How to loop through a HashMap in JSP?

java

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best-practices

model-citizens-of-mvc

byNikita Barsukov·Sep 22, 2024

Iterating a HashMap in JSP is best achieved with <c:forEach> tag from the JSTL library. It enables us to assign a variable for each loop iteration. Here's how to tap into keys and values:

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

<c:forEach var="entry" items="${map}">
    ${entry.key}: ${entry.value}<br>
</c:forEach>

Be aware to include the JSTL library in your project to be able to exploit the <c:forEach> tag.

Practical Applications

Use the HashMap entries to feed your <select> dropdown menu:

<select name="country">
  <c:forEach var="entry" items="${countries}">
    <option value="${entry.key}">${entry.value}</option>
  </c:forEach>
</select>

No Iterable Left Behind: Grab'Em in Servlet

Ensure your HashMap is all set and ready for JSP by setting it as an attribute of the request in your Servlet:

// Imagine you are threading a pearl necklace.
// Now forward it to princess JSP.
request.setAttribute("countries", countriesMap);
RequestDispatcher dispatcher = request.getRequestDispatcher("page.jsp");
dispatcher.forward(request, response);

Parking Your HashMap: The Application-Wide Access

If you fancy having a HashMap available application-wide, rely on ServletContextListener to initialize and plant it in the application scope:

// Park Your HashMap Here. It's a VIP spot!
public class ConfigListener implements ServletContextListener {
    public void contextInitialized(ServletContextEvent sce) {
        HashMap<String, String> countries = new HashMap<>();
        // ... Populate the HashMap...
        sce.getServletContext().setAttribute("countries", countries);
    }
}

Working with HashMap, Hands-on

Keeping the map neat and tidy with types

To make things clear, use specific data types such as Map<String, String> to maintain type safety of key-value pairs:

// It's like labeling your drawers. So organized!
Map<String, String> countriesMap = new HashMap<>();

Codeception: Accessing nested structures

For HashMap that holds complex objects, extend the EL notation to gain access to nested properties:

<c:forEach var="entry" items="${departments}">
  Department: ${entry.key}, Manager: ${entry.value.managerName}<br>
</c:forEach>

Best Practice Bureau

Stand aside, scriptlets

Shy away from scriptlets (<% code %>), which blur the separation between Java and JSP. Stick to JSTL and EL for a swanky, maintainable codebase.

Model Citizens of MVC

In MVC frameworks like Spring, you can define HashMaps as model attributes in the controller, making them accessible in JSP:

// HashMap, you are now a model. Strike a pose!
model.addAttribute("countries", countriesMap);

It's alive! Dynamic HTML Attributes

For dynamic HTML attributes powered by HashMap values, incorporate EL within the attribute values:

<c:forEach var="entry" items="${map}">
  <div id="country_${entry.key}" class="country">${entry.value}</div>
</c:forEach>

explain-codes / Java / How to loop through a HashMap in JSP?

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