SQLite多线程读写

前天去某某公司面试，被问到SQLite的多线程读写问题，突然发现都没注意到这个问题，翻了下以前做的项目，发现很少注意到这个问题，奇葩的是貌似很少出现过问题啊 = =！但是这个android.database.sqlite.SQLiteException: database is locked异常确实前几天在公司项目中出现过几次，后来就进行了研究总结，现在分享出来，水平有限，不妥或者错误的地方还望指出。

SQLite实质上是将数据写入一个文件，通常情况下，在应用的包名下面都能找到xxx.db的文件，拥有root权限的手机，可以通过adb shell，看到data/data/packagename/databases/xxx.db这样的文件。我们可以联系到另外一个关系型数据库H2，其实两者还是有很多相似的地方的，此处就不详细介绍H2了。

由此我们可以得知SQLite其实是文件级别的锁：多个线程可以同时读，但是同时只能有一个线程写。Android提供了SqliteOpenHelper类，加入Java的锁机制以便调用。如果多线程同时读写（注意：指的是不同的线程用使用不同的SqliteOpenHelper实例），后面的操作就会遇到android.database.sqlite.SQLiteException: database is locked这样的异常。很显然，解决这种问题就必须保持SQLite Connection的单例性，即保持始终只有一个SQLite连接实例，并且需要对所有的数据库操作方法加sychronized关键字，如下代码：

public class MyDatabaseHelper extends SQLiteOpenHelper {
        public static final String TAG = "MyDatabaseHelper";
        private static final String DB_NAME = "test.db";
        private static final int DB_VERSION = 1;

        private Context mContext;
        private static MyDatabaseHelper mInstance;

        private MyDatabaseHelper(Context context) {
                super(context,DB_NAME,null,DB_VERSION);
        }

        public synchronized static MyDatabaseHelper getInstance(Context context) {
                if (mInstance == null) {
                        mInstance = new MyDatabaseHelper(context);
                }
                return mInstance;
        }
        public synchronized static void destoryInstance() {
              if (mInstance != null) {
                    mInstance.close();
             }
       }
        @Override
        public void onCreate(SQLiteDatabase db) {
                // TODO Auto-generated method stub

        }

        @Override
        public void onUpgrade(SQLiteDatabase db,int oldVersion,int newVersion) {
                // TODO Auto-generated method stub

        }
        public synchronized void queryMethod() {
                SQLiteDatabase readableDatabase = getReadableDatabase();
                //read operation
        }
        
        public synchronizedvoid updateMethod() {
                SQLiteDatabase writableDatabase = getWritableDatabase();
                //update operation
        }
}

SqliteOpenHelper.java:

/**
 * 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,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 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) {}
}

（编辑：李大同）

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