我是 Android 开发新手，正在制作一个可以读取 rss xml 文件、将其存储在数据库中并可在本地访问的应用程序。当我这样做时，代码崩溃了，我不明白为什么。我尝试过一些事情，但一无所获。我真的很感激任何帮助。谢谢您
布局文件(maintwo.xml)
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="I'm screen 2 (main2.xml)"
android:textAppearance="?android:attr/textAppearanceLarge" />
</LinearLayout>
Activity 文件 (ActivityTwo.java)
import android.app.Activity;
import android.os.Bundle;
import android.widget.Button;
import android.widget.TextView;
import java.util.ArrayList;
public class ActivityTwo extends Activity{
//private DatabaseHelper helper;
private TextView tv;
private ArrayList<String> strings = new ArrayList<String>();
private HandleXML handleXML;
//DatabaseHelper helper;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.maintwo);
tv = (TextView)findViewById(R.id.textView1);
//helper = new DatabaseHelper(this);
//storeToDB();
//writeToFile();
getFromXML();
}
/*public void storeToDB() {
helper = new DatabaseHelper(this);
helper.insertContact("Something", "123");
helper.insertContact("Another", "456");
}
public void writeToFile() {
strings = helper.getAllContacts();
String x = strings.get(0);
String y = strings.get(2);
tv.setText(x + " " + y);
}*/
public void getFromXML() {
handleXML = new HandleXML("http://examplewebsite/file.xml");
handleXML.fetchXML();
strings = handleXML.getAll();
//strings = helper.getAllContacts();
String x = strings.get(0);
tv.setText(x);
}
}
RSS 阅读器 (HandleXML.java)
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserFactory;
import android.app.Activity;
import android.content.Context;
import android.util.Log;
public class HandleXML extends Activity{
private String title = "title";
private String link = "link";
private String description = "description";
private DatabaseHelper helper;
private String urlString = null;
private XmlPullParserFactory xmlFactoryObject;
public volatile boolean parsingComplete = true;
public HandleXML(String url){
helper = new DatabaseHelper(this);
this.urlString = url;
}
public ArrayList<String> getAll() {
return helper.getAllContacts();
}
/*public String getTitle(){
return title;
}
public String getLink(){
return link;
}
public String getDescription(){
return description;
}*/
public void parseXMLAndStoreIt(XmlPullParser myParser) {
int event;
int i =0;
String text=null;
try {
event = myParser.getEventType();
while (event != XmlPullParser.END_DOCUMENT) {
String name=myParser.getName();
switch (event){
case XmlPullParser.START_TAG:
break;
case XmlPullParser.TEXT:
text = myParser.getText();
break;
case XmlPullParser.END_TAG:
if(name.equals("title")){
title = text;
}
else if(name.equals("link")){
link = text;
}
else if(name.equals("description")){
description = text;
}
else{
}
break;
}
helper.insertContact(title + i, "123");
i++;
event = myParser.next();
}
parsingComplete = false;
} catch (Exception e) {
e.printStackTrace();
}
}
public void fetchXML(){
Thread thread = new Thread(new Runnable(){
@Override
public void run() {
try {
URL url = new URL(urlString);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setReadTimeout(10000 /* milliseconds */);
conn.setConnectTimeout(15000 /* milliseconds */);
conn.setRequestMethod("GET");
conn.setDoInput(true);
// Starts the query
conn.connect();
InputStream stream = conn.getInputStream();
xmlFactoryObject = XmlPullParserFactory.newInstance();
XmlPullParser myparser = xmlFactoryObject.newPullParser();
myparser.setFeature(XmlPullParser.FEATURE_PROCESS_NAMESPACES, false);
myparser.setInput(stream, null);
parseXMLAndStoreIt(myparser);
stream.close();
} catch (Exception e) {
}
}
});
thread.start();
}
}
用于创建数据库的文件(DatabaseHelper.java)
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.DatabaseUtils;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import java.util.ArrayList;
import java.util.HashMap;
public class DatabaseHelper extends SQLiteOpenHelper {
public static final String DATABASE_NAME = "MyDBName.db";
public static final String CONTACTS_TABLE_NAME = "contacts";
public static final String CONTACTS_COLUMN_ID = "id";
public static final String CONTACTS_COLUMN_NAME = "name";
public static final String CONTACTS_COLUMN_PHONE = "phone";
private HashMap hp;
public DatabaseHelper(Context context) {
super(context, DATABASE_NAME , null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL("create table contacts " + "(id integer primary key, name text,phone text)");
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
db.execSQL("DROP TABLE IF EXISTS contacts");
onCreate(db);
}
public boolean insertContact (String name, String phone)
{
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put("name", name);
contentValues.put("phone", phone);
db.insert("contacts", null, contentValues);
return true;
}
public Cursor getData(int id){
SQLiteDatabase db = this.getReadableDatabase();
Cursor res = db.rawQuery( "select * from contacts where id="+id+"", null );
return res;
}
public int numberOfRows(){
SQLiteDatabase db = this.getReadableDatabase();
int numRows = (int) DatabaseUtils.queryNumEntries(db, CONTACTS_TABLE_NAME);
return numRows;
}
public boolean updateContact (Integer id, String name, String phone)
{
SQLiteDatabase db = this.getWritableDatabase();
ContentValues contentValues = new ContentValues();
contentValues.put("name", name);
contentValues.put("phone", phone);
db.update("contacts", contentValues, "id = ? ", new String[] { Integer.toString(id) } );
return true;
}
public Integer deleteContact (Integer id)
{
SQLiteDatabase db = this.getWritableDatabase();
return db.delete("contacts",
"id = ? ",
new String[] { Integer.toString(id) });
}
public ArrayList getAllContacts()
{
ArrayList array_list = new ArrayList();
//hp = new HashMap();
SQLiteDatabase db = this.getReadableDatabase();
Cursor res = db.rawQuery( "select * from contacts", null );
res.moveToFirst();
while(res.isAfterLast() == false){
array_list.add(res.getString(res.getColumnIndex(CONTACTS_COLUMN_NAME)));
res.moveToNext();
}
return array_list;
}
}
崩溃报告
01-23 20:33:10.124 19847-19847/com.varvaniweb.example.helloworld E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.varvaniweb.example.helloworld, PID: 19847
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.varvaniweb.example.helloworld/com.varvaniweb.example.helloworld.ActivityTwo}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5021)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:827)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:256)
at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
at com.varvaniweb.example.helloworld.DatabaseHelper.getAllContacts(DatabaseHelper.java:87)
at com.varvaniweb.example.helloworld.HandleXML.getAll(HandleXML.java:33)
at com.varvaniweb.example.helloworld.ActivityTwo.getFromXML(ActivityTwo.java:45)
at com.varvaniweb.example.helloworld.ActivityTwo.onCreate(ActivityTwo.java:29)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1090)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
at android.app.ActivityThread.access$800(ActivityThread.java:135)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:136)
at android.app.ActivityThread.main(ActivityThread.java:5021)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:827)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:643)
at dalvik.system.NativeStart.main(Native Method)
最佳答案
问题是您正在自己创建一个 Activity (HandleXML) 并将其用作 DatabaseHelper< 所需的 :context/
handleXML = new HandleXML("http://examplewebsite/file.xml");
HandleXML 内部:
public HandleXML(String url) {
helper = new DatabaseHelper(this); // <-- PROBLEM: this does not have Context info
this.urlString = url;
}
您不应该自己实例化应用程序组件。系统有责任这样做——作为这样做的一部分，系统会附加有关应用程序及其代表您创建的组件的信息。因为您自己创建了它，所以它没有打开数据库(或执行任何操作，实际上)所需的Context信息。
更改您的 HandleXML 类，使其不扩展 Activity，而是为其提供一个 Context 来传递给 DatabaseHelper:
public class HandleXML {
...
public HandleXML(Context context, String url) {
helper = new DatabaseHelper(context);
this.urlString = url;
}
...
}
在您的 ActivityTwo 中调用它，如下所示:
handleXML = new HandleXML(this, "http://examplewebsite/file.xml");
关于java - Android 数据库崩溃，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/28121443/