Android数据库更新机制

Android源码：

1、通过下面的构造方法传入新的版本号

/**     * Create a helper object to create, open, and/or manage a database.     * This method always returns very quickly.  The database is not actually     * created or opened until one of {@link #getWritableDatabase} or     * {@link #getReadableDatabase} is called.     *     * @param context to use to open or create the database     * @param name of the database file, or null for an in-memory database     * @param factory to use for creating cursor objects, or null for the default     * @param version number of the database (starting at 1); if the database is older,     *     {@link #onUpgrade} will be used to upgrade the database; if the database is     *     newer, {@link #onDowngrade} will be used to downgrade the database     */    public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version) {        this(context, name, factory, version, null);    }    /**     * Create a helper object to create, open, and/or manage a database.     * The database is not actually created or opened until one of     * {@link #getWritableDatabase} or {@link #getReadableDatabase} is called.     *     * <p>Accepts input param: a concrete instance of {@link DatabaseErrorHandler} to be     * used to handle corruption when sqlite reports database corruption.</p>     *     * @param context to use to open or create the database     * @param name of the database file, or null for an in-memory database     * @param factory to use for creating cursor objects, or null for the default     * @param version number of the database (starting at 1); if the database is older,     *     {@link #onUpgrade} will be used to upgrade the database; if the database is     *     newer, {@link #onDowngrade} will be used to downgrade the database     * @param errorHandler the {@link DatabaseErrorHandler} to be used when sqlite reports database     * corruption, or null to use the default error handler.     */    public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version,            DatabaseErrorHandler errorHandler) {        if (version < 1) throw new IllegalArgumentException("Version must be >= 1, was " + version);        mContext = context;        mName = name;        mFactory = factory;        mNewVersion = version;        mErrorHandler = errorHandler;    }

2、当得到数据的读写对象时，进行当前版本号与前版本号的比较，以此确定执行sqlitehelper的oncreate或者onUpgrade或者onDowngrade

public SQLiteDatabase getWritableDatabase() {        synchronized (this) {            return getDatabaseLocked(true);        }    }public SQLiteDatabase getReadableDatabase() {        synchronized (this) {            return getDatabaseLocked(false);        }    }    private SQLiteDatabase getDatabaseLocked(boolean writable) {        if (mDatabase != null) {            if (!mDatabase.isOpen()) {                // Darn!  The user closed the database by calling mDatabase.close().                mDatabase = null;            } else if (!writable || !mDatabase.isReadOnly()) {                // The database is already open for business.                return mDatabase;            }        }        if (mIsInitializing) {            throw new IllegalStateException("getDatabase called recursively");        }        SQLiteDatabase db = mDatabase;        try {            mIsInitializing = true;            if (db != null) {                if (writable && db.isReadOnly()) {                    db.reopenReadWrite();                }            } else if (mName == null) {                db = SQLiteDatabase.create(null);            } else {                try {                    if (DEBUG_STRICT_READONLY && !writable) {                        final String path = mContext.getDatabasePath(mName).getPath();                        db = SQLiteDatabase.openDatabase(path, mFactory,                                SQLiteDatabase.OPEN_READONLY, mErrorHandler);                    } else {                        db = mContext.openOrCreateDatabase(mName, mEnableWriteAheadLogging ?                                Context.MODE_ENABLE_WRITE_AHEAD_LOGGING : 0,                                mFactory, mErrorHandler);                    }                } catch (SQLiteException ex) {                    if (writable) {                        throw ex;                    }                    Log.e(TAG, "Couldn't open " + mName                            + " for writing (will try read-only):", ex);                    final String path = mContext.getDatabasePath(mName).getPath();                    db = SQLiteDatabase.openDatabase(path, mFactory,                            SQLiteDatabase.OPEN_READONLY, mErrorHandler);                }            }            onConfigure(db);            final int version = db.getVersion();            if (version != mNewVersion) {                if (db.isReadOnly()) {                    throw new SQLiteException("Can't upgrade read-only database from version " +                            db.getVersion() + " to " + mNewVersion + ": " + mName);                }                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);            if (db.isReadOnly()) {                Log.w(TAG, "Opened " + mName + " in read-only mode");            }            mDatabase = db;            return db;        } finally {            mIsInitializing = false;            if (db != null && db != mDatabase) {                db.close();            }        }    }

数据库的更新机制似乎到此完结了。但是大家不晓得有没有注意到db.getVersion(); 我们把应用缓存数据库文件导出来会发现，并没有version之类的字段，那么数据库的版本又是通过什么标示的呢？

继续查看源码：

 /**     * Gets the database version.     *     * @return the database version     */    public int getVersion() {        return ((Long) DatabaseUtils.longForQuery(this, "PRAGMA user_version;", null)).intValue();    }

而PRAGMA 详细可参见 http://hi.baidu.com/dushm/item/6d8cc8d6faf1111e21e25026  是sqlite的一条编译指令

到此为止！