在 Java 中，我們會使用 public static final String A_CONST  = "const"; 的方式設定常數，但在 JSTL 中無法直接取用。以下是解決方法：

1. 由於 JSTL 可以取得 Map 或 JavaBean 的值，所以我們可以利用這個特性，建立一個 Class 繼承 HashMap 。然後用 relection 將其中的 fields 放入 Map 中。

   package com.utils;
   
   import java.lang.reflect.Field;
   import java.lang.reflect.Modifier;
   import java.util.HashMap;
   import java.util.Map;
   
   /**
    * Class to reveal java constants to JSTL Expression Language
    * Uses reflection to scan the declared fields of a Constants class
    * Adds these fields to the Map.
    * Map is unmodifiable after initialization.
    *
    */
   public class JSTLConstants extends HashMap {
   	private boolean initialised = false;
   
   	public JSTLConstants() {
   		Class c = this.getClass();
   		Field[] fields = c.getDeclaredFields();
   		for (int i = 0; i < fields.length; i++) {
   
   			Field field = fields[i];
   			int modifier = field.getModifiers();
   			if (Modifier.isFinal(modifier) && !Modifier.isPrivate(modifier))
   				try {
   					this.put(field.getName(), field.get(this));
   				}
   				catch (IllegalAccessException e) {}
   		}
   		initialised = true;
   	}
   
   	public void clear() {
   		if (!initialised)
   			super.clear();
   		else
   			throw new UnsupportedOperationException("Cannot modify this map");
   	}
   
   	public Object put(Object key, Object value) {
   		if (!initialised)
   			return super.put(key, value);
   		else
   			throw new UnsupportedOperationException("Cannot modify this map");
   	}
   
   	public void putAll(Map m) {
   		if (!initialised)
   			super.putAll(m);
   		else
   			throw new UnsupportedOperationException("Cannot modify this map");
   	}
   
   	public Object remove(Object key) {
   		if (!initialised)
   			return super.remove(key);
   		else
   			throw new UnsupportedOperationException("Cannot modify this map");
   	}
   }
   

2. 然後我們的 Constants Class 再繼承上面的 Class：

   package com.utils;
   
   import java.util.*;
   public class JSTLConstantsTest extends JSTLConstants {
   
   	public static final String name = "Evnafets";
   	public static final int age = 28;
   	private static final String bad = "bad";
   	public static final String programmingSkill = "Awesome";
   	public static final String[] daysOfTheWeek = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun" };
   	public int temp = 100;
   
   	public static void main(String[] args) {
   		Map m = new JSTLConstantsTest();
   
   		System.out.println("Tee constants are ");
   		for (Iterator it = m.entrySet().iterator(); it.hasNext();) {
   			Map.Entry me = (Map.Entry) it.next();
   			System.out.println(me.getKey() + " = " + me.getValue());
   		}
   	}
   }
   

3. 那麼在 JSP 頁面上我們就可以這樣取用：

   <%@ taglib prefix="c" uri="http://java.sun.com/jstl/core" %>
   <jsp:useBean id="Constants" class="com.utils.JSTLConstantsTest"/>
   <c:out value="${Constants.name}"/>
   <c:out value="${Constants.age}"/>
   <c:forEach var="day" items="${Constants.daysOfTheWeek}">
     <c:out value="${day}"/>
   </c:forEach>
   

在討論串中另有針對無法繼承的 Interface 或 third party class 的改進法，但基本上我想我個人應該用不到，所以就不整理了。