exception ex){ex.printStackTrace();}
String sql="select * from userinfo where username='"+username+"'and password='"+password+"'";
stmt=(Statement)conn.createStatement();
rs=(ResultSet)stmt.executeQuery(sql);

if(rs.next())
bool=true;
else
bool=false;

}
catch(SQLException e)
{
System.out.print("SQL Exception occur.Message is:");
System.out.print(e.getMessage());
}
return bool;
}
产生的错误：
exception

java.lang.NullPointerException
mypack.SQLQuery.validate(SQLQuery.java:38)
mypack.CheckA.doPost(CheckA.java:43)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)

上面的代码是SQLQuery.java中的，我被这个问题困扰将近一天了，始终是不知道该怎样解决？

[suihu]

把异常信息粘出来啊！

[lip009]

请确信conn该连接不是NULL

[小瑾～]

这是出现的异常信息：
Class not found error occur.Message is:com.mysql.jdbc.Driverjava.sql.SQLException: No suitable driver found for jdbc:mysql://localhost:3306/student
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at mypack.SQLQuery.validate(SQLQuery.java:34)
at mypack.CheckA.doPost(CheckA.java:43)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:172)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:875)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Unknown Source)
2008-9-22 16:32:15 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet CheckA threw exception
java.lang.NullPointerException
at mypack.SQLQuery.validate(SQLQuery.java:38)
at mypack.CheckA.doPost(CheckA.java:43)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:172)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:875)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Unknown Source)

[Daly罗]

还有中可能是字符编码问题造成ResultSet对象rs为Null；

[Atai-Lu]

没有mysql的.jar文件？

[ChocooM]

[Quote=引用 3 楼 Geothermal 的回复:]
这是出现的异常信息：
Class not found error occur.Message is:com.mysql.jdbc.Driverjava.sql.SQLException: No suitable driver found for jdbc:mysql://localhost:3306/student
[/Quote]

没有合适的驱动，重新下载一个驱动

[lip009]

驱动程序没有，连接conn为NULL

[小瑾～]

是的，我也想到了，可能是驱动的事，我又另外单独写了一个数据库查询操作，在JavaApplication下调试没有问题，我重新下载一个驱动试一下

[非典型射手]

代码有问题,

try 
{ 
conn=DriverManager.getConnection(url,user,passwordOfDB); 
} 
catch(Exception ex){ex.printStackTrace();} String sql="select * from userinfo where username='"+username+"'and                password='"+password+"'"; 
stmt=(Statement)conn.createStatement(); 
                rs=(ResultSet)stmt.executeQuery(sql); 

异常信息只是捕获没做任何处理是不对的.之所以出现空指针异常就是因为前面的异常捕获处理不当.
首先conn获取的时候出现了No suitable driver found for jdbc:mysql://localhost:3306/student的异常,但是此时将异常信息捕获后会继续后面的程序.这个时候conn仍然为null,所以就会出现空指针异常了

[小瑾～]

我重新下载了驱动还是不行，看来不是驱动的事

对于9楼的看法
具体的该怎样做才能使conn不为空，这个异常该怎样处理

[Xiaoyang]

LZ 是没有合适的驱动了！！ 自己多试试吧。。

[非典型射手]

[Quote=引用 10 楼 Geothermal 的回复:]
我重新下载了驱动还是不行，看来不是驱动的事

对于9楼的看法
具体的该怎样做才能使conn不为空，这个异常该怎样处理
[/Quote]

根据异常信息看，就是因为驱动没找到才导致conn为null的,你再仔细看看驱动是否已经加好,有可能是没在path中

至于异常怎么处理就要看你的程序想怎么处理了,至少你这里的程序,出现这样的异常就不应该执行下去了,直接将异常抛到最外层或是将方法return 掉吧

[小瑾～]

驱动已经加上了，没有问题，还是出现那个问题

[yunyetui]

No suitable driver found for jdbc:mysql://localhost:3306/student

没有合适的驱动，也许是数据库版本和驱动不合适，请搜索一下MYSQL的驱动和版本，是有区别的。