getWritableDatabase()和getReadableDatabase()方法区别
2014-12-20 15:29
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(1) getWritableDatabase();
(2)getReadableDatabase();
看看这两个方法官网的解释是怎样的:
所以打开一个数据库的最佳方法是:
(2)getReadableDatabase();
看看这两个方法官网的解释是怎样的:
<span style="font-size:24px;">/** * Create and/or open a database that will be used for reading and writing. * 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 it. * * <p>Errors such as bad permissions or a full disk may cause this operation * to fail, but future attempts may succeed if the problem is fixed.</p> * * @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 && mDatabase.isOpen() && !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); } int version = db.getVersion(); if (version != mNewVersion) { db.beginTransaction(); try { if (version == 0) { onCreate(db); } 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. * * @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 && mDatabase.isOpen()) { 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); 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(); } } </span>读读英文都知道(1)(2)方法都是返回一个可读可写的数据库【注意前提是磁盘空间不是满的】,如果磁盘空间是满的的话那就不一样了,如果磁盘是满的(1)方法就会抛出异常,(2)不会抛出异常而会放回一个只读的数据库,在未来的某一刻,如果磁盘的空间问题解决了,她就会返回一个刻度可写的数据库。
所以打开一个数据库的最佳方法是:
SQLiteDatabase db = null; try{ db = mySQLiteOpenHelper.getWritableDatabase(); }catch(SQLiteException se){ db = mySQLiteOpenHelper.getReadableDatabase(); }
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