Android getReadableDatabase() 和 getWritableDatabase()分析对比

Android getReadableDatabase() 和 getWritableDatabase()分析对比

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();
    }
  }

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