Question

假设当用户访问我的网站时，他会看到之前存储在数据库中的大量图像。我知道如何在JSP Scriptlet中做到这一点，并且我还知道当用户使用servlet提交表单时如何从JSTL中的数据库中获取和检索数据。但是如果没有用户提交表单，我不知道如何在JSTL中完成它。

Answer 1

是的，您可以使用JSTL＆amp; EL。对于数据库访问，请使用JSTL SQL Tag library。

如何在JSP中显示存储在数据库中的图像？

我希望您使用BLOB类型列将图像存储在数据库中。只需点击一个传递记录ID的Servlet，然后发送byte[]作为响应。

我为每张图片创建了单独的请求，以便提供更好的用户体验。

注意：最好在Servlet中移动数据库代码。

JSP：

<%@ taglib prefix="sql" uri="http://java.sun.com/jsp/jstl/sql"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>

<sql:setDataSource var="webappDataSource"
    driver="com.mysql.jdbc.Driver" url="jdbc:mysql://localhost:3306/test"
    user="root" password="root" />

<sql:query dataSource="${webappDataSource}"
    sql="select id,username from users" var="result" />

<table width="100%" border="1">
    <c:forEach var="row" items="${result.rows}">
        <tr>
            <td>${row.id}</td>
            <td>${row.username}</td>
            <td>
               <img src="${pageContext.servletContext.contextPath }/photoServlet?id=${row.id}" />
            </td>
        </tr>
    </c:forEach>
</table>

Servlet（PhotoServlet.java）：

protected void doGet(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {

    final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
    final String DB_URL = "jdbc:mysql://localhost:3306/test";
    final String User = "root";
    final String Password = "root";
    try {
        Class.forName(JDBC_DRIVER);
        Connection conn = DriverManager.getConnection(DB_URL, User, Password);

        PreparedStatement stmt = conn.prepareStatement("select photo from users where id=?");
        stmt.setLong(1, Long.valueOf(request.getParameter("id")));
        ResultSet rs = stmt.executeQuery();
        if (rs.next()) {
            response.getOutputStream().write(rs.getBytes("photo"));
        }
        conn.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

的web.xml：

<servlet>
    <servlet-name>PhotoServlet</servlet-name>
    <servlet-class>com.server.servlet.PhotoServlet</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>PhotoServlet</servlet-name>
    <url-pattern>/photoServlet</url-pattern>
</servlet-mapping>

表结构:(用户）

+----------+-------------+------+-----+---------+-------+
| Field    | Type        | Null | Key | Default | Extra |
+----------+-------------+------+-----+---------+-------+
| id       | int(11)     | NO   | PRI | NULL    |       |
| username | varchar(30) | YES  |     | NULL    |       |
| password | varchar(20) | YES  |     | NULL    |       |
| photo    | blob        | YES  |     | NULL    |       |
+----------+-------------+------+-----+---------+-------+

Answer 2

如果您的问题是关于使用JSTL标记在JSP页面上显示数据库内容，并且您使用的是本机样式JDBC数据库连接（不是jpa或hibernate），请查看：

   // make a class for you database table mapping
   // assume you have id,name,salary in employee database table , make a Java class for it

       class Employee{
          int id;
          String name;
          float salary;     

          // now getter setter for fields
          public int getId(){
             return this.id;
           }
          public void setId(int id){
             this.id = id;
          }

          public String getName(){
             return this.name;
           }
          public void setName(String name){
             this.name = name;
          }

        public float getSalary(){
             return this.salary;
           }
          public void setSalary(int salary){
             this.salary = salary;
          }
       }



        // now in your servlet class make a list of Employee class and set data from data base


        List<Employee> empList = new ArrayList<Employee>();
        // asume your db connection and query is here
        ResultSet rs = your_result_Set
        while(rs.next()){
            Employee emp = new Employee();
            emp.setId(rs.getInt("id"));
            emp.setName(rs.getString("name"));
            em.setSalary(rs.getFloat("slary"));
            empList.add(em);
        }



    request.setAttribute("empList",empList);
    request.getRequestDispacher("your_jsp_path").forward(request,response);
 // now in your jsp add <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %> on top of jsp
      <table>
     <c:forEach items="${empList}"  var="e" >
                     <tr>
                               <td>${e.id}</td>
                                <td>${e.name}</td>
                                <td>${e.salary}</td>
                     </tr>
     </c:forEach>
     </table>

使用servlet和jsp自定义MVC。看看使用servlet 3.0或更高版本

   import java.io.IOException;
     import java.io.PrintWriter;
     import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
   import javax.servlet.http.HttpServletResponse;


       @WebServlet(name="login", urlPatterns={"/login"})
      public class NewServlet extends HttpServlet {

      protected void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
    response.setContentType("text/html;charset=UTF-8");
    try{

        // do your login logic here 

        request.setAttribute("message","login failed");

        request.getRequestDispatcher("/WEB-INF/jsp/login.jsp").forward(request,  response);





    }
    catch(Exception exp){
    }

} 


@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
    processRequest(request, response);
} 

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
     processRequest(request, response);
}


@Override
public String getServletInfo() {
    return "Short description";
}// </editor-fold>

}

             // now in login.jsp placed in WEB-INF/jsp

              <span>${message}</span><!-- message is request attribute here assigned in servlet-->

现在每当你调用登录url请求时，第一个登陆servlet后转移到jsp

从数据库中检索图像并使用JSTL在JSP中显示它

2 个答案: