在Java中使用DOM解析器时出现Null异常

Question

I am making an Android app and for it I have to do some XML parsing. I followed an example, which works, but when I try to adapt the example for my usage I get a null pointer exception I don't understand. This should hold all relevant bits of code. The issue is with the line near the bottom of this snippit between the Log.e("5", "5") and the corresponding "6" with Element docEle = dom.getDocumentElement();

Any ideas?

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);

        }
    }
}

Here's the 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

The problem is that you never initialize the parameter dom , because your public void parseReportXML() method is throwing an exception.

Take a look at your stacktrace and find what's the exception when you call:

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

I bet my fingers that the path is not correct ;)

edit: The stacktrace you posted is telling you that db can't find the document. So, yes, the path was incorrect. Check some examples of loading a file in Java, is not that easy sometimes.