Android之SQLite开发（2）—SQLiteOpenHelper类

SQLiteOpenHelper是Android提供的一个管理数据库的工具类，可用于管理数据库的创建和版本更新。SQLiteOpenHelper是一个抽象类，所以我们想要使用它，就必须创建它的子类，并且拓展它的onCreate(SQLiteDatabase db)和onUpdate(SQLiteDatabase db,int oldVersion,int newVersion)方法。

SQLiteOpenHelper包含如下常用方法：

构造函数

/**
     * 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;
    }

 onCreate(SQLiteDatabase db)

<span style="font-family:SimSun;">/**
     * Called when the database is created for the first time. This is where the
     * creation of tables and the initial population of the tables should happen.
     *
     * @param db The database.
     */
    public abstract void onCreate(SQLiteDatabase db);</span>

onUpdate(SQLiteDatabase db,int oldVersion,int newVersion)

<span style="font-size:18px;">  /**
     * Called when the database needs to be upgraded. The implementation
     * should use this method to drop tables, add tables, or do anything else it
     * needs to upgrade to the new schema version.
     *...</span><pre name="code" class="java"><span style="font-size:18px;">     *</span>...
     * @param db The database.
     * @param oldVersion The old database version.
     * @param newVersion The new database version.
     */
    public abstract void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion);

getWritableDatabase()

 <span style="font-size:18px;"> /**
     * 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.
     *......
     * @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);
        }
    }</span>

getReadableDatabase()

<span style="font-size:18px;">/**
     * 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 SQLiteDatabase getReadableDatabase() {
        synchronized (this) {
            return getDatabaseLocked(false);
        }
    }</span>

onClose()

<span style="font-family:SimSun;font-size:18px;">  /**
     * Close any open database object.
     */
    public synchronized void close() {
        if (mIsInitializing) throw new IllegalStateException("Closed during initialization");

        if (mDatabase != null && mDatabase.isOpen()) {
            mDatabase.close();
            mDatabase = null;
        }
    }</span>

onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion)

<span style="font-family:SimSun;font-size:18px;">   /**
     * Called when the database needs to be downgraded. This is strictly similar to
     * {@link #onUpgrade} method, but is called whenever current version is newer than requested one.
     * However, this method is not abstract, so it is not mandatory for a customer to
     * implement it. If not overridden, default implementation will reject downgrade and
     * throws SQLiteException
     *
     * <p>
     * This method executes within a transaction.  If an exception is thrown, all changes
     * will automatically be rolled back.
     * </p>
     *
     * @param db The database.
     * @param oldVersion The old database version.
     * @param newVersion The new database version.
     */
    public void onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        throw new SQLiteException("Can't downgrade database from version " +
                oldVersion + " to " + newVersion);
    }</span>

其中，这个类有两个构造函数，我们这只要看SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version,
            DatabaseErrorHandler errorHandler)就行了，

context用于打开或新建数据库的上下文，

name 数据库文件的名称，如果为空，那么使用的就是内存中的数据库了，

factory 用于新建cursor对象的，为空的话就是用默认的，

version 数据库的版本号（从1开始），如果当前的数据库是旧的，那么会调用onUpgrade来更新数据库，反之，会调用onDowngrade方法来回退数据库

errorHandler 当sqlite记录数据库损毁会使用它，一般设为null用默认的。

onCreate(SQLiteDatabase db)：只在数据库第一次创建时才会被调用，通常在这个方法里面会做一些建表和数据初始化的操作。

onUpdate(SQLiteDatabase db,int oldVersion,int newVersion)：当数据库需要升级的时候会调用这个方法，在这个方法中，应该做一些数据库表结构的更新操作。oldVersion代表数据库之前的版本号，newVersion代表当前数据库的版本号。那么在哪里指定数据库的版本号呢？当我们创建SQLiteOpenHelper对象时，必须指定一个version参数，该参数就决定了所使用的数据库的版本——也就是说，数据库的版本是由我们自己来控制的，只要某次创建SQLiteOpenHelper对象时指定的版本号高于之前的版本号，就会触发这个方法。特别指出，这个方法是在一个事物中执行的，也就是说这方法执行的过程中一旦发生了异常，那么所有的改变都会自动的回滚。

onDowngrade(SQLiteDatabase db, int oldVersion, int newVersion) 当数据库需要降级的时候会调用。严格的来说，这个方法和onUpdate方法是一样的。不过它和onUpdate的区别在于：

1、onDowngrade 用于版本回退，也就是当newVersion小于oldVersion的时候会调用，

2、onDowngrade不是抽象方法，没有强制我们去实现它，默认的它会抛出SQLiteException异常。

3、onDowngrade要求的api版本最低是10，10之前是没有的。

getWritableDatabase()新建或者打开一个用于读写的数据库，第一次被调用的时候，数据库会被打开，并且onCreate，onUpgrade或onOpen（注：如果数据库已经被打开，就会调用这个方法，如果还没有被打开，调用的是onUpgrade或者onDowngrade）会被调用。

getReadableDatabase()新建或者打开一个数据库。如果没有问题的话，调用该方法得到的数据库和通过调用getWritableDatabase()方法来得到的数据库是一样的。但如果磁盘满了，得到的只是一个只读的数据库，倘若使用getWritableDatabase()打开数据库就会报错。

onClose()  关闭所有的数据库对象。但我们完成了数据库操作之后，必须去关闭数据库连接，否则会发生内存泄漏。

总的来说SQLiteOpenHelper类就这么些东西，挺简单的。

Android之Sqlite开发（1）—简介