core/java/android/database/sqlite/DatabaseConnectionPool.java

/*
 * Copyright (C) 20010 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.
 */

package android.database.sqlite;

import android.content.res.Resources;
import android.os.SystemClock;
import android.util.Log;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Random;

/**
 * A connection pool to be used by readers.
 * Note that each connection can be used by only one reader at a time.
 */
/* package */ class DatabaseConnectionPool {

    private static final String TAG = "DatabaseConnectionPool";

    /** The default connection pool size. */
    private volatile int mMaxPoolSize =
        Resources.getSystem().getInteger(com.android.internal.R.integer.db_connection_pool_size);

    /** The connection pool objects are stored in this member.
     * TODO: revisit this data struct as the number of pooled connections increase beyond
     * single-digit values.
     */
    private final ArrayList<PoolObj> mPool = new ArrayList<PoolObj>(mMaxPoolSize);

    /** the main database connection to which this connection pool is attached */
    private final SQLiteDatabase mParentDbObj;

    /** Random number generator used to pick a free connection out of the pool */
    private Random rand; // lazily initialized

    /* package */ DatabaseConnectionPool(SQLiteDatabase db) {
        this.mParentDbObj = db;
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "Max Pool Size: " + mMaxPoolSize);
        }
    }

    /**
     * close all database connections in the pool - even if they are in use!
     */
    /* package */ synchronized void close() {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "Closing the connection pool on " + mParentDbObj.getPath() + toString());
        }
        for (int i = mPool.size() - 1; i >= 0; i--) {
            mPool.get(i).mDb.close();
        }
        mPool.clear();
    }

    /**
     * get a free connection from the pool
     *
     * @param sql if not null, try to find a connection inthe pool which already has cached
     * the compiled statement for this sql.
     * @return the Database connection that the caller can use
     */
    /* package */ synchronized SQLiteDatabase get(String sql) {
        SQLiteDatabase db = null;
        PoolObj poolObj = null;
        int poolSize = mPool.size();
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            assert sql != null;
            doAsserts();
        }
        if (getFreePoolSize() == 0) {
            // no free ( = available) connections
            if (mMaxPoolSize == poolSize) {
                // maxed out. can't open any more connections.
                // let the caller wait on one of the pooled connections
                // preferably a connection caching the pre-compiled statement of the given SQL
                if (mMaxPoolSize == 1) {
                    poolObj = mPool.get(0);
                } else {
                    for (int i = 0; i < mMaxPoolSize; i++) {
                        if (mPool.get(i).mDb.isInStatementCache(sql)) {
                            poolObj = mPool.get(i);
                            break;
                        }
                    }
                    if (poolObj == null) {
                        // there are no database connections with the given SQL pre-compiled.
                        // ok to return any of the connections.
                        if (rand == null) {
                            rand = new Random(SystemClock.elapsedRealtime());
                        }
                        poolObj = mPool.get(rand.nextInt(mMaxPoolSize));
                    }
                }
                db = poolObj.mDb;
            } else {
                // create a new connection and add it to the pool, since we haven't reached
                // max pool size allowed
                db = mParentDbObj.createPoolConnection((short)(poolSize + 1));
                poolObj = new PoolObj(db);
                mPool.add(poolSize, poolObj);
            }
        } else {
            // there are free connections available. pick one
            // preferably a connection caching the pre-compiled statement of the given SQL
            for (int i = 0; i < poolSize; i++) {
                if (mPool.get(i).isFree() && mPool.get(i).mDb.isInStatementCache(sql)) {
                    poolObj = mPool.get(i);
                    break;
                }
            }
            if (poolObj == null) {
                // didn't find a free database connection with the given SQL already
                // pre-compiled. return a free connection (this means, the same SQL could be
                // pre-compiled on more than one database connection. potential wasted memory.)
                for (int i = 0; i < poolSize; i++) {
                    if (mPool.get(i).isFree()) {
                        poolObj = mPool.get(i);
                        break;
                    }
                }
            }
            db = poolObj.mDb;
        }

        assert poolObj != null;
        assert poolObj.mDb == db;

        poolObj.acquire();
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "END get-connection: " + toString() + poolObj.toString());
        }
        return db;
        // TODO if a thread acquires a connection and dies without releasing the connection, then
        // there could be a connection leak.
    }

    /**
     * release the given database connection back to the pool.
     * @param db the connection to be released
     */
    /* package */ synchronized void release(SQLiteDatabase db) {
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            assert db.mConnectionNum > 0;
            doAsserts();
            assert mPool.get(db.mConnectionNum - 1).mDb == db;
        }

        PoolObj poolObj = mPool.get(db.mConnectionNum - 1);

        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "BEGIN release-conn: " + toString() + poolObj.toString());
        }

        if (poolObj.isFree()) {
            throw new IllegalStateException("Releasing object already freed: " +
                    db.mConnectionNum);
        }

        poolObj.release();
        if (Log.isLoggable(TAG, Log.DEBUG)) {
            Log.d(TAG, "END release-conn: " + toString() + poolObj.toString());
        }
    }

    /**
     * Returns a list of all database connections in the pool (both free and busy connections).
     * This method is used when "adb bugreport" is done.
     */
    /* package */ synchronized ArrayList<SQLiteDatabase> getConnectionList() {
        ArrayList<SQLiteDatabase> list = new ArrayList<SQLiteDatabase>();
        for (int i = mPool.size() - 1; i >= 0; i--) {
            list.add(mPool.get(i).mDb);
        }
        return list;
    }

    /**
     * package level access for testing purposes only. otherwise, private should be sufficient.
     */
    /* package */ int getFreePoolSize() {
        int count = 0;
        for (int i = mPool.size() - 1; i >= 0; i--) {
            if (mPool.get(i).isFree()) {
                count++;
            }
        }
        return count++;
    }

    /**
     * only for testing purposes
     */
    /* package */ ArrayList<PoolObj> getPool() {
        return mPool;
    }

    @Override
    public String toString() {
        StringBuilder buff = new StringBuilder();
        buff.append("db: ");
        buff.append(mParentDbObj.getPath());
        buff.append(", totalsize = ");
        buff.append(mPool.size());
        buff.append(", #free = ");
        buff.append(getFreePoolSize());
        buff.append(", maxpoolsize = ");
        buff.append(mMaxPoolSize);
        for (PoolObj p : mPool) {
            buff.append("\n");
            buff.append(p.toString());
        }
        return buff.toString();
    }

    private void doAsserts() {
        for (int i = 0; i < mPool.size(); i++) {
            mPool.get(i).verify();
            assert mPool.get(i).mDb.mConnectionNum == (i + 1);
        }
    }

    /** only used for testing purposes. */
    /* package */ synchronized void setMaxPoolSize(int size) {
        mMaxPoolSize = size;
    }

    /** only used for testing purposes. */
    /* package */ synchronized int getMaxPoolSize() {
        return mMaxPoolSize;
    }

    /** only used for testing purposes. */
    /* package */ boolean isDatabaseObjFree(SQLiteDatabase db) {
        return mPool.get(db.mConnectionNum - 1).isFree();
    }

    /** only used for testing purposes. */
    /* package */ int getSize() {
        return mPool.size();
    }

    /**
     * represents objects in the connection pool.
     * package-level access for testing purposes only.
     */
    /* package */ static class PoolObj {

        private final SQLiteDatabase mDb;
        private boolean mFreeBusyFlag = FREE;
        private static final boolean FREE = true;
        private static final boolean BUSY = false;

        /** the number of threads holding this connection */
        // @GuardedBy("this")
        private int mNumHolders = 0;

        /** contains the threadIds of the threads holding this connection.
         * used for debugging purposes only.
         */
        // @GuardedBy("this")
        private HashSet<Long> mHolderIds = new HashSet<Long>();

        public PoolObj(SQLiteDatabase db) {
            mDb = db;
        }

        private synchronized void acquire() {
            if (Log.isLoggable(TAG, Log.DEBUG)) {
                assert isFree();
                long id = Thread.currentThread().getId();
                assert !mHolderIds.contains(id);
                mHolderIds.add(id);
            }

            mNumHolders++;
            mFreeBusyFlag = BUSY;
        }

        private synchronized void release() {
            if (Log.isLoggable(TAG, Log.DEBUG)) {
                long id = Thread.currentThread().getId();
                assert mHolderIds.size() == mNumHolders;
                assert mHolderIds.contains(id);
                mHolderIds.remove(id);
            }

            mNumHolders--;
            if (mNumHolders == 0) {
                mFreeBusyFlag = FREE;
            }
        }

        private synchronized boolean isFree() {
            if (Log.isLoggable(TAG, Log.DEBUG)) {
                verify();
            }
            return (mFreeBusyFlag == FREE);
        }

        private synchronized void verify() {
            if (mFreeBusyFlag == FREE) {
                assert mNumHolders == 0;
            } else {
                assert mNumHolders > 0;
            }
        }

        /**
         * only for testing purposes
         */
        /* package */ synchronized int getNumHolders() {
            return mNumHolders;
        }

        @Override
        public String toString() {
            StringBuilder buff = new StringBuilder();
            buff.append(", conn # ");
            buff.append(mDb.mConnectionNum);
            buff.append(", mCountHolders = ");
            synchronized(this) {
                buff.append(mNumHolders);
                buff.append(", freeBusyFlag = ");
                buff.append(mFreeBusyFlag);
                for (Long l : mHolderIds) {
                    buff.append(", id = " + l);
                }
            }
            return buff.toString();
        }
    }
}