Question

我创建了一个应用程序，将某些数据存储在数据库中（SQLite），但我遇到的问题是，在存储数据时，我得到NullPointerException，（Logcat附加）。我面临的问题是SQLiteDatabse db=this.getWritableDatabase();

我已经搜索了几乎所有的解决方案，但没有人帮忙，如果有人能提供帮助，那将是一个很大的帮助。

由于

package com.example.neuronsdk;

import android.content.ContentValues;
import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;


public class DatabaseHandler extends SQLiteOpenHelper {
	
	private static final int DATABASE_VERSION = 2;
	private static final String DATABASE_NAME = "NeuronDatabase";
	
	private static final String TABLE_EVENTS = "Events";
	private static final String TAG = "DatabaseHandler.class";
	private static final String KEY_ID = "id";
	private static final String KEY_EVENTS = "eventsText";
	private static final String KEY_START_TIME="start_time";
	private static final String KEY_END_TIME="end_time";
	
	String url="www.neuronme.com/android_sdk_data/";
	
	SQLiteDatabase dbHelper=null;
	DatabaseHandler dbh;
	private Context context;
	
	
	
	
	
	private static final String NEURON_DATABASE ="CREATE TABLE " + TABLE_EVENTS + "("   +
																KEY_EVENTS + " TEXT NOT NULL, " + KEY_START_TIME + " INTEGER," +  
																KEY_END_TIME + " INTEGER );";					
	public DatabaseHandler(Context context) {
		super(context, DATABASE_NAME, null, DATABASE_VERSION);
		this.context=context;
	}
 
	@Override
	public void onCreate(SQLiteDatabase db) {
		// TODO Auto-generated method stub
		db.execSQL(NEURON_DATABASE);
		Log.d(TAG, "Database Created");	
		}

	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
		// TODO Auto-generated method stub
		 db.execSQL("DROP TABLE IF EXISTS " + TABLE_EVENTS);
		 this.onCreate(db);
	}
	
	 public void open() throws SQLException {
		// close();
		//dbh=new DatabaseHandler(this.context);
		dbHelper = this.getWritableDatabase();
	    }
	
	public void addEvent(String eventText){
		ContentValues values = new ContentValues();
		Log.d("Database", "Adding value in database"+ eventText);
		open();
		if(eventText != null)
		{
		values.put(KEY_EVENTS, eventText);
		dbHelper.insert(TABLE_EVENTS, null, values);
		close();
		}
	}
	
	public void addStartTime(Long startTime){
		ContentValues values = new ContentValues();
		Log.d("Database", "Adding start time in database"+ startTime);
		close();
		//dbHelper=this.getWritableDatabase();
		values.put(KEY_START_TIME, startTime);
		dbHelper.insert(TABLE_EVENTS, null, values);	
		close();
	}
	
	public void addEndTime(Long endTime){
		ContentValues values = new ContentValues();
		Log.d("Database", "Adding End time in database"+ endTime);
		close();
	//	dbHelper=getWritableDatabase();
		values.put(KEY_END_TIME, endTime);
		dbHelper.insert(TABLE_EVENTS, null, values);
		close();
		}
}

Logcat的错误报告是：

07-16 07:08:32.435: E/AndroidRuntime(1869): FATAL EXCEPTION: main
07-16 07:08:32.435: E/AndroidRuntime(1869): Process: com.example.test_application, PID: 1869
07-16 07:08:32.435: E/AndroidRuntime(1869): java.lang.NullPointerException
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.neuronsdk.DatabaseHandler.open(DatabaseHandler.java:59)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.neuronsdk.DatabaseHandler.addEvent(DatabaseHandler.java:72)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.neuronsdk.NeuronActivity.onClick(NeuronActivity.java:28)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.example.test_application.MainActivity$1.onClick(MainActivity.java:47)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.view.View.performClick(View.java:4438)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.view.View$PerformClick.run(View.java:18422)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.os.Handler.handleCallback(Handler.java:733)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.os.Handler.dispatchMessage(Handler.java:95)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.os.Looper.loop(Looper.java:136)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at android.app.ActivityThread.main(ActivityThread.java:5017)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at java.lang.reflect.Method.invokeNative(Native Method)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at java.lang.reflect.Method.invoke(Method.java:515)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-16 07:08:32.435: E/AndroidRuntime(1869):     at dalvik.system.NativeStart.main(Native Method)

Answer 1

您正在null传递Context：

Context context = null;
//this.context=context;
dbh=new DatabaseHandler(context);

您需要传递有效Context，例如Application或Activity。

Answer 2

首先在构造函数中指定上下文值

public DatabaseHandler(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    this.context=context;
}

然后在你的open方法中替换

dbh=new DatabaseHandler(context);

与

dbh=new DatabaseHandler(this.context);

Answer 3

将构造函数更改为：

template <typename T>
void func_1(std::initializer_list<T>&& a) {
    // do something
} 

template <typename... T>
void func(T&&... a) {
    func_1({std::forward<T>(a)...});
} 

int main() {
    func(1, 2, 3);
    // func(1, 2, 3, 4.0); // OK doesn't compile
}

将open（）方法更改为：

public DatabaseHandler(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    this.context = context;
}

实施closeDB（）方法

public void open() throws SQLException {
         close();
       this.getWritableDatabase();
    }

使用应用程序上下文并使用已存在的实例。也就是说，不是调用构造函数，而是调用实例。见下文：

public void closeDB() {
    SQLiteDatabase db = this.getReadableDatabase();
    if (db != null && db.isOpen())
        db.close();
}

android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked（SQLiteOpenHelper.java:224）中的java.lang.NullPointerException

3 个答案: