編輯:關於Android編程
有時候為了需要,會將數據庫保存到外部存儲或者SD卡中(對於這種情況可以通過加密數據來避免數據被破解),比如一個應用支持多個數據,每個數據都需要有一個對應的數據庫,並且數據庫中的信息量特別大時,這顯然更應該將數據庫保存在外部存儲或者SD卡中,因為RAM的大小是有限的;其次在寫某些測試程序時將數據庫保存在SD卡更方便查看數據庫中的內容。
Android通過SQLiteOpenHelper創建數據庫時默認是將數據庫保存在'/data/data/應用程序名/databases'目錄下的,只需要在繼承SQLiteOpenHelper類的構造函數中傳入數據庫名稱就可以了,但如果將數據庫保存到指定的路徑下面,都需要通過重寫繼承SQLiteOpenHelper類的構造函數中的context,因為:在閱讀SQLiteOpenHelper.java的源碼時會發現:創建數據庫都是通過Context的openOrCreateDatabase方法實現的,如果我們需要在指定的路徑下創建數據庫,就需要寫一個類繼承Context,並復寫其openOrCreateDatabase方法,在openOrCreateDatabase方法中指定數據庫存儲的路徑即可,下面為類SQLiteOpenHelper中getWritableDatabase和getReadableDatabase方法的源碼,SQLiteOpenHelper就是通過這兩個方法來創建數據庫的。
/** * Create and/or open a database that will be used for reading and writing. * The first time this is called, the database will be opened and * {@link #onCreate}, {@link #onUpgrade} and/or {@link #onOpen} will be * called. * * <p>Once opened successfully, the database is cached, so you can * call this method every time you need to write to the database. * (Make sure to call {@link #close} when you no longer need the database.) * Errors such as bad permissions or a full disk may cause this method * to fail, but future attempts may succeed if the problem is fixed.</p> * * <p class="caution">Database upgrade may take a long time, you * should not call this method from the application main thread, including * from {@link android.content.ContentProvider#onCreate ContentProvider.onCreate()}. * * @throws SQLiteException if the database cannot be opened for writing * @return a read/write database object valid until {@link #close} is called */ public synchronized SQLiteDatabase getWritableDatabase() { if (mDatabase != null) { if (!mDatabase.isOpen()) { // darn! the user closed the database by calling mDatabase.close() mDatabase = null; } else if (!mDatabase.isReadOnly()) { return mDatabase; // The database is already open for business } } if (mIsInitializing) { throw new IllegalStateException("getWritableDatabase called recursively"); } // If we have a read-only database open, someone could be using it // (though they shouldn't), which would cause a lock to be held on // the file, and our attempts to open the database read-write would // fail waiting for the file lock. To prevent that, we acquire the // lock on the read-only database, which shuts out other users. boolean success = false; SQLiteDatabase db = null; if (mDatabase != null) mDatabase.lock(); try { mIsInitializing = true; if (mName == null) { db = SQLiteDatabase.create(null); } else { db = mContext.openOrCreateDatabase(mName, 0, mFactory, mErrorHandler); } int version = db.getVersion(); if (version != mNewVersion) { db.beginTransaction(); try { if (version == 0) { onCreate(db); } else { if (version > mNewVersion) { onDowngrade(db, version, mNewVersion); } else { onUpgrade(db, version, mNewVersion); } } db.setVersion(mNewVersion); db.setTransactionSuccessful(); } finally { db.endTransaction(); } } onOpen(db); success = true; return db; } finally { mIsInitializing = false; if (success) { if (mDatabase != null) { try { mDatabase.close(); } catch (Exception e) { } mDatabase.unlock(); } mDatabase = db; } else { if (mDatabase != null) mDatabase.unlock(); if (db != null) db.close(); } } } /** * Create and/or open a database. This will be the same object returned by * {@link #getWritableDatabase} unless some problem, such as a full disk, * requires the database to be opened read-only. In that case, a read-only * database object will be returned. If the problem is fixed, a future call * to {@link #getWritableDatabase} may succeed, in which case the read-only * database object will be closed and the read/write object will be returned * in the future. * * <p class="caution">Like {@link #getWritableDatabase}, this method may * take a long time to return, so you should not call it from the * application main thread, including from * {@link android.content.ContentProvider#onCreate ContentProvider.onCreate()}. * * @throws SQLiteException if the database cannot be opened * @return a database object valid until {@link #getWritableDatabase} * or {@link #close} is called. */ public synchronized SQLiteDatabase getReadableDatabase() { if (mDatabase != null) { if (!mDatabase.isOpen()) { // darn! the user closed the database by calling mDatabase.close() mDatabase = null; } else { return mDatabase; // The database is already open for business } } if (mIsInitializing) { throw new IllegalStateException("getReadableDatabase called recursively"); } try { return getWritableDatabase(); } catch (SQLiteException e) { if (mName == null) throw e; // Can't open a temp database read-only! Log.e(TAG, "Couldn't open " + mName + " for writing (will try read-only):", e); } SQLiteDatabase db = null; try { mIsInitializing = true; String path = mContext.getDatabasePath(mName).getPath(); db = SQLiteDatabase.openDatabase(path, mFactory, SQLiteDatabase.OPEN_READONLY, mErrorHandler); if (db.getVersion() != mNewVersion) { throw new SQLiteException("Can't upgrade read-only database from version " + db.getVersion() + " to " + mNewVersion + ": " + path); } onOpen(db); Log.w(TAG, "Opened " + mName + " in read-only mode"); mDatabase = db; return mDatabase; } finally { mIsInitializing = false; if (db != null && db != mDatabase) db.close(); } }
通過上面的分析可以寫出一個自定義的Context類,該類繼承Context即可,但由於Context中有除了openOrCreateDatabase方法以外的其它抽象函數,所以建議使用非抽象類ContextWrapper,該類繼承自Context,自定義的DatabaseContext類源碼如下:
public class DatabaseContext extends ContextWrapper { public DatabaseContext(Context context){ super( context ); } /** * 獲得數據庫路徑,如果不存在,則創建對象對象 * @param name * @param mode * @param factory */ @Override public File getDatabasePath(String name) { //判斷是否存在sd卡 boolean sdExist = android.os.Environment.MEDIA_MOUNTED.equals(android.os.Environment.getExternalStorageState()); if(!sdExist){//如果不存在, return null; }else{//如果存在 //獲取sd卡路徑 String dbDir= FileUtils.getFlashBPath(); dbDir += "DB";//數據庫所在目錄 String dbPath = dbDir+"/"+name;//數據庫路徑 //判斷目錄是否存在,不存在則創建該目錄 File dirFile = new File(dbDir); if(!dirFile.exists()){ dirFile.mkdirs(); } //數據庫文件是否創建成功 boolean isFileCreateSuccess = false; //判斷文件是否存在,不存在則創建該文件 File dbFile = new File(dbPath); if(!dbFile.exists()){ try { isFileCreateSuccess = dbFile.createNewFile();//創建文件 } catch (IOException e) { e.printStackTrace(); } }else{ isFileCreateSuccess = true; } //返回數據庫文件對象 if(isFileCreateSuccess){ return dbFile; }else{ return null; } } } /** * 重載這個方法,是用來打開SD卡上的數據庫的,android 2.3及以下會調用這個方法。 * * @param name * @param mode * @param factory */ @Override public SQLiteDatabase openOrCreateDatabase(String name, int mode, SQLiteDatabase.CursorFactory factory) { SQLiteDatabase result = SQLiteDatabase.openOrCreateDatabase(getDatabasePath(name), null); return result; } /** * Android 4.0會調用此方法獲取數據庫。 * * @see android.content.ContextWrapper#openOrCreateDatabase(java.lang.String, int, * android.database.sqlite.SQLiteDatabase.CursorFactory, * android.database.DatabaseErrorHandler) * @param name * @param mode * @param factory * @param errorHandler */ @Override public SQLiteDatabase openOrCreateDatabase(String name, int mode, CursorFactory factory, DatabaseErrorHandler errorHandler) { SQLiteDatabase result = SQLiteDatabase.openOrCreateDatabase(getDatabasePath(name), null); return result; } }
在繼承SQLiteOpenHelper的子類的構造函數中,用DatabaseContext的實例替代context即可:
DatabaseContext dbContext = new DatabaseContext(context); super(dbContext, mDatabaseName, null, VERSION);
基於Android如何實現將數據庫保存到SD卡的全部內容就給大家介紹這麼多,同時也非常感謝大家一直以來對本站網站的支持,謝謝。
代理模式(Proxy)一、 什麼是代理模式先來看看官方的說法,代理模式就是為其他對象提供一種代理,以控制對這個對象的訪問。 看來這個官方的說法的確有點官方,看了還是讓
說起架構的話,稍微有點寫程序經驗的人來說,都可以理解架構對於整個服務的重要性。架構最核心的三個點就是:穩定性、擴展性、性能。一個好的架構主要通過這三點來看。會不會宕機,你
在本文當中,我將會與大家分享一個封裝了PopupWindow實現彈出菜單的類,並說明它的實現與使用。 因對界面的需求,android原生的彈出菜單已不能滿足我們的需求,自
Activities提供了一種方便管理的創建、保存、回復的對話框機制,例如 onCreateDialog(int), onPrepareDialog(int, Dialo