java - Get value from hashmap based on key to JSTL

Java - Get value from hashmap based on key to JSTL

To access a HashMap value based on a key in JSTL (JavaServer Pages Standard Tag Library), you typically need to pass the HashMap from your servlet or controller to the JSP page, and then use JSTL's c:forEach or c:choose tags to retrieve the value associated with the key. Here's how you can achieve this:

Example Scenario

Assume you have a HashMap in your servlet or controller:

import java.util.HashMap;
import javax.servlet.http.*;

public class MyServlet extends HttpServlet {
    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) {
        // Create a HashMap and populate it
        HashMap<String, String> myMap = new HashMap<>();
        myMap.put("key1", "value1");
        myMap.put("key2", "value2");
        
        // Set the HashMap as an attribute in the request
        request.setAttribute("myMap", myMap);
        
        // Forward to your JSP page
        RequestDispatcher dispatcher = request.getRequestDispatcher("yourJspPage.jsp");
        dispatcher.forward(request, response);
    }
}

Accessing HashMap in JSP Using JSTL

In your JSP page (yourJspPage.jsp), you can use JSTL to iterate through the HashMap and retrieve values based on keys:

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html>
<html>
<head>
    <title>HashMap Example</title>
</head>
<body>
    <h1>HashMap Example</h1>
    
    <%-- Get the HashMap from request attribute --%>
    <c:forEach var="entry" items="${myMap}">
        <c:choose>
            <c:when test="${entry.key == 'key1'}">
                Key 'key1' found, value is: ${entry.value}
            </c:when>
            <c:otherwise>
                Key not found or does not match specified key.
            </c:otherwise>
        </c:choose>
    </c:forEach>
</body>
</html>

Explanation

  1. Setting Attribute in Servlet: In the servlet, the HashMap (myMap) is set as a request attribute using request.setAttribute("myMap", myMap);. This makes the myMap available to the JSP page.

  2. Taglib Declaration: The JSTL core taglib is declared at the top of the JSP page to use JSTL tags (<c:forEach>, <c:choose>, etc.).

  3. Accessing HashMap in JSTL:

    • <c:forEach var="entry" items="${myMap}"> iterates through each entry in the myMap.
    • <c:choose> and <c:when> are used to check if the key matches 'key1'.
    • ${entry.key} accesses the key of the current entry.
    • ${entry.value} accesses the value associated with the key.

  4. Output: Depending on the key specified in <c:when test="${entry.key == 'key1'}">, it will display the corresponding value or indicate that the key was not found.

Notes

  • Ensure that you have the appropriate JSTL library (jstl.jar) included in your project's classpath.
  • Replace 'key1' with the actual key you want to retrieve from the HashMap.
  • This approach allows you to dynamically access and display values from a HashMap in your JSP page using JSTL, facilitating separation of concerns between presentation (JSP) and business logic (Java servlet).

By following this approach, you can effectively retrieve values from a HashMap based on keys in JSTL within your JSP pages in a Java web application.

Examples

  1. How to access a HashMap value in JSTL using a key?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />
<c:set var="key" value="myKey" />

<c:out value="${myMap[key]}" />

    Description: This JSP code snippet demonstrates accessing a value from a HashMap (myMap) stored in the request scope (requestScope.myHashMap) using JSTL. Replace "myKey" with your actual key value.

  2. How to iterate over a HashMap and display values in JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />

<c:forEach items="${myMap}" var="entry">
    Key: ${entry.key}, Value: ${entry.value}<br/>
</c:forEach>

    Description: This code iterates over a HashMap (myMap) stored in the request scope and displays each key-value pair using JSTL.

  3. How to check if a key exists in a HashMap using JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />
<c:set var="keyToCheck" value="myKey" />

<c:if test="${myMap.containsKey(keyToCheck)}">
    Key 'myKey' exists in the HashMap.
</c:if>

    Description: This example checks if "myKey" exists as a key in the HashMap (myMap) stored in the request scope using JSTL.

  4. How to retrieve all keys from a HashMap in JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />

<c:forEach var="key" items="${myMap.keySet()}">
    Key: ${key}<br/>
</c:forEach>

    Description: This snippet retrieves all keys from a HashMap (myMap) stored in the request scope using JSTL and iterates through them.

  5. How to retrieve all values from a HashMap in JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />

<c:forEach var="value" items="${myMap.values()}">
    Value: ${value}<br/>
</c:forEach>

    Description: This code fetches all values from a HashMap (myMap) stored in the request scope using JSTL and iterates through them.

  6. How to get the size of a HashMap in JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />

Size of HashMap: <c:out value="${fn:length(myMap)}" />

    Description: This snippet calculates and displays the size of a HashMap (myMap) stored in the request scope using JSTL's fn:length function.

  7. How to fetch a specific HashMap entry by index in JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />
<c:set var="index" value="0" />

<c:forEach var="entry" items="${myMap}" varStatus="loop">
    <c:if test="${loop.index eq index}">
        Key: ${entry.key}, Value: ${entry.value}<br/>
    </c:if>
</c:forEach>

    Description: This example retrieves a specific entry (index=0) from a HashMap (myMap) stored in the request scope using JSTL.

  8. How to fetch and display a HashMap value by key conditionally in JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />
<c:set var="keyToFetch" value="myKey" />

<c:choose>
    <c:when test="${myMap.containsKey(keyToFetch)}">
        Value for 'myKey': <c:out value="${myMap[keyToFetch]}" />
    </c:when>
    <c:otherwise>
        Key 'myKey' does not exist in the HashMap.
    </c:otherwise>
</c:choose>

    Description: This code checks if "myKey" exists in the HashMap (myMap) stored in the request scope using JSTL and displays its value if present.

  9. How to fetch a HashMap value based on a request parameter in JSTL?

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

<c:set var="myMap" value="${requestScope.myHashMap}" />
<c:set var="paramKey" value="${param.key}" />

<c:out value="${myMap[paramKey]}" />

    Description: This example retrieves a value from a HashMap (myMap) stored in the request scope using a request parameter (param.key) in JSTL.

  10. How to fetch and display all HashMap entries in JSTL sorted by key or value?

    <%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page import="java.util.HashMap" %>
<%@ page import="java.util.Map" %>
<%@ page import="java.util.TreeMap" %>

<c:set var="myMap" value="${requestScope.myHashMap}" />
<c:set var="sortedMap" value="<%= new TreeMap(myMap) %>" />

<c:forEach items="${sortedMap}" var="entry">
    Key: ${entry.key}, Value: ${entry.value}<br/>
</c:forEach>

    Description: This snippet demonstrates sorting and displaying all entries from a HashMap (myMap) stored in the request scope sorted by key using a TreeMap in JSTL.

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