如何在JSP中循环访问HashMap？ [英] How to loop through a HashMap in JSP?

问题描述

如何在JSP中循环 HashMap ？

  <％
 HashMap< String，String> countries = MainUtils.getCountries（l）; 
％> 
 
  <％
 //这里我需要循环遍历国家。 
％> 
< / select> 
  
 
 
解决方案
就像您在普通Java中一样代码。 
 
 
  for（Map.Entry< String，String> entry：countries.entrySet（））{
 String key = entry.getKey（）; 
 String value = entry.getValue（）; 
 // ... 
} 
  
 
 
 
 
 
 
 然而， scriptlets （JSP文件中的原始Java代码，那些<％％> 东西）被视为不良练习。我建议安装 JSTL （只需删除JAR文件在 / WEB-INF / lib 中声明所需 taglibs ）。它有一个 < c：forEach>  标签，可以迭代其他标签 Map  s。每次迭代都会为您提供  Map.Entry  返回，后者又具有 getKey（）和 getValue（） 
 
下面是一个基本的例子： 
 
 
 <％@ taglib prefix =curi =http://java.sun.com/jsp/jstl/core％> 
 
< c：forEach items =$ {map}var =entry> 
 Key = $ {entry.key}，value = $ {entry.value}< br> 
< / c：forEach> 
  
因此您的特定问题可以按照以下方式解决：
 
 
 <％@ taglib prefix =curi =http://java.sun.com/jsp/jstl/core > 
 
 < c：forEach items =$ {countries}var =country> 
 < / c：forEach> 
< / select> 
  

您需要一个 Servlet 或一个 ServletContextListener 将 $ {countries} 放置在所需的范围内。如果这个列表应该是基于请求的，那么使用 Servlet 的 doGet（）：

  protected void doGet（HttpServletRequest请求，HttpServletResponse响应）{
 Map< String，String> countries = MainUtils.getCountries（）; 
 request.setAttribute（国家，国家）; 
 request.getRequestDispatcher（/ WEB-INF / page.jsp）。forward（request，response）; 
 
 $ / code> 

或者如果这个列表应该是一个应用程序范围内的常量，那么使用 ServletContextListener 的 contextInitialized（），这样它将只加载一次并保存在内存中：

  public void contextInitialized（ServletContextEvent event）{
 Map< String，String> countries = MainUtils.getCountries（）; 
 event.getServletContext（）。setAttribute（countries，countries）; 
 
 $ / code> 

在这两种情况下， countries 将通过<$ c在 EL 中提供$ b> $ {countries} 。

希望这有助于您。 还有：

• 使用JSTL迭代List和Map的元素< c：forEach>标记


• >如何在< c：forEach> 中迭代嵌套的地图 如何使用JSTL迭代HashMap中的ArrayList？


• 使用特殊的自动启动servlet来初始化启动和共享应用程序数据

How can I loop through a HashMap in JSP?

<%
    HashMap<String, String> countries = MainUtils.getCountries(l);
%>

<select name="country">
    <% 
        // Here I need to loop through countries.
    %>
</select>

解决方案

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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