我需要将一个SQLite数据库从assets /复制到app的数据库/目录中，但正如我将在下面给出的那样，我不断获得空指针异常。下面是一些可能相关的Logcat和代码：
[ 11-24 21:51:19.694 1652:0x67e E/AndroidRuntime ]
FATAL EXCEPTION: AsyncTask #1
java.lang.RuntimeException: An error occured while executing doInBackground()
at android.os.AsyncTask$3.done(AsyncTask.java:200)
at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:274)
at java.util.concurrent.FutureTask.setException(FutureTask.java:125)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:308)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1088)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:581)
at java.lang.Thread.run(Thread.java:1019)
Caused by: java.lang.NullPointerException
at net.xx.wildedibles.full.common.DataBaseHelper.<init>(DataBaseHelper.java:27)
at net.xx.wildedibles.full.common.DBAdapter.<init>(DBAdapter.java:21)
at net.xx.wildedibles.full.intro.LoadingFragment$LoadingTask.doInBackground(LoadingFragment.java:75)
at net.xx.wildedibles.full.intro.LoadingFragment$LoadingTask.doInBackground(LoadingFragment.java:68)
at android.os.AsyncTask$2.call(AsyncTask.java:185)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:306)
... 4 more
DatabaseHelper的第27行：
DB_PATH = context.getDatabasePath(DB_NAME).toString();
现在我知道有几种方法可以计算数据库的路径，而且我已经尝试了所有这些，但没有工作。仍然得到相同的NPE。
但是这里有点好奇：我决定做adb shell并查看应用程序的目录结构，以便更好地了解我出错的地方。这是导航到data / data / package.name /：
# cd net.xx.wildedibles.full
# ls
lib
#
为什么我看不到/ databases /，以及其他所有东西？当我进入lib /时，里面什么也没有。正如你所看到的，我在看的时候是根。鉴于此，我当然可以理解为什么我会得到一个空指针......
非常感谢您提供的任何建议！
根据要求，这是完整的代码：
这是一个DBAdapter类：
public class DBAdapter
{
protected static final String TAG = "DataAdapter";
private final Context mContext;
private SQLiteDatabase mDb;
private DataBaseHelper mDbHelper;
public DBAdapter(Context context)
{
mContext = context;
mDbHelper = new DataBaseHelper(mContext);
}
public DBAdapter createDatabase() throws SQLException
{
try
{
mDbHelper.createDataBase();
}
catch (IOException mIOException)
{
Log.e(TAG, mIOException.toString() + " UnableToCreateDatabase");
throw new Error("UnableToCreateDatabase");
}
return this;
}
public DBAdapter open() throws SQLException
{
try
{
mDbHelper.openDataBase();
mDbHelper.close();
mDb = mDbHelper.getReadableDatabase();
}
catch (SQLException mSQLException)
{
Log.e(TAG, "open >>"+ mSQLException.toString());
throw mSQLException;
}
return this;
}
public void close()
{
mDbHelper.close();
}
public Cursor getData(String query)
{
try
{
Cursor mCur = mDb.rawQuery(query, null);
if (mCur!=null)
{
mCur.moveToNext();
}
return mCur;
}
catch (SQLException mSQLException)
{
Log.e(TAG, "getData >>"+ mSQLException.toString());
throw mSQLException;
}
}
}
以下是整个Helper类：
public class DataBaseHelper extends SQLiteOpenHelper{
private static String TAG = "DataBaseHelper"; //tag for logcat
private static String DB_PATH;
private static String DB_NAME ="foraging.db";// Database name
private SQLiteDatabase mDataBase;
private final Context mContext;
public DataBaseHelper(Context context){
super(context, DB_NAME, null, 1);
if(android.os.Build.VERSION.SDK_INT >= 4.2){
DB_PATH = context.getDatabasePath(DB_NAME).toString();
}
else
{
DB_PATH = "/data/data/" + context.getPackageName() + "/databases/";
}
this.mContext = context;
}
public void onCreate(SQLiteDatabase db) {
}
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
//db.execSQL("DROP TABLE IF EXISTS" + DATABASE_TABLE);
onCreate(db);
}
public void createDataBase() throws IOException
{
//If database does not exist, copy from assets/
boolean mDataBaseExist = checkDataBase();
if(!mDataBaseExist)
{
this.getReadableDatabase();
this.close();
try {
//Copy the database from assests
copyDataBase();
Log.e(TAG, "createDatabase database created");
}
catch (IOException mIOException)
{
throw new Error("ErrorCopyingDataBase");
}
}
}
private boolean checkDataBase()
{
File dbFile = new File(DB_PATH + DB_NAME);
//Log.v("dbFile", dbFile + " "+ dbFile.exists());
return dbFile.exists();
}
//Copy the database from assets
private void copyDataBase() throws IOException
{
InputStream mInput = mContext.getAssets().open(DB_NAME);
String outFileName = DB_PATH + DB_NAME;
OutputStream mOutput = new FileOutputStream(outFileName);
byte[] mBuffer = new byte[1024];
int mLength;
while ((mLength = mInput.read(mBuffer))>0)
{
mOutput.write(mBuffer, 0, mLength);
}
mOutput.flush();
mOutput.close();
mInput.close();
}
public boolean openDataBase() throws SQLException
{
String mPath = DB_PATH + DB_NAME;
//Log.v("mPath", mPath);
mDataBase = SQLiteDatabase.openDatabase(mPath, null, SQLiteDatabase.CREATE_IF_NECESSARY);
//mDataBase = SQLiteDatabase.openDatabase(mPath, null, SQLiteDatabase.NO_LOCALIZED_COLLATORS);
return mDataBase != null;
}
@Override
public void close()
{
if(mDataBase != null)
mDataBase.close();
super.close();
}
}
答案 0 :(得分：0)
直接从the horse's mouth第一次致电getReadableDatabase()或getWriteableDatabase()，才会创建您的数据库：
public SQLiteDatabase getReadableDatabase（）
创建和/或打开数据库。 ...
如果您希望从您在assets文件夹中的预构建数据库中复制您创建的数据库（我喜欢这个想法，因为您不必在Java中使用SQL中的所有CREATE语句）我需要先调用getReadableDatabase()或getWriteableDatabase() 然后复制文件。可以在this answer中找到一个例子。
编辑：只是在您的编辑中看到您使用了我链接的相同代码。请注意，答案中的代码不会调用getDatabasePath，而是尝试手动创建路径，因为它知道它可能不存在。
getDatabasePath正在投掷NPE，因为正如its description中所述，它返回the absolute path on the filesystem where a database created。您的文件尚未创建！