getReadableDatabase() 和 getWritableDatabase()

原文地址：http://blog.csdn.net/alex_zhuang/article/details/7342840

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。

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。

源码如下：

 /**
      * 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();
         }
     }