如何使用JSTL遍历可能无限的集合的集合

Question

Preface: I am a long-time fan of SO, read stuff here almost everyday -- but this is my first question -- thank you to everyone who has made SO such a fantastic resource!

Environment: Spring/JSTL/Java ; the constraints of this project are such that I cannot accept a solution which requires abandoning Spring/Java, though I could probably implement any standard view in place of JSTL.

Note: This question is not about how to iterate over a simple collection in JSTL; I know how to do that. Additionally, I have searched SO and Google for an answer to this problem and have yet to find an answer. I am willing to admit that maybe that indicates that I am approaching the problem incorrectly -- if so, I would appreciate an explanation of a better way to approach it.

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Problem: I have a data-service from which I obtain a collection of collections. I do not control this service, nor am I able to define it -- someone else is in charge of that, and it can change at any time. Each item in the collection can be another collection (which I must parse), so the entire collection is theoretically unbounded. The collection is basically like this:

• Item name : String
  • Item : Object (can be a String or another Collection)
    (this sequence is potentially infinite)

Essentially each item is a "service" that my client offers to their customers -- for example: "Configure your institutional-email", and each collection is a "category" of related services -- for example: "Email Services Offered". But the category of "Email Services Offered" could actually be an item/object/collection of a larger item/object/collection that is more general -- for example: "Communication Services Offered".

Current Solution: In my Spring Controller I currently get this collection as a Map and place it onto my Model. In my View (JSTL) I (currently) iterate through 2-levels of the collection, where I assume (the one thing I can assume about this) that the first level is all going to be sub-collections (ie: 'categories'), and where I also assume (incorrectly) that the second level is all going to be items (ie: 'services').

<ul>    
  <c:forEach items="${serviceCategories} var="category">  // iterate through each collection

    <li>${category.key}  //the user-displayable title of the 'category' ('collection')

      <ul>
        <c:forEach items="${category.value}" var="service">  // iterate through each item
          <li>${service.key}</li>  //the user-displayable title of the 'service' ('item')
        </c:if>
      </ul>

    </li>

  </c:if>
</ul>

So I guess my question is that, given such a data-structure, how do I represent it in a view that can manage this??? I am bound by the client's desire to have this structure represented in a tree-like structure.

If I could manipulate the view using pure-java then I would expect one way to achieve this would be to use recursion... something like this, I guess:

/** NOTE:  I am not suggesting this is a great example of recursion, nor that it even
 *  would result in correct output; it was one-off'd just to provide something of an 
 *  example of how I imagine this could be done in Java if I had to do it this way.
 */
private StringBuilder buildString( Map<String, Object> dataStruct ) {

  StringBuilder view = new StringBuilder();

  Set<String> keys = dataStruct.getKeys();
  for( String key : keys ) {

    Object value = dataStruct.get( key );
    if( value instanceof String ) {

      view = key + "[" + value + "]";
      return;
    }

    String subView = buildString( value );
    view = key + "[" + subView + "]";
}

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Gosh, I hope that is all clear-as-mud and not overwhelming. I also hope I am simply overlooking (or ignorant of) a more elegant-solution that is readily recognized by someone who can help me out.

Thank you for your time! :D

Answer 1

Anytime you have hierarchical data that has unknown depth, and you want to display it, I think a good option is to turn it into XML and bind the XML to some type of tree display. For example, in a similar situation in one of my applications (also Java/Spring) I use XStream to turn the collection into XML (you simply point XStream at the root object and it follows all of the paths for you), and then I display the results using the jQuery plugin jsTree .

If you don't want to use javascript, an alternate approach would be to use something like Jakob Jenkov's Tree Tag .

Answer 2

另一种选择是创建一个JSP简单标记（ .tag文件），该标记以递归方式调用自身。