Android SQLiteOpenHelper 的一些小小分析

  • Android,sqlite,数据库,

就目前而言,SQLiteOpenHelper 的 onCreate() 方法是在第一次调用getWritableDatabase() 方法的时候被调用。

写一个子类,继承于SQLiteOpenHelper,使用的时候调用他的构造方法。构造方法有两个。

public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version) {
    this(context, name, factory, version, new DefaultDatabaseErrorHandler());
}

public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version,
        DatabaseErrorHandler errorHandler) {
    if (version < 1) throw new IllegalArgumentException("Version must be >= 1, was " + version);
    if (errorHandler == null) {
        throw new IllegalArgumentException("DatabaseErrorHandler param value can't be null.");
    }

    mContext = context;
    mName = name;
    mFactory = factory;
    mNewVersion = version;
    mErrorHandler = errorHandler;
}

一般使用第一个。

下面是getWritableDatabase() 方法的源码

public synchronized SQLiteDatabase getWritableDatabase() {
    if (mDatabase != null) {
        if (!mDatabase.isOpen()) {
            // darn! the user closed the database by calling mDatabase.close()
            mDatabase = null;
        } else if (!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, mErrorHandler);
        }

        int version = db.getVersion();
        if (version != mNewVersion) {
            db.beginTransaction();
            try {
                if (version == 0) {
                    onCreate(db);
                } else {
                    if (version > mNewVersion) {
                        onDowngrade(db, version, mNewVersion);
                    } 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();
        }
    }
}

下面是getReadableDatabase()方法的源码

 public synchronized SQLiteDatabase getReadableDatabase() {
    if (mDatabase != null) {
        if (!mDatabase.isOpen()) {
            // darn! the user closed the database by calling mDatabase.close()
            mDatabase = null;
        } else {
            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,
                mErrorHandler);
        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();
    }
}

从上面的源码可以看出

1 。 这个类是对Context.openOrCreateDatabase() 的封装(Google官网本来就是那么说的,有点废话了)。

2 。 在大部分情况下,getReadableDatabase() 和 getWritableDatabase() 效果是一样的。返回的是同一个mDatabase对象。特殊情况是指内存空间不足,不能继续往数据库里写东西类似的情况。如果调用getWritableDatabase()方法,会抛出一个异常。如果调用getReadableDatabase()的话,会尝试返回一个只读的Database对象。当然还是可能抛异常。

3 。 onCreate()方法在第一次调用getWritableDatabase()方法的时候被调用。当然如果第一次调用getReadableDatabase()方法也会调用onCreate()方法。

- EOF -

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