How to get Map value with the null key inside JSP

It seems that the only way of getting the value for the null key using standard EL implementation is to call get() method on the map (considering that you said keyObject.keyProp resolves to Integer object):

<c:out value="${map.get(keyObject.keyProp)}" />

I tested this solution and it works.

Actually, in this case, you can easily do without <c:out />, just use plain EL where you need it, e.g.

${map.get(keyObject.keyProp)}

Simple example:

TestMapServlet.java

package com.example;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;

import java.util.Map;
import java.util.HashMap;

import java.io.IOException;

public class TestMapServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
                    throws ServletException, IOException {
        Map<Integer, Object> map = new HashMap<Integer, Object>();
        Integer noValueInt = null;
        Integer one = new Integer(1);
        Integer two = new Integer(2);

        map.put(noValueInt, "Default object for null Integer key");
        map.put(one, "Object for key = Integer(1)");
        map.put(two, "Object for key = Integer(2)");

        request.setAttribute("map", map);
        request.setAttribute("noValueInt", noValueInt);
        request.setAttribute("one", one);
        request.setAttribute("two", two);

        request.getRequestDispatcher("/test-map.jsp").forward(request, response);
    }
}

test-map.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core"%>
<!DOCTYPE html>
<html>
<head>
    <title>Home Page</title>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
    <h1>Testing access to java.util.Map using just EL</h1>
    <p><b>\${map.get(noValueInt)}</b>: ${map.get(noValueInt)}</p>
    <p><b>\${map[one]}</b>: ${map[one]}</p>
    <p><b>\${map[two]}</b>: ${map[two]}</p>

    <h1>Testing access to java.util.Map using JSTL and EL</h1>
    <p><b>&lt;c:out value="\${map.get(noValueInt)}" /&gt; </b>: <c:out value="${map.get(noValueInt)}" /></p>
    <p><b>&lt;c:out value="\${map[one]}" /&gt; </b>: <c:out value="${map[one]}" /></p>
    <p><b>&lt;c:out value="\${map[two]}" /&gt; </b>: <c:out value="${map[two]}" /></p>

    <h2>Printing java.util.Map keys and values (when Key = null, the <i>null</i> won't be shown)</h2>
    <c:forEach items="${map}" var="entry">
        <p><b>Key</b> = "${entry.key}", <b>Value</b> = "${entry.value}"</p>
    </c:forEach>
</body>
</html>

web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                        http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">

    <servlet>
        <servlet-name>Test Map Servlet</servlet-name>
        <servlet-class>com.example.TestMapServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Test Map Servlet</servlet-name>
        <url-pattern>/TestMap.do</url-pattern>
    </servlet-mapping>

</web-app>

IMPORTANT NOTE
To be able to invoke methods with arguments using EL you must use minimum Servlet version 3.0.
Quote from here: https://stackoverflow.com/tags/el/info

Since EL 2.2, which is maintained as part of Servlet 3.0 / JSP 2.2 (Tomcat 7, Glassfish 3, JBoss AS 6, etc), it's possible to invoke non-getter methods, if necessary with arguments.

Apart from the above solution you could use custom Unified Expression Language implementation such as JUEL that has an alternative solution.

An explanation why it is not possible (in the standard implementation) to access map value by the null key using [] and the custom solution can be found in Java Unified Expression Language (JUEL) documentation (emphasis in paragraphs is mine):

2.5. Advanced Topics
...
Enabling/Disabling null Properties
The EL specification describes the evaluation semantics of base[property]. If property is null, the specification states not to resolve null on base. Rather, null should be returned if getValue(...) has been called and a PropertyNotFoundException should be thrown else. As a consequence, it is impossible to resolve null as a key in a map. However, JUEL's expression factory may be configured to resolve null like any other property value. To enable (disable) null as an EL property value, you may set property javax.el.nullProperties to true (false).
Assume that identifier map resolves to a java.util.Map.

If feature javax.el.nullProperties has been disabled, evaluating ${base[null]} as an rvalue (lvalue) will return null (throw an exception).

If feature javax.el.nullProperties has been enabled, evaluating ${base[null]} as an rvalue (lvalue) will get (put) the value for key null in that map. The default is not to allow null as an EL property value.

...

Hope this will help.

Simple example:

2.5. Advanced Topics

Enabling/Disabling null Properties

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