Servlet JSP getparameter getattribute返回null

Question

I'm having trouble with passing servlet variables to jsp. 我在将servlet变量传递给jsp时遇到麻烦。

Of course, I also have the web.xml set for the servlet already 当然，我已经为servlet设置了web.xml

<servlet>
    <servlet-name>databaseServlet</servlet-name>
    <servlet-class>Servlet.databaseServlet</servlet-class>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>databaseServlet</servlet-name>
    <url-pattern>/dbServlet</url-pattern>
</servlet-mapping>

The result is all of the name, owner, species and sex values are null. 结果是所有名称，所有者，物种和性别值均为空。 Can someone help me with this? 有人可以帮我弄这个吗？ Thanks 谢谢

PS: I also tried to use request.getSession().setAttribute in the servlet, didn't work either PS：我也尝试在servlet中使用request.getSession（）。setAttribute，也没有用

PPS: So if I make the following changes: PPS：因此，如果我进行以下更改：

databaseServlet.java databaseServlet.java

package Servlet;

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

@SuppressWarnings("serial")

public class databaseServlet extends HttpServlet {
private Connection conn;
private Statement statement;

String name;
String owner;
String species;
String sex;
String birth;
String death;

public void init(ServletConfig config) throws ServletException {
    try {
        Class.forName("com.mysql.jdbc.Driver");
        conn = DriverManager.getConnection(
                "jdbc:mysql://localhost/STUDENTS",
                "root",
                "");
        statement = conn.createStatement();

        String sql = "SELECT name, owner, species, sex, birth, death FROM pet";
        ResultSet rs = statement.executeQuery(sql);

        //STEP 5: Extract data from result set
        while(rs.next()){
            //Retrieve by column name
            name  = rs.getString("name");
            owner = rs.getString("owner");
            species = rs.getString("species");
            sex = rs.getString("sex");
            birth = rs.getString("birth");
            death = rs.getString("death");
        }
        rs.close();
    }
    catch (Exception e) {
        e.printStackTrace();
    }
}

protected void doPost (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.setAttribute("NAME", "Hello");
    System.out.println(name);
    request.setAttribute("OWNER",owner);
    request.setAttribute("SPECIES",species);
    request.setAttribute("SEX", sex);
    RequestDispatcher dispatcher=getServletConfig().getServletContext().getRequestDispatcher("/dbServlet.jsp");
    dispatcher.forward(request,  response);
}

} }

and this is my new jsp: 这是我的新jsp：

<body>
Name="${databaseServlet.NAME}" 
Owner="${databaseServlet.OWNER}" 
Species="<%= request.getAttribute("SPECIES") %>"
Sex="<%= request.getSession().getAttribute("SEX") %>"

</body>

both Name and Owner returns empty string, while Species and Sex still returns NULL Name和Owner都返回空字符串，而Species和Sex仍返回NULL

basically what I'm trying to do is to access MySQL database to retrieve variables from a table, and display it using JSP 基本上我想做的是访问MySQL数据库以从表中检索变量，并使用JSP显示它

Answer 1

Make sure that you have valid values for name , owner , species and sex when setting request attribute inside doPost method. 在doPost方法中设置请求属性时，请确保您具有有效的name ， owner ， species和sex值。

Use EL syntax like 使用EL语法，例如

${NAME}
${OWNER}
${SPECIES}
${SEX}

Do not write scriptlets in JSP , because scriptlets shouldn't be used in JSPs for more than a decade. 不要在JSP中编写scriptlet ，因为scriptlet在JSP中不应使用超过十年。 Learn the JSP EL , the JSTL , and use servlet for the Java code. 学习JSP EL和JSTL ，并将servlet用于Java代码。 How to avoid Java Code in JSP-Files? 如何避免JSP文件中的Java代码？

Answer 2

Try using 尝试使用


RequestDispatcher dispatcher=getServletConfig().getServletContext().getRequestDispatcher("/dbServlet.jsp");

I guess it should work. 我想应该可以。 And use request.getAttribute() only, it will return Object type, you will have to cast it 并仅使用request.getAttribute（），它将返回Object类型，您必须将其强制转换

Answer 3

Just give a scope to your attributes, and you can change your code like this : 只需给您的属性指定作用域，就可以像这样更改代码：

protected void doPost (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    request.getSession().setAttribute("NAME", "Hello");
    System.out.println(name);
    request.getSession().setAttribute("OWNER",owner);
    request.getSession().setAttribute("SPECIES",species);
    request.getSession().setAttribute("SEX", sex);
    RequestDispatcher dispatcher=getServletConfig().getServletContext().getRequestDispatcher("/dbServlet.jsp");
    dispatcher.forward(request,  response);
}

And when you handling the attribute make it sessionScope like : 处理属性时，将其设为sessionScope，如：

<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<body>
<c:set var="myName" value="NAME" />
<c:set var="myOwner" value="OWNER" />
Name="${sessionScope[myName]}" 
Owner="${sessionScope[myOwner]}" 
       //etc...
</body>

I hope this helps you. 我希望这可以帮助你。

Servlet JSP getparameter getattribute返回null

问题描述

3 个解决方案

解决方案1
0 2014-01-08 07:37:21

解决方案2
0 2014-01-08 07:46:18

解决方案3
0 2017-05-23 00:35:24

Servlet JSP getparameter getattribute返回null

问题描述

3 个解决方案

解决方案1 0 2014-01-08 07:37:21

解决方案2 0 2014-01-08 07:46:18

解决方案3 0 2017-05-23 00:35:24

解决方案1
0 2014-01-08 07:37:21

解决方案2
0 2014-01-08 07:46:18

解决方案3
0 2017-05-23 00:35:24