Android getWritableDatabase（）抛出NullPointerException

调用getWritableDatabase（）时遇到问题。它正在返回NullPointerException。最奇怪的是，在我注释掉android清单中的几行代码后，这个错误才开始出现。现在项目回到了原来的位置（我取消了对这些部分的注释，它仍然抛出一个NullPointerException）。所以我检查了我的上下文并尝试了这个.getBaseContext（）和这个.getApplicationContext（）

使用getApplicationContext（）可以将以下内容输出到logcat：

08-12 09:24:20.042: E/AndroidRuntime(5572): FATAL EXCEPTION: main
08-12 09:24:20.042: E/AndroidRuntime(5572): java.lang.RuntimeException: Unable to instantiate service com.productivity.uptimeapp.NetService: java.lang.NullPointerException
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2395)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.access$1600(ActivityThread.java:139)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1313)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.os.Looper.loop(Looper.java:137)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.main(ActivityThread.java:5021)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.reflect.Method.invokeNative(Native Method)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.reflect.Method.invoke(Method.java:511)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at dalvik.system.NativeStart.main(Native Method)
08-12 09:24:20.042: E/AndroidRuntime(5572): Caused by: java.lang.NullPointerException
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:101)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at com.productivity.uptimeapp.NetService.<init>(NetService.java:45)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.Class.newInstanceImpl(Native Method)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.Class.newInstance(Class.java:1319)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2392)
08-12 09:24:20.042: E/AndroidRuntime(5572):     ... 10 more

08-12 09:28:57.058: E/AndroidRuntime(5833): FATAL EXCEPTION: main
08-12 09:28:57.058: E/AndroidRuntime(5833): java.lang.RuntimeException: Unable to instantiate service com.productivity.uptimeapp.NetService: java.lang.NullPointerException
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2395)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.access$1600(ActivityThread.java:139)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1313)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.os.Looper.loop(Looper.java:137)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.main(ActivityThread.java:5021)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.reflect.Method.invokeNative(Native Method)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.reflect.Method.invoke(Method.java:511)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at dalvik.system.NativeStart.main(Native Method)
08-12 09:28:57.058: E/AndroidRuntime(5833): Caused by: java.lang.NullPointerException
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at com.productivity.uptimeapp.NetService.<init>(NetService.java:46)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.Class.newInstanceImpl(Native Method)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.Class.newInstance(Class.java:1319)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2392)
08-12 09:28:57.058: E/AndroidRuntime(5833):     ... 10 more

08-12 09:24:20.042:E/AndroidRuntime（5572）：致命异常：主
08-12 09:24:20.042:E/AndroidRuntime（5572）：java.lang.RuntimeException:无法实例化服务com.productivity.uptimeapp.NetService:java.lang.NullPointerException
08-12 09:24:20.042:E/AndroidRuntime（5572）：在android.app.ActivityThread.handleCreateService（ActivityThread.java:2395）上
08-12 09:24:20.042:E/AndroidRuntime（5572）：在android.app.ActivityThread.access$1600（ActivityThread.java:139）
08-12 09:24:20.042:E/AndroidRuntime（5572）：在android.app.ActivityThread$H.handleMessage（ActivityThread.java:1313）
08-12 09:24:20.042:E/AndroidRuntime（5572）：在android.os.Handler.dispatchMessage（Handler.java:99）上
08-12 09:24:20.042:E/AndroidRuntime（5572）：在android.os.Looper.loop（Looper.java:137）上
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于android.app.ActivityThread.main（ActivityThread.java:5021）
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于java.lang.reflect.Method.Invokenactive（本机方法）
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于java.lang.reflect.Method.invoke（Method.java:511）
08-12 09:24:20.042:E/AndroidRuntime（5572）：在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run（ZygoteInit.java:789）
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于com.android.internal.os.ZygoteInit.main（ZygoteInit.java:556）
08-12 09:24:20.042:E/AndroidRuntime（5572）：在dalvik.system.NativeStart.main（本机方法）
08-12 09:24:20.042:E/AndroidRuntime（5572）：由以下原因引起：java.lang.NullPointerException
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于android.content.ContextWrapper.getApplicationContext（ContextWrapper.java:101）
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于com.productivity.uptimeapp.NetService.（NetService.java:45）
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于java.lang.Class.newInstanceImpl（本机方法）
08-12 09:24:20.042:E/AndroidRuntime（5572）：位于java.lang.Class.newInstance（Class.java:1319）
08-12 09:24:20.042:E/AndroidRuntime（5572）：在android.app.ActivityThread.handleCreateService（ActivityThread.java:2392）上
2008-12 09:24:20.042:E/AndroidRuntime（5572）：。。。10多

使用getBaseContext（）可以将以下内容输出到logcat：

08-12 09:24:20.042: E/AndroidRuntime(5572): FATAL EXCEPTION: main
08-12 09:24:20.042: E/AndroidRuntime(5572): java.lang.RuntimeException: Unable to instantiate service com.productivity.uptimeapp.NetService: java.lang.NullPointerException
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2395)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.access$1600(ActivityThread.java:139)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1313)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.os.Looper.loop(Looper.java:137)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.main(ActivityThread.java:5021)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.reflect.Method.invokeNative(Native Method)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.reflect.Method.invoke(Method.java:511)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at dalvik.system.NativeStart.main(Native Method)
08-12 09:24:20.042: E/AndroidRuntime(5572): Caused by: java.lang.NullPointerException
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:101)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at com.productivity.uptimeapp.NetService.<init>(NetService.java:45)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.Class.newInstanceImpl(Native Method)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at java.lang.Class.newInstance(Class.java:1319)
08-12 09:24:20.042: E/AndroidRuntime(5572):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2392)
08-12 09:24:20.042: E/AndroidRuntime(5572):     ... 10 more

08-12 09:28:57.058: E/AndroidRuntime(5833): FATAL EXCEPTION: main
08-12 09:28:57.058: E/AndroidRuntime(5833): java.lang.RuntimeException: Unable to instantiate service com.productivity.uptimeapp.NetService: java.lang.NullPointerException
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2395)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.access$1600(ActivityThread.java:139)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1313)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.os.Handler.dispatchMessage(Handler.java:99)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.os.Looper.loop(Looper.java:137)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.main(ActivityThread.java:5021)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.reflect.Method.invokeNative(Native Method)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.reflect.Method.invoke(Method.java:511)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:789)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:556)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at dalvik.system.NativeStart.main(Native Method)
08-12 09:28:57.058: E/AndroidRuntime(5833): Caused by: java.lang.NullPointerException
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at com.productivity.uptimeapp.NetService.<init>(NetService.java:46)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.Class.newInstanceImpl(Native Method)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at java.lang.Class.newInstance(Class.java:1319)
08-12 09:28:57.058: E/AndroidRuntime(5833):     at android.app.ActivityThread.handleCreateService(ActivityThread.java:2392)
08-12 09:28:57.058: E/AndroidRuntime(5833):     ... 10 more

08-12 09:28:57.058:E/AndroidRuntime（5833）：致命异常：主
08-12 09:28:57.058:E/AndroidRuntime（5833）：java.lang.RuntimeException:无法实例化服务com.productivity.uptimeapp.NetService:java.lang.NullPointerException
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于android.app.ActivityThread.handleCreateService（ActivityThread.java:2395）
08-12 09:28:57.058:E/AndroidRuntime（5833）：在android.app.ActivityThread.access$1600（ActivityThread.java:139）
08-12 09:28:57.058:E/AndroidRuntime（5833）：在android.app.ActivityThread$H.handleMessage（ActivityThread.java:1313）
08-12 09:28:57.058:E/AndroidRuntime（5833）：在android.os.Handler.dispatchMessage（Handler.java:99）上
08-12 09:28:57.058:E/AndroidRuntime（5833）：在android.os.Looper.loop（Looper.java:137）上
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于android.app.ActivityThread.main（ActivityThread.java:5021）
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于java.lang.reflect.Method.Invokenactive（本机方法）
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于java.lang.reflect.Method.invoke（Method.java:511）
08-12 09:28:57.058:E/AndroidRuntime（5833）：在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run（ZygoteInit.java:789）
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于com.android.internal.os.ZygoteInit.main（ZygoteInit.java:556）
08-12 09:28:57.058:E/AndroidRuntime（5833）：在dalvik.system.NativeStart.main（本机方法）
08-12 09:28:57.058:E/AndroidRuntime（5833）：由以下原因引起：java.lang.NullPointerException
08-12 09:28:57.058:E/AndroidRuntime（5833）：在android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked（SQLiteOpenHelper.java:224）
08-12 09:28:57.058:E/AndroidRuntime（5833）：在android.database.sqlite.SQLiteOpenHelper.getWritableDatabase（SQLiteOpenHelper.java:164）
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于com.productivity.uptimeapp.NetService.（NetService.java:46）
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于java.lang.Class.newInstanceImpl（本机方法）
08-12 09:28:57.058:E/AndroidRuntime（5833）：位于java.lang.Class.newInstance（Class.java:1319）
08-12 09:28:57.058:E/AndroidRuntime（5833）：在android.app.ActivityThread.handleCreateService（ActivityThread.java:2392）
2008-12 09:28:57.058:E/AndroidRuntime（5833）：。。。10多

这是我的相关NetService代码。这是我第一次活动的名字：

package com.productivity.uptimeapp;

import java.io.BufferedReader;
import java.io.DataOutputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;
import java.util.Iterator;

import org.json.JSONException;
import org.json.JSONObject;
import org.json.JSONTokener;

import android.app.Service;
import android.content.Intent;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.os.AsyncTask;
import android.os.Binder;
import android.os.IBinder;
import android.util.Log;

public class NetService extends Service{

    private static final String TAG = NetService.class.getSimpleName();

    //variables for ServerGet
    ServerSocket serverSocket;
    private static Socket clientSocket;
    //server port
    private static final int PORT = 8888;
    private static InputStreamReader isReader;
    private static DataOutputStream dos;
    private static PrintWriter printWriter;
    private static BufferedReader bReader;
    private String mess;
    //...end ServerGet variables

    ServerGet myFeed;
    Client out;

    DbHelper dbHelper = new DbHelper(this.getBaseContext()); 
    SQLiteDatabase db = dbHelper.getWritableDatabase();

    private final IBinder iBinder = new LocalBinder();

    public class LocalBinder extends Binder {
        public NetService getService() {
            // Return this instance of LocalService so clients can call public methods
            return NetService.this;
        }
    }

    @Override
    public IBinder onBind(Intent arg0) {
        //creates new AsyncTask
        myFeed = new ServerGet();
        //starts new AsyncTask
        myFeed.execute();

        return iBinder;

    }

    //ServerGet class asynchronously gets new strings
    private class ServerGet extends AsyncTask <Void, String, Integer>{
         //This contains my Server which gets messages from a client and responds accordingly   
    }
}

package com.productivity.uptimeapp；
导入java.io.BufferedReader；
导入java.io.DataOutputStream；
导入java.io.IOException；
导入java.io.InputStreamReader；
导入java.io.PrintWriter；
导入java.net.ServerSocket；
导入java.net.Socket；
导入java.net.SocketException；
导入java.util.Iterator；
导入org.json.JSONException；
导入org.json.JSONObject；
导入org.json.JSONTokener；
导入android.app.Service；
导入android.content.Intent；
导入android.database.SQLException；
导入android.database.sqlite.SQLiteDatabase；
导入android.os.AsyncTask；
导入android.os.Binder；
导入android.os.IBinder；
导入android.util.Log；
公共类NetService扩展服务{
私有静态最终字符串标记=NetService.class.getSimpleName（）；
//ServerGet的变量
服务器套接字服务器套接字；
DbHelper dbHelper = new DbHelper(this.getBaseContext()); 
SQLiteDatabase db = dbHelper.getWritableDatabase();