从Internet下载SQLite数据库并加载到Android应用程序中

Question

对于我的Android应用程序，我想使用一个大型数据库（大约45 MB）。

一种解决方案是将（拆分的）数据库包含在assets文件夹中，并在第一次启动时将其复制到数据库目录。

但是这会占用磁盘空间两次 - 一次在资产文件夹中，文件无法删除，一次在数据库目录中，它已被复制到。

所以我宁愿在第一次启动时从Internet（webserver）下载数据库。我怎么能这样做？我可以下载完整的SQLite文件并将其保存到数据库目录中吗？或者我应该使用用于填充数据库的JSON数据文件？

Answer 1

  

一种解决方案是将（拆分的）数据库包含在assets文件夹中，并在第一次启动时将其复制到数据库目录。

它不必拆分，只是拉链。有关示例，请参阅SQLiteAssetHelper。

  

我怎么能这样做？

使用HttpUrlConnection。或者，使用HttpClient。

  

我可以下载完整的SQLite文件并将其保存到数据库目录吗？

是。使用getDatabasePath()获取正确的本地路径。

  

或者我应该使用用于填充数据库的JSON数据文件？

你可以，但对于45MB，这将是非常缓慢的。

Answer 2

被问到我最终得到了什么解决方案，这是我使用的代码（大致）。它可能不再完整，也不优雅或干净。但也许这对你有帮助。

<强> MyActivity.java

public class MyActivity extends Activity {

    private static final String SD_CARD_FOLDER = "MyApp";
    private static final String DB_DOWNLOAD_PATH = "http://www.example.org/downloads/dictionary.sqlite";
    private Database mDB = null;
    private DatabaseDownloadTask mDatabaseDownloadTask = null;
    private DatabaseOpenTask mDatabaseOpenTask = null;

    private class DatabaseDownloadTask extends AsyncTask<Context, Integer, Boolean> {

        @Override
        protected void onPreExecute() {
            mProgressDialog = new ProgressDialog(MyActivity.this);
            mProgressDialog.setTitle(getString(R.string.please_wait));
            mProgressDialog.setMessage(getString(R.string.downloading_database));
            mProgressDialog.setIndeterminate(false);
            mProgressDialog.setMax(100);
            mProgressDialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);
            mProgressDialog.setCancelable(false);
            mProgressDialog.show();
            getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
        }

        @Override
        protected Boolean doInBackground(Context... params) {
            try {
                File dbDownloadPath = new File(Database.getDatabaseFolder());
                if (!dbDownloadPath.exists()) {
                    dbDownloadPath.mkdirs();
                }
                HttpParams httpParameters = new BasicHttpParams();
                HttpConnectionParams.setConnectionTimeout(httpParameters, 5000);
                HttpConnectionParams.setSoTimeout(httpParameters, 5000);
                DefaultHttpClient client = new DefaultHttpClient(httpParameters);
                HttpGet httpGet = new HttpGet(DB_DOWNLOAD_PATH);
                InputStream content = null;
                try {
                    HttpResponse execute = client.execute(httpGet);
                    if (execute.getStatusLine().getStatusCode() != 200) { return null; }
                    content = execute.getEntity().getContent();
                    long downloadSize = execute.getEntity().getContentLength();
                    FileOutputStream fos = new FileOutputStream(Database.getDatabaseFolder()+Database.DATABASE_NAME+".sqlite");
                    byte[] buffer = new byte[256];
                    int read;
                    long downloadedAlready = 0;
                    while ((read = content.read(buffer)) != -1) {
                        fos.write(buffer, 0, read);
                        downloadedAlready += read;
                        publishProgress((int) (downloadedAlready*100/downloadSize));
                    }
                    fos.flush();
                    fos.close();
                    content.close();
                    return true;
                }
                catch (Exception e) {
                    if (content != null) {
                        try {
                            content.close();
                        }
                        catch (IOException e1) {}
                    }
                    return false;
                }
            }
            catch (Exception e) {
                return false;
            }
        }

        protected void onProgressUpdate(Integer... values) {
            if (mProgressDialog != null) {
                if (mProgressDialog.isShowing()) {
                    mProgressDialog.setProgress(values[0]);
                }
            }
        }

        @Override
        protected void onPostExecute(Boolean result) {
            if (mProgressDialog != null) {
                mProgressDialog.dismiss();
                mProgressDialog = null;
            }
            if (result.equals(Boolean.TRUE)) {
                Toast.makeText(MyActivity.this, getString(R.string.database_download_success), Toast.LENGTH_LONG).show();
                mDatabaseOpenTask = new DatabaseOpenTask();
                mDatabaseOpenTask.execute(new Context[] { MyActivity.this });
            }
            else {
                Toast.makeText(getApplicationContext(), getString(R.string.database_download_fail), Toast.LENGTH_LONG).show();
                finish();
            }
        }

    }

    private class DatabaseOpenTask extends AsyncTask<Context, Void, Database> {

        @Override
        protected Database doInBackground(Context ... ctx) {
            try {
                String externalBaseDir = Environment.getExternalStorageDirectory().getAbsolutePath();
                // DELETE OLD DATABASE ANFANG
                File oldFolder = new File(Environment.getExternalStorageDirectory().getAbsolutePath()+"/"+SD_CARD_FOLDER);
                File oldFile = new File(oldFolder, "dictionary.sqlite");
                if (oldFile.exists()) {
                    oldFile.delete();
                }
                if (oldFolder.exists()) {
                    oldFolder.delete();
                }
                // DELETE OLD DATABASE ENDE
                File newDB = new File(Database.getDatabaseFolder()+"dictionary.sqlite");
                if (newDB.exists()) {
                    return new Database(ctx[0]);
                }
                else {
                    return null;
                }
            }
            catch (Exception e) {
                return null;
            }
        }

        @Override
        protected void onPreExecute() {
            mProgressDialog = ProgressDialog.show(MainActivity.this, getString(R.string.please_wait), "Loading the database! This may take some time ...", true);
        }

        @Override
        protected void onPostExecute(Database newDB) {
            if (mProgressDialog != null) {
                mProgressDialog.dismiss();
                mProgressDialog = null;
            }
            if (newDB == null) {
                mDB = null;
                AlertDialog.Builder downloadDatabase = new AlertDialog.Builder(MyActivity.this);
                downloadDatabase.setTitle(getString(R.string.downloadDatabase));
                downloadDatabase.setCancelable(false);
                downloadDatabase.setMessage(getString(R.string.wantToDownloadDatabaseNow));
                downloadDatabase.setPositiveButton(getString(R.string.download), new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.dismiss();
                        mDatabaseDownloadTask = new DatabaseDownloadTask();
                        mDatabaseDownloadTask.execute();
                    }
                });
                downloadDatabase.setNegativeButton(getString(R.string.cancel), new DialogInterface.OnClickListener() {
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.dismiss();
                        finish();
                    }
                });
                downloadDatabase.show();
            }
            else {
                mDB = newDB;
            }
        }
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        if (mDatabaseDownloadTask != null) {
            if (mDatabaseDownloadTask.getStatus() != AsyncTask.Status.FINISHED) {
                mDatabaseDownloadTask.cancel(true);
            }
        }
        if (mDatabaseOpenTask != null) {
            if (mDatabaseOpenTask.getStatus() != AsyncTask.Status.FINISHED) {
                mDatabaseOpenTask.cancel(true);
            }
        }
        if (mProgressDialog != null) {
            mProgressDialog.dismiss();
            mProgressDialog = null;
        }
        if (mDB != null) {
            mDB.close();
            mDB = null;
        }
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        if (!Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED)) {
            Toast.makeText(getApplicationContext(), getString(R.string.sd_card_not_found), Toast.LENGTH_LONG).show();
            finish();
        }
        mDatabaseOpenTask = new DatabaseOpenTask();
        mDatabaseOpenTask.execute(new Context[] { this });
    }

}

<强> Database.java

public class Database扩展了SQLiteOpenHelper {

private static final String DATABASE_NAME = "dictionary";
private String DATABASE_PATH = null;
private static final int DATABASE_VERSION = 1;
private static final String PACKAGE_NAME = "com.my.package";
private SQLiteDatabase db;

public Database(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
    DATABASE_PATH = getDatabaseFolder()+DATABASE_NAME+".sqlite";
    db = getWritableDatabase();
}

public static String getDatabaseFolder() {
    return Environment.getExternalStorageDirectory().getAbsolutePath()+"/Android/data/"+PACKAGE_NAME+"/databases/";
}

@Override
public synchronized SQLiteDatabase getWritableDatabase() {
    try {
        if (db != null) {
            if (db.isOpen()) {
                return db;
            }
        }
        return SQLiteDatabase.openDatabase(DATABASE_PATH, null, SQLiteDatabase.OPEN_READWRITE | SQLiteDatabase.NO_LOCALIZED_COLLATORS);
    }
    catch (Exception e) {
        return null;
    }
}

@Override
public synchronized void close() {
     if (db != null) {
         db.close();
         db = null;
     }
     super.close();
}

@Override
public void onCreate(SQLiteDatabase db) { }

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { }

}

Answer 3

我认为如果您的数据库那么大，JSON方法会更好。

我不是100％肯定，但我相信当您发布应用程序更新时，您的设备将下载整个应用程序。如果您将45mb文件与应用程序捆绑在一起，这意味着每次推送更新时，您的用户将无法下载45mb文件。不是个好主意。

您可以做的是包含结构合理的数据库，而您的应用程序中没有数据。当用户打开应用程序时，它可以连接到您的Web服务器并获取JSON数据来填充数据库。这样，当用户更新您的应用程序时，他们不会因为下载新的大文件而陷入困境。更新不会消灭现有数据库。

您甚至可以通过JSON来获取并获取数据库的一部分，直到用户拥有所有内容。这样一来，如果你正在进行怪物查询并且他们失去了与互联网的连接，那么就不会发生太糟糕的事情。