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How to loop arraylist |
Purpose of this tutorial is to show how to iterate/loop through an ArrayList of objects/dtos and access dto fields and display data.
This is most commonly needed where list is fetched from database and then it is displayed in a JSP.
Steps
1. Create data transfer object for testing.
2. Create jsp to use simple for loop in first scenario and then use JSTL tags in second
Also see
3. Create servlet to create ArrayList for testing
4. Add servlet to web.xml
5. Run
com.company.servlet.User.java
package com.company.servlet;
import java.io.Serializable;
public class User implements Serializable{
private String firstName;
private String lastName;
private String userId;
private String userRole;
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getUserId() {
return userId;
}
public void setUserId(String userId) {
this.userId = userId;
}
public String getUserRole() {
return userRole;
}
public void setUserRole(String userRole) {
this.userRole = userRole;
}
}
/jsp/iterator.jsp
<%@ page import="com.company.servlet.User" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<jsp:useBean id="cartoonUsers" class="java.util.ArrayList" scope="request"/>
<html>
<body>
<table bgcolor="#EFEFEF" border="1" >
<tr>
<td colspan="4"><Strong>1: Data displayed using for loop and <%=> tag</Strong></td>
</tr>
<tr>
<td><Strong>First Name</Strong></td>
<td><Strong>Last Name</Strong></td>
<td><Strong>User Id</Strong></td>
<td><Strong>User Role</Strong></td>
</tr>
<%
for(int i=0; i<cartoonUsers.size(); i++)
{
User user = (User)(cartoonUsers.get(i));
%>
<tr>
<td><%=user.getFirstName()%></td>
<td><%=user.getLastName()%></td>
<td><%=user.getUserId()%></td>
<td><%=user.getUserRole()%></td>
</tr>
<%
}
%>
</table>
<br/>
<br/>
<br/>
<table bgcolor="#EFEFEF" border="1" >
<tr>
<td colspan="4"><Strong>2: Data displayed using JSTL tags</Strong></td>
</tr>
<tr>
<td><Strong>First Name</Strong></td>
<td><Strong>Last Name</Strong></td>
<td><Strong>User Id</Strong></td>
<td><Strong>User Role</Strong></td>
</tr>
<c:forEach var="user" items="${cartoonUsers}" >
<tr>
<td>${user.firstName}</td>
<td>${user.lastName}</td>
<td>${user.userId}</td>
<td>${user.userRole}</td>
</tr>
</c:forEach>
</table>
</body>
</html>
Following tags are used <jsp:useBean>, <c:forEach> in this JSP
com.company.servlet.IteratorServlet.java
package com.company.servlet;
import java.util.ArrayList;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class IteratorServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws javax.servlet.ServletException, java.io.IOException {
process(request, response);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws javax.servlet.ServletException, java.io.IOException {
process(request, response);
}
public void process(HttpServletRequest request, HttpServletResponse response)
throws javax.servlet.ServletException, java.io.IOException {
System.out.println("Inside IteratorServlet::process()");
//Let us create an ArrayList with multiple User objects for testing
User user1 = new User();
user1.setFirstName("Donald");
user1.setLastName("Duck");
user1.setUserId("ddonald");
user1.setUserRole("duck");
User user2 = new User();
user2.setFirstName("Micky");
user2.setLastName("Mouse");
user2.setUserId("mmouse");
user2.setUserRole("mouse");
User user3 = new User();
user3.setFirstName("Pluto");
user3.setLastName("Dog");
user3.setUserId("pdog");
user3.setUserRole("dog");
ArrayList<User> cartoonUsers = new ArrayList<User>();
cartoonUsers.add(user1);
cartoonUsers.add(user2);
cartoonUsers.add(user3);
request.setAttribute("cartoonUsers", cartoonUsers);
request.getRequestDispatcher
("/jsp/iterator.jsp").forward(request,
response);
}
}
web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4"
xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<display-name>ServletApp</display-name>
<welcome-file-list>
<welcome-file>index.html</welcome-file>
</welcome-file-list>
<servlet>
<display-name>IteratorServlet</display-name>
<servlet-name>IteratorServlet</servlet-name>
<servlet-class>com.company.servlet.IteratorServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>IteratorServlet</servlet-name>
<url-pattern>/IteratorServlet</url-pattern>
</servlet-mapping>
</web-app>
Now launch page
Please note that both these implementations are XSS vulnerable in their current form. You should avaoid using
<%=%> tag unless absolutly necessary. Use <c:out> tag with escapeXml="true" to display user entered data and protect applications from XSS attack.
Following section describes various errors you may encounter while using JSTL tags
org.apache.jasper.JasperException:
According to TLD or attribute directive in tag file, attribute items does not accept any expressions
Resolution : Check <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%> line in your JSP. It should be
uri="http://java.sun.com/jsp/jstl/core" and not uri="http://java.sun.com/jstl/core"
org.apache.jasper.JasperException:
The absolute uri: http://java.sun.com/jsp/jstl/core cannot be resolved in either web.xml or
the jar files deployed with this application
Resolution : If you are running it in Tomcat check standard.jar and jstl.jar is either available in applications /WEB-INF/lib/ directory
or put it in TOMCAT classpath.
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