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How to loop through a HashMap in JSP?

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长发绾君心
长发绾君心 2020-11-22 02:18

How can I loop through a HashMap in JSP?

<%
    HashMap countries = MainUtils.getCountries(l);
%>
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  • 轮回少年
    2020-11-22 02:31

    Just the same way as you would do in normal Java code.

    for (Map.Entry<String, String> entry : countries.entrySet()) {
    String key = entry.getKey();
    String value = entry.getValue();
    // ...
}

    However, scriptlets (raw Java code in JSP files, those <% %> things) are considered a poor practice. I recommend to install JSTL (just drop the JAR file in /WEB-INF/lib and declare the needed taglibs in top of JSP). It has a <c:forEach> tag which can iterate over among others Maps. Every iteration will give you a Map.Entry back which in turn has getKey() and getValue() methods.

    Here's a basic example:

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

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

    Thus your particular issue can be solved as follows:

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

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

    You need a Servlet or a ServletContextListener to place the ${countries} in the desired scope. If this list is supposed to be request-based, then use the Servlet's doGet():

    protected void doGet(HttpServletRequest request, HttpServletResponse response) {
    Map<String, String> countries = MainUtils.getCountries();
    request.setAttribute("countries", countries);
    request.getRequestDispatcher("/WEB-INF/page.jsp").forward(request, response);
}

    Or if this list is supposed to be an application-wide constant, then use ServletContextListener's contextInitialized() so that it will be loaded only once and kept in memory:

    public void contextInitialized(ServletContextEvent event) {
    Map<String, String> countries = MainUtils.getCountries();
    event.getServletContext().setAttribute("countries", countries);
}

    In both cases the countries will be available in EL by ${countries}.

    Hope this helps.

    See also:

    • Iterate over elements of List and Map using JSTL <c:forEach> tag
    • How to iterate over a nested map in <c:forEach>
    • How to iterate an ArrayList inside a HashMap using JSTL?
    • Using special auto start servlet to initialize on startup and share application data
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  • 情书的邮戳
    2020-11-22 02:42

    Depending on what you want to accomplish within the loop, iterate over one of these instead:

    • countries.keySet()
    • countries.entrySet()
    • countries.values()
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