我已经挣扎了3天左右，最后我决定冒风险问你们。问题是，我从标题中获得了NullPointerException。使用调试器，我发现了我的问题的根源，所以我确切地知道是什么原因导致nullpointer：它的变量＆＃34; doc＆＃34;类型为Document，您可以在下面的代码中找到它。对不起，有太多的话，这是代码
这是我的班级DomXmlParser的代码
public class DomXmlParser {
Document doc;
String urlLink;
InputStream is;
public DomXmlParser(String url){
this.urlLink = url;
}
public Document getDoc(){
return doc;
}
public void parseXML(InputStream is) {
try {
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
doc = builder.parse(is);
} catch (Exception e) {
e.printStackTrace();
}
}
public void getXML(){
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
try{
URL url = new URL(urlLink);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");
conn.setReadTimeout(15000);
conn.setConnectTimeout(15000);
conn.setDoInput(true);
conn.connect();
is = conn.getInputStream();
Log.d("InputStreamContents", is.toString());
parseXML(is);
is.close();
}catch(Exception e){
e.printStackTrace();
}
}
});
thread.start();
}
}
因此，在MainActivity中，我调用getXml（）方法，然后调用getDoc（）。
public void parseDom(){
DomXmlParser parser = new DomXmlParser(xmlUrl);
parser.getXML();
doc = parser.getDoc();
NodeList list = doc.getElementsByTagName("item");
for (int i = 0; i < list.getLength(); i++){
Node item = list.item(i);
if (item.getNodeType()==Node.ELEMENT_NODE){
Element data = (Element) item;
NodeList dataList = data.getChildNodes();
for (int j = 0; j < dataList.getLength(); i++){
Node p = dataList.item(j);
if (p.getNodeType()==Node.ELEMENT_NODE){
Element title = (Element) p;
Element description = (Element) p;
FeedItem fi = new FeedItem();
fi.setName(title.getTextContent());
fi.setStatus(description.getTextContent());
feedItems.add(fi);
}
}
}
}
}
我调试了我的应用程序，看起来getDoc（）方法返回null。它也说行
NodeList list = doc.getElementsByTagName("item");
返回null。因此，DomXmlParser类的Document doc变量为null。我确信，InputStream不是null，当我尝试通过XmlPullParser解析XML时，一切正常，因此XML文件或链接都可以。希望你能帮助我们，因为我和谷歌都不知道该做什么。
更新：正如下面的答案中所建议的那样，我删除了Thread，并且刚刚创建了一个在MainActivity中扩展了AsyncTask的类，所以现在我没有那个NullPointer，但我有另一个问题：Out 144字节分配的内存。我不知道该怎么做。
日志
10-29 00:17:13.139 3258-3258/com.motoharu.cleaningapp E/AndroidRuntime﹕ FATAL EXCEPTION: main
java.lang.NullPointerException
at com.motoharu.cleaningapp.HomeFragment.parseDom(HomeFragment.java:151)
at com.motoharu.cleaningapp.HomeFragment.initControls(HomeFragment.java:78)
at com.motoharu.cleaningapp.HomeFragment.onStart(HomeFragment.java:70)
at android.support.v4.app.Fragment.performStart(Fragment.java:1528)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:972)
at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1121)
at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682)
at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1484)
at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:482)
at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
at android.support.v4.view.ViewPager.populate(ViewPager.java:1073)
at android.support.v4.view.ViewPager.populate(ViewPager.java:919)
at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441)
at android.view.View.measure(View.java:16030)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:314)
at android.view.View.measure(View.java:16030)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302)
at android.view.View.measure(View.java:16030)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:314)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2196)
at android.view.View.measure(View.java:16030)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1915)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1109)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1290)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1009)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5508)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749)
at android.view.Choreographer.doCallbacks(Choreographer.java:562)
at android.view.Choreographer.doFrame(Choreographer.java:532)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735)
at android.os.Handler.handleCallback(Handler.java:730)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:213)
at android.app.ActivityThread.main(ActivityThread.java:5225)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:741)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557)
at dalvik.system.NativeStart.main(Native Method)
答案 0 :(得分：1)
问题是因为getXML正在产生一个新线程，该线程构造Document引用的doc对象。因此，在调用parser.getXML()之后，代码尝试使用doc方法访问getDoc()变量，但该变量仍未由该线程初始化。
解决方案是等待线程完成。这可以简单地使用join来完成。例如，您可以在调用Thread方法时创建新的getXML，而不是在方法本身内创建它。
public void getXML(){
try {
URL url = new URL(urlLink);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("POST");
conn.setReadTimeout(15000);
conn.setConnectTimeout(15000);
conn.setDoInput(true);
conn.connect();
is = conn.getInputStream();
Log.d("InputStreamContents", is.toString());
parseXML(is);
is.close();
}catch(Exception e){
e.printStackTrace();
}
}
在parseDom
final DomXmlParser parser = new DomXmlParser(xmlUrl);
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
parser.getXML();
}
});
thread.start();
try {
thread.join(); // wait until above thread finishes
} catch (InterruptedException e) {
// handle the exception here
}
doc = parser.getDoc();
答案 1 :(得分：1)