实现接口,代码:
public List querAll() throws Exception
{
List<User> all= new ArrayList<User>();
String sql="SELECT UserId, UserName, UserPwd, UserQx FROM User";
PreparedStatement pstmt = null;
DateBaseConn dbc=null;
try
{
dbc = new DateBaseConn();
pstmt = dbc.getConnection().prepareStatement(sql);
ResultSet rs = pstmt.executeQuery();
while (rs.next())
{
User user= new User();
user.setUserId(rs.getString(1));
user.setUserName(rs.getString(2));
user.setUserPwd(rs.getString(3));
user.setUserQx(rs.getString(4));
all.add(User);
}
rs.close();
pstmt.close();
}
catch (Exception e)
{
throw new Exception("操作出现异常") ;
}
finally
{
dbc.close();
}
return all; }调用:jsp中调用
<%
UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
Iterator iter = allList.iterator() ;
while(iter.hasNext())
{
User user = (User)iter.next() ;
// 进行循环打印,打印出所有的内容,以表格形式
// 从数据库中取出内容
%>
<td><%=user.getUserId() %></td>
<td><%=user.getUserName() %></td>
<td><%=user.getUserPwd() %></td>
<td><%=user.getUserQx() %></td>
<%
}
%>
public List querAll() throws Exception
{
List<User> all= new ArrayList<User>();
String sql="SELECT UserId, UserName, UserPwd, UserQx FROM User";
PreparedStatement pstmt = null;
DateBaseConn dbc=null;
try
{
dbc = new DateBaseConn();
pstmt = dbc.getConnection().prepareStatement(sql);
ResultSet rs = pstmt.executeQuery();
while (rs.next())
{
User user= new User();
user.setUserId(rs.getString(1));
user.setUserName(rs.getString(2));
user.setUserPwd(rs.getString(3));
user.setUserQx(rs.getString(4));
all.add(User);
}
rs.close();
pstmt.close();
}
catch (Exception e)
{
throw new Exception("操作出现异常") ;
}
finally
{
dbc.close();
}
return all; }调用:jsp中调用
<%
UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
Iterator iter = allList.iterator() ;
while(iter.hasNext())
{
User user = (User)iter.next() ;
// 进行循环打印,打印出所有的内容,以表格形式
// 从数据库中取出内容
%>
<td><%=user.getUserId() %></td>
<td><%=user.getUserName() %></td>
<td><%=user.getUserPwd() %></td>
<td><%=user.getUserQx() %></td>
<%
}
%>
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那你去看看构造函数里面怎么写的。还有接口是不能被实例化的(就算实例化也没有意义),
只有申明一个接口然后用它的实现类去实例化。
比如:List l = new List(); 错误
List l = new ArrayList(); 正确
应该是从数据库里根本就没查出数据.
<body>
<a href="web/list.jsp">查询全部用户</a>
</body>调用list.jsp页面
查询并显示全部用户:
<body>
<%
UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
Iterator iter = allList.iterator() ;
while(iter.hasNext())
{
User user = (User)iter.next() ;
// 进行循环打印,打印出所有的内容,以表格形式
// 从数据库中取出内容
%>
<h1><%=user.getUserId() %></h1>
<h1> <%=user.getUserName() %> </h1>
<h1> <%=user.getUserPwd() %> </h1>
<h1> <%=user.getUserQx() %> </h1>
<%
}
%>
</body>
</html>在User user = (User)iter.next() ;
时出现空异常!请问一下什么问题!
public List querAll() throws Exception
{
List<User> all= new ArrayList<User>();
String sql="SELECT UserId, UserName, UserPwd, UserQx FROM User";
PreparedStatement pstmt = null;
DateBaseConn dbc=null;
try
{
dbc = new DateBaseConn();
pstmt = dbc.getConnection().prepareStatement(sql);
ResultSet rs = pstmt.executeQuery();
while (rs.next())
{
User user= new User();
user.setUserId(rs.getString(1));
System.out.println(rs.getString(1));
user.setUserName(rs.getString(2));
System.out.println(rs.getString(2));
user.setUserPwd(rs.getString(3));
System.out.println(rs.getString(3));
user.setUserQx(rs.getString(4));
System.out.println(rs.getString(4));
all.add(User);
}
rs.close();
pstmt.close();
}
catch (Exception e)
{
throw new Exception("操作出现异常") ;
}
finally
{
dbc.close();
}
return all; }打出的东西!
2008-4-16 15:45:47 org.apache.catalina.startup.Catalina start
信息: Server startup in 8547 ms
1
bill
bill
1
2
franck
franck
1
3
suzi
suzi
1
4
by
by
1
而在list.jsp 中出现空异常,百思不得其解,讲高手指教:
</head>
<body>
<%
UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
Iterator iter = allList.iterator() ;
while(iter.hasNext())
{
User user = (User)iter.next() ;
// 进行循环打印,打印出所有的内容,以表格形式
// 从数据库中取出内容
%>
<h1><%=user.getUserId() %></h1>
<h1> <%=user.getUserName() %> </h1>
<h1> <%=user.getUserPwd() %> </h1>
<h1> <%=user.getUserQx() %> </h1>
<%
}
%>
</body>
</html>
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:372)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
root cause java.lang.NullPointerException
org.apache.jsp.web.list_jsp._jspService(list_jsp.java:94)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
note The full stack trace of the root cause is available in the Apache Tomcat/5.0.27 logs.
UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
Iterator iter = allList.iterator() ;
while(iter.hasNext())
{
User user = (User)iter.next() ;
// 进行循环打印,打印出所有的内容,以表格形式
// 从数据库中取出内容
out.write(" \r\n");
out.write(" <h1>");
94 out.print(user.getUserId() );
out.write("</h1> \r\n");
out.write(" <h1> ");
out.print(user.getUserName() );
out.write(" </h1> \r\n");
out.write(" <h1> ");
out.print(user.getUserPwd() );
out.write(" </h1> \r\n");
out.write(" <h1> ");
out.print(user.getUserQx() );
out.write(" </h1> \r\n");
out.write(" ");
}
out.write(" \r\n");
out.write("\r\n");
out.write(" \r\n");
out.write(" </body>\r\n");
out.write("</html>\r\n");
public List querAll() throws Exception
{
List<User> all= new ArrayList<User>();
。。 return all;
}
建议在迭代List时,先判断List是否为空 UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
if(allList !=null&&allList .size()>0){
Iterator iter = allList.iterator() ;
while(iter.hasNext())
{
User user = (User)iter.next() ;
// 进行循环打印,打印出所有的内容,以表格形式
// 从数据库中取出内容 out.write(" \r\n");
out.write(" <h1>");
94 out.print(user.getUserId() );
out.write(" </h1> \r\n");
out.write(" <h1> ");
out.print(user.getUserName() );
out.write(" </h1> \r\n");
out.write(" <h1> ");
out.print(user.getUserPwd() );
out.write(" </h1> \r\n");
out.write(" <h1> ");
out.print(user.getUserQx() );
out.write(" </h1> \r\n");
out.write(" ");
}
out.write(" \r\n");
out.write("\r\n");
out.write(" \r\n");
out.write(" </body>\r\n");
out.write(" </html>\r\n");
org.apache.jsp.web.list_jsp._jspService(list_jsp.java:94)
到tomcat上面找到
楼上的
user.setUserId(rs.getString(1));
System.out.println(rs.getString(1));
user.setUserName(rs.getString(2));
System.out.println(rs.getString(2));
user.setUserPwd(rs.getString(3));
System.out.println(rs.getString(3));
user.setUserQx(rs.getString(4));
System.out.println(rs.getString(4));
是不是set的时候就有错误了!
怎么不用jstl语言?
你在页面中打印你的list 看是不是未空,又没有set出去
request.setAttribute("list",list);
UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
楼主把这2行弄到页面的头部去
再检查user这个BEAN里的get和set方法是否重写了
净扯淡,list是空的话Iterator iter = allList.iterator() ; 就报错了
或者user里的那个字段是空的 数据库字段有空值
public List querAll() throws Exception
{
List <User> all= new ArrayList <User>();
String sql="SELECT UserId, UserName, UserPwd, UserQx FROM User";
PreparedStatement pstmt = null;
DateBaseConn dbc=null; try
{
dbc = new DateBaseConn();
pstmt = dbc.getConnection().prepareStatement(sql);
ResultSet rs = pstmt.executeQuery();
while (rs.next())
{
User user= new User();
user.setUserId(rs.getString(1));
user.setUserName(rs.getString(2));
user.setUserPwd(rs.getString(3));
user.setUserQx(rs.getString(4));
all.add(User);
}
rs.close();
pstmt.close();
}
catch (Exception e)
{
throw new Exception("操作出现异常") ;
}
finally
{
dbc.close();
} return all; } 调用:jsp中调用
<%
UserDaoImpl usdoim =new UserDaoImpl();
List allList = usdoim.querAll();
Iterator iter = allList.iterator() ;
while(iter.hasNext())
{
User user = (User)iter.next() ; //出错的地方 空异常
// 进行循环打印,打印出所有的内容,以表格形式
// 从数据库中取出内容
%>
<td> <%=user.getUserId() %> </td>
<td> <%=user.getUserName() %> </td>
<td> <%=user.getUserPwd() %> </td>
<td> <%=user.getUserQx() %> </td>
<%
}
%>