我的XML不是導出元素。爲什麼？

Question

我一直在研究一種將數據轉換爲基於XML的Collada .DAE格式的工具。不幸的是，有一件事一直在阻止我：我的導出XML沒有任何我的元素！

這是代碼。我已經使它易於閱讀，因此您不必經歷閱讀過程中的許多麻煩。

public class DAEExport { 
    private static boolean alreadyConstructed = false; 
    private static Document doc = null; 
    private static Element root = null; 
    private static Element lib_images_base_element = null; 
    private static Element lib_geometry_base_element = null; 
    private static Element lib_control_base_element = null; 
    private static Element lib_visual_scene_base_element = null; 

    public static void AppendData() { 
     //Normally this method would have the data to append as its args, but I'm not worried about that right now. 
     //Furthermore, ASSUME THIS RUNS ONLY ONCE (It runs once in the test code I'm using to run this method)! I know that it won't work if called multiple times, as the below variables for the document builder and such wouldn't exist the second time around 
     try { 
      if (!alreadyConstructed) { 
       alreadyConstructed = true; 
       DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance(); 
       DocumentBuilder docBuilder = docFactory.newDocumentBuilder(); 
       Document document = docBuilder.newDocument(); 

       Element rootElement = document.createElement("SomeGenericElement"); 
       rootElement.appendChild(document.createTextNode("Generic test contents"); 

       document.appendChild(rootElement); 

       doc = document; 
      } 
     } catch (Exception e) { 
      e.printStackTrace(); 
     } 
    } 

    public static void Build(File _out) { 
     try { 
      TransformerFactory transformerFactory = TransformerFactory.newInstance(); 
      Transformer transformer = transformerFactory.newTransformer(); 
      DOMSource source = new DOMSource(doc); 
      StreamResult result = new StreamResult(System.out); 


      transformer.transform(source, result); 

      alreadyConstructed = false; 
      doc = null; 
     } catch (Exception e) { 
      e.printStackTrace(); 
     } 
    } 
} 

好了，所以這裏是我的問題：儘管通過調用AppendData()，然後調用Build()打印數據添加這些元素的文件，我只得到了以下數據：

<?xml version="1.0" encoding="UTF-8"?> - 無元素。只是基本的標題。就是這個。

我不知道是否因爲一些愚蠢的錯誤，我忘記了過去的時間或其他東西。任何答案爲什麼我的元素消失？

Answer 1

目前，您的doc對象未從AppendData()方法傳遞到Build()方法。所有Build()用途均爲聲明中的空文檔：doc = null。因此，你的結果是空的節點（TransformFactory添加XML標題）。

考慮將doc對象從AppendData()返回到其他方法的類級別（請注意將void更改爲返回的對象類型）。然後，您重新定義doc並把它傳遞到Build()：

public static Document AppendData() { 
    ... 
    doc = document 
    return(doc) 
} 

或者，撥打Build()內AppendData，路過doc作爲參數：

public static void AppendData() { 
    ... 
    doc = document 
    Build(doc, outfile) 
} 

public static void Build(Document doc, File _out) { 
    ... 
}