Question

javaweb一直报空指针异常，java文件单独运行时没有问题的，但是Jsp一引用就报取不到值

jsp代码：
try {DBmanage DB = new DBmanage();
String sql = "select top 3 * from admin";
ResultSet rs = DB.doQuery(sql);
request.setCharacterEncoding("utf-8");
while (rs.next()) {
%><%
<p><%=rs.getString(1)%></p>
<p><%=rs.getString(2)%></p>
<p><%=rs.getString(3)%></p>
}
DB.closeConnection();
} catch (SQLException e) {
out.println("conn:" + e.getMessage());
}
%>
java代码:
public class DBmanage {
public Connection con=null;
public Statement stmt=null;
public ResultSet rs=null;
private static String dbDriver="com.microsoft.sqlserver.jdbc.SQLServerDriver";
private static String dburl="jdbc:sqlserver://localhost:1433;DatabaseName=student";
private static String dbname="sa";
private static String dbpassword="12345";

public static Connection getConnection(){
Connection conn=null;

try {
Class.forName(dbDriver);
conn=DriverManager.getConnection(dburl, dbname, dbpassword);
System.out.println("连接成功");
} catch (Exception e) {
// TODO: handle exception
System.err.println("连接失败");
}
return conn;
}

public ResultSet doQuery(String sql){
try {
con=DBmanage.getConnection();
stmt=con.createStatement();
rs=stmt.executeQuery(sql);
} catch (SQLException e) {
// TODO: handle exception
e.printStackTrace();
System.out.println("连接失败"+e.getMessage());
}
return rs;
}

报错：
org.apache.jasper.JasperException: An exception occurred processing JSP page /index.jsp at line 21

18: try {
19: DBmanage DB = new DBmanage();
20: String sql = "select top 3 * from admin";
21: ResultSet rs = DB.doQuery(sql);
22: %>
23: <meta http-equiv="pragma" content="no-cache">
24: <meta http-equiv="cache-control" content="no-cache">

Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:588)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:481)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

root cause
java.lang.NullPointerException
db.DBmanage.doQuery(DBmanage.java:32)
org.apache.jsp.index_jsp._jspService(index_jsp.java:140)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:443)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

Answer 1

连接数据库成功了么

检查 DBmanage的代码
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