getWritableDatabase()和getReadableDatabase()方法区别

Android使用getWritableDatabase()和getReadableDatabase()方法都可以获取一个用于操作数据库的SQLiteDatabase实例。

(getReadableDatabase()方法中会调用getWritableDatabase()方法)。

其中getWritableDatabase() 方法以读写方式打开数据库，一旦数据库的磁盘空间满了，数据库就只能读而不能写，倘若使用的是getWritableDatabase() 方法就会出错。所以在平时使用时多使用getWritableDatabase()
。

getReadableDatabase()方法则是先以读写方式打开数据库，如果数据库的磁盘空间满了，就会打开失败，当打开失败后会继续尝试以只读方式打开数据库。如果该问题成功解决，则只读数据库对象就会关闭，然后返回一个可读写的数据库对象。

/**

     * 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();

        }

    }