Java 如何在 Servlet 上下文中获取所有属性名称（是否嵌套）并迭代它是地图还是列表？
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原文地址: http://stackoverflow.com/questions/6613535/
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How to get all attributes names(nested or not) in Servlet Context and iterate if it's a map or list?
提问by hamahama
I've tried to get attributeNames of a ill-maintained context, then use those names with reflection.
我试图获取维护不良的上下文的属性名称，然后通过反射使用这些名称。
Here's some pseudo code for a rough idea. E.g. I have an ArrayList and a HashMap in the context.
这是一些粗略的伪代码。例如，我在上下文中有一个 ArrayList 和一个 HashMap。
enum = getServletContext().getAttributeNames();
for (; enum.hasMoreElements(); ) {
String name = (String)enum.nextElement();
// Get the value of the attribute
Object value = getServletContext().getAttribute(name);
if (value instanceof HashMap){
HashMap hmap = (HashMap) value;
//iterate and print key value pair here
}else if(value instanceof ArrayList){
//do arraylist iterate here and print
}
}
采纳答案by Bohemian
Here's code that will do what you want:
这是可以执行您想要的操作的代码：
Enumeration<?> e = getServletContext().getAttributeNames();
while (e.hasMoreElements())
{
String name = (String) e.nextElement();
// Get the value of the attribute
Object value = getServletContext().getAttribute(name);
if (value instanceof Map) {
for (Map.Entry<?, ?> entry : ((Map<?, ?>)value).entrySet()) {
System.out.println(entry.getKey() + "=" + entry.getValue());
}
} else if (value instanceof List) {
for (Object element : (List)value) {
System.out.println(element);
}
}
}
Notes:
笔记：
- Always favour referring to the abstract interface over concrete implementations. In this case, check for
ListandMap(interfaces), rather thanArrayListandHashMap(specific implementations); consider what will happen if the context hands you aLinkedListrather than anArrayList, or aMapthat's not aHashMap- your code would (unnecessarily) explode - Use
while (condition)rather thanfor (;condition;)- it's just ugly - If you know the types of your Collections, specify them. For example, web contexts usually give you a
Map<String, Object>:
- 始终倾向于引用抽象接口而不是具体实现。在这种情况下，检查
List和Map（接口），而不是ArrayList和HashMap（具体实现）；考虑如果上下文给你 aLinkedList而不是ArrayList，或者Map不是 a会发生什么HashMap- 你的代码会（不必要地）爆炸 - 使用
while (condition)而不是for (;condition;)- 它只是丑陋 - 如果您知道集合的类型，请指定它们。例如，网络上下文通常会给你一个
Map<String, Object>：
so the code could become
所以代码可以变成
for (Map.Entry<String, Object> entry : ((Map<String, Object>)value).entrySet()) {
String entryKey = entry.getKey();
Object entryValue = entry.getValue();
}
