SQLiteOpenHelper 源码

package com.tongyan.common.db;

/**
 * Copyright (C) 2007 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */


import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteException;
import android.util.Log;

/***
 * A helper class to manage database creation and version management.
 * You create a subclass implementing {@link #onCreate}, {@link #onUpgrade} and
 * optionally {@link #onOpen}, and this class takes care of opening the database
 * if it exists, creating it if it does not, and upgrading it as necessary.
 * Transactions are used to make sure the database is always in a sensible state.
 * <p>For an example, see the NotePadProvider class in the NotePad sample application,
 * in the <em>samples/</em> directory of the SDK.</p>
 */
public abstract class SQLiteOpenHelper {
    private static final String TAG = SQLiteOpenHelper.class.getSimpleName();

    private final Context mContext;
    private final String mName;
    private final CursorFactory mFactory;
    private final int mNewVersion;

    private SQLiteDatabase mDatabase = null;
    private boolean mIsInitializing = false;

    /***
     * 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.
     *
     * @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
     */
    public SQLiteOpenHelper(Context context, String name, CursorFactory factory, int version) {
        if (version < 1) throw new IllegalArgumentException("Version must be >= 1, was " + version);

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

    /***
     * 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();
        }
    }

    /***
     * 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;
        }
    }

    /***
     * 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);

    /***
     * 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.
     *
     * <p>The SQLite ALTER TABLE documentation can be found
     * <a href="http://sqlite.org/lang_altertable.html">here</a>. If you add new columns
     * you can use ALTER TABLE to insert them into a live table. If you rename or remove columns
     * you can use ALTER TABLE to rename the old table, then create the new table and then
     * populate the new table with the contents of the old table.
     *
     * @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);

    /***
     * Called when the database has been opened.
     * Override method should check {@link SQLiteDatabase#isReadOnly} before
     * updating the database.
     *
     * @param db The database.
     */
    public void onOpen(SQLiteDatabase db) {}
}