getReadableDatebase() 和getwriteableDatebase()的解读

1. 两个方法干嘛的？
两个方法都是用于获取数据库的读写对象，并不是字面上一个获取读取数据库的对象，另一个获取写
数据库的对象。
2. getWritableDatabase()
源码注释：
/** * 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 SQLiteDatabase getWritableDatabase() {    synchronized (this) {        return getDatabaseLocked(true);    }}
它会调用并返回一个可以读写数据库的对象
在第一次调用时会调用onCreate的方法
当数据库存在时会调用onOpen方法
结束时调用onClose方法
3. getReadableDatabase()
源码注释：
/** * 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 SQLiteDatabase getReadableDatabase() {    synchronized (this) {        return getDatabaseLocked(false);    }}
它会调用并返回一个可以读写数据库的对象
在第一次调用时会调用onCreate的方法
当数据库存在时会调用onOpen方法
结束时调用onClose方法
4. 区别
是不是上面两个总结一样？
然后事实呢？
两个方法都是返回读写数据库的对象，但是当磁盘已经满了时，getWritableDatabase会抛异常，而getReadableDatabase不会报错，它此时不会返回读写数据库的对象，而是仅仅返回一个读数据库的对象。
getReadableDatabase会在问题修复后继续返回一个读写的数据库对象。
两者都是数据库操作，可能存在延迟等待，所以尽量不要在主线程中调用。