Android数据库源码分析(3)-连接缓存池SQLiteConnectionPool

本系列主要关注安卓数据库的线程行为,分为四个部分:
(1)SQLiteOpenHelper的getReadableDatabase和getWritableDatabase
(2)SQLiteDatabase的实现以及多线程行为
(3)连接缓存池SQLiteConnectionPool
(4)SQLiteDatabase多线程实践

本篇主要关注SQLiteConnectionPool(连接池)在并发下的行为。

上文提到,SQLiteDatabase会在每个线程中使用一个SQLiteSession,而SQLiteSession会共用一个SQLiteConnectionPool对象,并通过SQLiteConnectionPoolacquireConnectionreleaseConnection方法来获取和释放数据库连接(一个SQLiteConnection对象)。

public SQLiteConnection acquireConnection(String sql, int connectionFlags,
        CancellationSignal cancellationSignal) {
    return waitForConnection(sql, connectionFlags, cancellationSignal);//看名字是要等待什么锁了
}

connectionFlags是用SQLiteDatabase.getThreadDefaultConnectionFlags的返回值一路传下来的,这个方法在前文讨论过这个方法,会记录两件事:1.数据库是只读还是可写;2.当前是否主线程。

waitForConnection方法比较长,我们一段一段地看。

1 尝试立即获取连接

    //是否可写连接。可写的连接同一时间只能存在一个。
    final boolean wantPrimaryConnection =
            (connectionFlags & CONNECTION_FLAG_PRIMARY_CONNECTION_AFFINITY) != 0;

    final ConnectionWaiter waiter;
    final int nonce;
    synchronized (mLock) {//加锁。留意这一段代码中加锁部分并未结束。
        throwIfClosedLocked();

        // Abort if canceled.
        if (cancellationSignal != null) {
            cancellationSignal.throwIfCanceled();
        }

        // Try to acquire a connection.
        SQLiteConnection connection = null;
        if (!wantPrimaryConnection) {
            //尝试获取只读连接
            connection = tryAcquireNonPrimaryConnectionLocked(
                    sql, connectionFlags); // might throw
        }
        if (connection == null) {
            //尝试获取可写连接
            connection = tryAcquirePrimaryConnectionLocked(connectionFlags); // might throw
        }
        if (connection != null) {
            return connection;
        }

到这里是尝试直接获取连接。尝试的方法有tryAcquireNonPrimaryConnectionLockedtryAcquirePrimaryConnectionLocked。只读时只需要 non primary connection,而需要写时要primary connection。
先看tryAcquirePrimaryConnectionLocked

// Might throw.
private SQLiteConnection tryAcquirePrimaryConnectionLocked(int connectionFlags) {
    // If the primary connection is available, acquire it now.
    SQLiteConnection connection = mAvailablePrimaryConnection;//同时只能存在一个可写连接,用一个成员变量mAvailablePrimaryConnection缓存空闲连接
    if (connection != null) {//有缓存返回即可。finishAcquirePrimaryConnection会把connection放到mAcquiredConnections中。mAcquiredConnections存储正在使用的连接。
        mAvailablePrimaryConnection = null;//不再空闲
        finishAcquireConnectionLocked(connection, connectionFlags); // might throw
        return connection;
    }

    // Make sure that the primary connection actually exists and has just been acquired.
    //如果上一个if造成了不再空闲,则mAcquiredConnections中就会有一个primary connection,这里就会返回null。上一层的waitForConnection接到null会进入等待状态,这个后面讨论。
    for (SQLiteConnection acquiredConnection : mAcquiredConnections.keySet()) {
        if (acquiredConnection.isPrimaryConnection()) {
            return null;
        }
    }

    //如果没有在上面返回null,那么这一定是第一次请求primary connnection,或者有一个连接泄露了(未recycle的情况下finalize),这时候就需要用openConnectionLocked去新开一个连接。
    // Uhoh.  No primary connection!  Either this is the first time we asked
    // for it, or maybe it leaked?
    connection = openConnectionLocked(mConfiguration,
            true /*primaryConnection*/); // might throw
    finishAcquireConnectionLocked(connection, connectionFlags); // might throw
    return connection;
}

然后看tryAcquireNonPrimaryConnectionLocked

// Might throw.
private SQLiteConnection tryAcquireNonPrimaryConnectionLocked(
        String sql, int connectionFlags) {
    // Try to acquire the next connection in the queue.
    SQLiteConnection connection;
    //只读连接可以有多个,用一个ArrayList缓存了所有空闲连接
    final int availableCount = mAvailableNonPrimaryConnections.size();
    if (availableCount > 1 && sql != null) {
        // If we have a choice, then prefer a connection that has the
        // prepared statement in its cache.
        // 如上面的英文注释说的,如果有不止一个连接可选,那么挑选缓存了相同sql语句的那个。可能SQLiteConnection对此有优化?
        for (int i = 0; i < availableCount; i++) {
            connection = mAvailableNonPrimaryConnections.get(i);
            if (connection.isPreparedStatementInCache(sql)) {//如果有相同sql,返回
                mAvailableNonPrimaryConnections.remove(i);
                finishAcquireConnectionLocked(connection, connectionFlags); // might throw
                return connection;
            }
        }
    }
    if (availableCount > 0) {
        // Otherwise, just grab the next one.
        //没有挑到,随便给一个
        connection = mAvailableNonPrimaryConnections.remove(availableCount - 1);
        finishAcquireConnectionLocked(connection, connectionFlags); // might throw
        return connection;
    }

    // 一个空闲连接都没有。
    // Expand the pool if needed.
    int openConnections = mAcquiredConnections.size();
    if (mAvailablePrimaryConnection != null) {
        openConnections += 1;
    }
    // 上面在计算有多少已打开连接(空闲+使用中)。这里肯定没有空闲non primary连接了,而如果有空闲primary连接,则要 += 1。
    if (openConnections >= mMaxConnectionPoolSize) {
        // 超过数据库连接限制,放弃治疗。连接限制与数据库底层实现有关。
        return null;
    }
    // 没超限,还能再开一个连接。所以开连接并返回。
    connection = openConnectionLocked(mConfiguration,
            false /*primaryConnection*/); // might throw
    finishAcquireConnectionLocked(connection, connectionFlags); // might throw
    return connection;
}

在这一步中,进行了从缓存中取得连接的尝试;而如果无法取得连接,也进行了打开连接的尝试。如果再无法打开的话,就会拿到一个null了。后续就需要进行等待。

2 等待获取连接

        // 留意这里还在上一个锁mLock中
        // No connections available.  Enqueue a waiter in priority order.
        final int priority = getPriority(connectionFlags);//主线程中的连接优先级更高,记得吗?
        final long startTime = SystemClock.uptimeMillis();
        // waiter是一个ConnectionWaiter对象。它同时也是一个链表,有一个同类的mNext成员变量。
        // obtainConnectionWaiterLocked会去复用(取链表头)或者新建一个对象。
        waiter = obtainConnectionWaiterLocked(Thread.currentThread(), startTime,
                priority, wantPrimaryConnection, sql, connectionFlags);
        ConnectionWaiter predecessor = null;
        // 按照优先级向mConnectionWaiterQueue添加waitor对象。mConnectionWaiterQueue不是复用池,而是有效的等待队列(也是链表)。
        ConnectionWaiter successor = mConnectionWaiterQueue;
        while (successor != null) {
            if (priority > successor.mPriority) {
                waiter.mNext = successor;
                break;
            }
            predecessor = successor;
            successor = successor.mNext;
        }
        if (predecessor != null) {
            predecessor.mNext = waiter;
        } else {
            mConnectionWaiterQueue = waiter;
        }

        nonce = waiter.mNonce;//观察recycleConnectionWaiterLocked方法,mNonce在waiter每次被复用完成回收时自增1
    }//锁mLock结束

到这里就是把需要等待的连接信息封装到ConnectionWaiter中,并将ConnectionWaiter对象放到一个链表里。那么什么时候会结束等待并返回呢?继续看代码:

    // Set up the cancellation listener.
    if (cancellationSignal != null) {
        cancellationSignal.setOnCancelListener(new CancellationSignal.OnCancelListener() {
            @Override
            public void onCancel() {
                synchronized (mLock) {
                    if (waiter.mNonce == nonce) {//nonce的作用在这里体现。防止waiter对象复用造成误取消。
                        cancelConnectionWaiterLocked(waiter);
                    }
                }
            }
        });
    }

这一段用于额外处理取消信号的。在等待连接过程中取消,就可以把这一个waiter去除了。

接下来:

    try {
        // Park the thread until a connection is assigned or the pool is closed.
        // Rethrow an exception from the wait, if we got one.
        long busyTimeoutMillis = CONNECTION_POOL_BUSY_MILLIS;
        long nextBusyTimeoutTime = waiter.mStartTime + busyTimeoutMillis;
        for (;;) {//循环开始
            // Detect and recover from connection leaks.
            // 管理泄露连接的。如果一个SQLiteConnection在finalize时还未关闭,则会置泄露状态。
            // mConnectionLeaked是一个AtomicBoolean。
            if (mConnectionLeaked.compareAndSet(true, false)) {
                synchronized (mLock) {
                    wakeConnectionWaitersLocked();//有泄露连接被关闭的话,最大连接限制下就可能有位置空出来,这时候就可以尝试分配一个连接
                }
            }

            // Wait to be unparked (may already have happened), a timeout, or interruption.
            // 等待。那么unpark在哪里?在wakeConnectionWaitersLocked中。这个方法在上面泄露测试时调用过。
            // 还有cancelConnectionWaiterLocked中,取消等待自然要唤醒线程处理一下。
            LockSupport.parkNanos(this, busyTimeoutMillis * 1000000L);

            // Clear the interrupted flag, just in case.
            Thread.interrupted();

            // Check whether we are done waiting yet.
            synchronized (mLock) {
                throwIfClosedLocked();

                //等到了一个Connection。这个mAssignedConnection是何时赋值的呢?
                //也是在wakeConnectionWaitersLocked中赋值的。
                final SQLiteConnection connection = waiter.mAssignedConnection;
                final RuntimeException ex = waiter.mException;
                if (connection != null || ex != null) {
                    recycleConnectionWaiterLocked(waiter);//回收waiter,会造成mNonce自增1
                    if (connection != null) {
                        return connection;
                    }
                    throw ex; // rethrow!
                }

                //没拿到连接,继续等。
                final long now = SystemClock.uptimeMillis();
                if (now < nextBusyTimeoutTime) {
                    busyTimeoutMillis = now - nextBusyTimeoutTime;
                } else {
                    logConnectionPoolBusyLocked(now - waiter.mStartTime, connectionFlags);
                    busyTimeoutMillis = CONNECTION_POOL_BUSY_MILLIS;
                    nextBusyTimeoutTime = now + busyTimeoutMillis;
                }
            }
        }//循环结束
    } finally {
        // Remove the cancellation listener.
        if (cancellationSignal != null) {
            cancellationSignal.setOnCancelListener(null);
        }
    }
}

在这一步中,用ConnectionWaiter来封装等待中的连接信息,并按优先级放入一个链表,随后进入等待状态。获取到连接后,等待状态结束,返回连接。

3 连接的释放

这里我们可以先预估以下:释放连接时需要把被释放的连接放回到空闲连接集合,并进行unpark操作,通知正在等待连接的线程。

代码如下:

public void releaseConnection(SQLiteConnection connection) {
    synchronized (mLock) {
        AcquiredConnectionStatus status = mAcquiredConnections.remove(connection);//从活跃连接池中移除
        if (status == null) {
            throw new IllegalStateException("Cannot perform this operation "
                    + "because the specified connection was not acquired "
                    + "from this pool or has already been released.");
        }

        if (!mIsOpen) {
            closeConnectionAndLogExceptionsLocked(connection);
        } else if (connection.isPrimaryConnection()) {
            if (recycleConnectionLocked(connection, status)) {
                assert mAvailablePrimaryConnection == null;
                mAvailablePrimaryConnection = connection;//放回可写连接mAvailablePrimaryConnection
            }
            wakeConnectionWaitersLocked();//通知其它线程
        } else if (mAvailableNonPrimaryConnections.size() >= mMaxConnectionPoolSize - 1) {
            closeConnectionAndLogExceptionsLocked(connection);
        } else {
            if (recycleConnectionLocked(connection, status)) {
                mAvailableNonPrimaryConnections.add(connection);//放回空闲只读连接池
            }
            wakeConnectionWaitersLocked();//通知其它线程
        }
    }
}

4 总结

综上所述,SQLiteConnectionPool提供数据库连接的流程如下:
(1)从缓存中获取一个空闲的连接。若有多个空闲连接,优先挑选执行过相同SQL的那个。注意如果是写操作的话,则会返回一个primary connection,并将其它尝试获得primary connection的线程阻塞,直到当前线程结束使用连接。而只读的操作则可以同时存在多个,并可以和写操作的连接共存。
(2)如果缓存中没有连接,检查底层数据库是否可以容纳更多连接。如果可以,新建一个连接并返回。
(3)如果底层数据库不再允许增加连接,则进入等待。到超时或者有其它连接被释放结束等待。如果此时可以获取连接,则返回连接。如果不能,进入新一轮等待。

5 多线程下的transaction

了解了以上的特性之后,transaction的多线程行为就比较好理解了。
以下是SQLiteDatabasebeginTransaction方法:

private void beginTransaction(SQLiteTransactionListener transactionListener,
        boolean exclusive) {
    acquireReference();
    try {
        getThreadSession().beginTransaction(
                exclusive ? SQLiteSession.TRANSACTION_MODE_EXCLUSIVE :
                        SQLiteSession.TRANSACTION_MODE_IMMEDIATE,
                transactionListener,
                getThreadDefaultConnectionFlags(false /*readOnly*/), null);
    } finally {
        releaseReference();
    }
}

留意flags参数传入的readOnly为false,所以SQLiteSession会从SQLiteConnectionPool中获取一个独占的连接。并且在SQLiteSession执行其它SQL语句的情况下,执行完成会将连接释放回连接池,而beginTransaction操作则不会,而是持有这一个连接直至同一线程内调用endTransaction。这里再贴一遍SQLiteSession.execute源码:

public void execute(String sql, Object[] bindArgs, int connectionFlags,
        CancellationSignal cancellationSignal) {
    if (sql == null) {
        throw new IllegalArgumentException("sql must not be null.");
    }

    if (executeSpecial(sql, bindArgs, connectionFlags, cancellationSignal)) {
        return;
    }

    acquireConnection(sql, connectionFlags, cancellationSignal); // might throw
    try {
        mConnection.execute(sql, bindArgs, cancellationSignal); // might throw
    } finally {
        //这里释放了连接(其实是交还给连接池)
        releaseConnection(); // might throw
    }
}

所以,当有一个线程在transaction过程中时,其它线程的写操作和beginTransaction操作都会被阻塞住,直至当前线程的transaction完成才会按照优先级挑选一个线程继续。

 

Android数据库源码分析(3)-连接缓存池SQLiteConnectionPool

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