在Java中使用DOM解析器時出現Null異常

Question

我正在制作一個Android應用程序，為此我必須進行一些XML解析。 我遵循了一個有效的示例，但是當我嘗試使該示例適合我的用法時，我得到了一個空指針異常，我無法理解。 這應該包含所有相關的代碼位。 問題在於此代碼段底部附近的Log.e("5", "5")和Element docEle = dom.getDocumentElement();的相應"6" Element docEle = dom.getDocumentElement();

有任何想法嗎？

public class Parser {

//Constructor
ArrayList<Response> responseList;
ArrayList<ResourceType> resourceTypeList;
List myEmpls;
Document dom;

public Parser()
{
    responseList = new ArrayList<Response>();
    resourceTypeList = new ArrayList<ResourceType>();
    myEmpls = new ArrayList();
}

public void addRequest(int reportId)
{
    Request request = new Request(reportId);
    Response response = new Response(request);
        //Form XML in LoadReport Request
        //Send XML
        //Receive and parse 
    parseReportResponse();
        //Input information into response
        //Form XML in GetRoomData request
        //Send XML
        //Receive and parse
        //Input information into response
    responseList.add(response);
    Log.e("5", "5");
}
public void parseReportResponse()
{
    parseReportXML();
    parseReportDocument();
}   

public void parseReportXML()
{
    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    try {
        DocumentBuilder db = dbf.newDocumentBuilder();
        dom = db.parse("LoadReportResponse.xml");

    }catch(ParserConfigurationException pce) {
        pce.printStackTrace();
    }catch(SAXException se) {
        se.printStackTrace();
    }catch(IOException ioe) {
        ioe.printStackTrace();
    }
}
public void parseReportDocument()
{
    Log.e("5", "5");
    Element docEle = dom.getDocumentElement();
    Log.e("6", "6");
    NodeList nl = docEle.getElementsByTagName("Room");
    if(nl != null && nl.getLength() > 0) 
    {
        for(int i=0; i<nl.getLength(); i++) 
        {
                Element el = (Element)nl.item(i);
                //Employee e = getEmployee(el);
                Room room = getNewRoom(el);
                myEmpls.add(room);

        }
    }
}

這是logcat

06-24 15:16:08.209: E/AndroidRuntime(6017): FATAL EXCEPTION: main
06-24 15:16:08.209: E/AndroidRuntime(6017): java.lang.RuntimeException: Unable to start activity ComponentInfo{[Stuff I have to hide].MainActivity}: java.lang.NullPointerException
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.os.Looper.loop(Looper.java:137)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.ActivityThread.main(ActivityThread.java:5039)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at java.lang.reflect.Method.invokeNative(Native Method)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at java.lang.reflect.Method.invoke(Method.java:511)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at dalvik.system.NativeStart.main(Native Method)
06-24 15:16:08.209: E/AndroidRuntime(6017): Caused by: java.lang.NullPointerException
06-24 15:16:08.209: E/AndroidRuntime(6017):     at com.example.[I have to hide].Parser.parseReportDocument(Parser.java:81)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at com.example.[I have to hide].Parser.parseReportResponse(Parser.java:59)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at com.example.[I have to hide].Parser.addRequest(Parser.java:41)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at com.example.[I have to hide].MainActivity.onCreate(MainActivity.java:79)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.Activity.performCreate(Activity.java:5104)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
06-24 15:16:08.209: E/AndroidRuntime(6017):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
06-24 15:16:08.209: E/AndroidRuntime(6017):     ... 11 more

Answer 1

問題是您永遠不會初始化參數dom ，因為您的public void parseReportXML()方法會引發異常。

查看一下您的堆棧跟蹤，並在調用時查找異常：

dom =db.parse("LoadReportResponse.xml");

我打賭路徑不正確 ;）

編輯：您發布的stacktrace告訴您db找不到文檔。 因此，是的，路徑不正確。 檢查一些用Java加載文件的示例，有時並不容易。