《HikariCP数据库连接池实战》学习笔记（二）：获取连接原理

这篇文章是关于HikariCP源码的一些碎片分析记录。

ConcurrentBag(final IBagStateListener listener)

ConcurrentBag具有无锁设计、ThreadLocal缓存、队列窃取、直接切换优化四大特点。

CopyOnWriteArrayList：负责存放ConcurrentBag中全部用于出借的资源。
ThreadLocal：用于加速线程本地化资源访问。
SynchronousQueue：用于存在资源等待线程时第一手资源交接。这里采用的是公平模式。底层实现是TransferQueue。SynchronousQueue无存储空间，每一个put操作必须等待一个take操作或者poll操作，否则不能添加新的元素。

private final CopyOnWriteArrayList<T> sharedList;
private final ThreadLocal<List<Object>> threadList;
private final SynchronousQueue<T> handoffQueue;

public ConcurrentBag(final IBagStateListener listener)
   {
      this.listener = listener;
      this.weakThreadLocals = useWeakThreadLocals();

      this.handoffQueue = new SynchronousQueue<>(true);
      this.waiters = new AtomicInteger();
      this.sharedList = new CopyOnWriteArrayList<>();
      if (weakThreadLocals) {
         this.threadList = ThreadLocal.withInitial(() -> new ArrayList<>(16));
      }
      else {
         this.threadList = ThreadLocal.withInitial(() -> new FastList<>(IConcurrentBagEntry.class, 16));
      }
   }

HikariPool(final HikariConfig config)

HikariPool的有参构造函数做了以下事情：
(1)实例化ConcurrentBag。
(2)初始化HouseKeeping线程池，该线程池用于销毁空闲连接。
(3)初始化添加连接线程池，关闭连接线程池。

public HikariPool(final HikariConfig config)
   {
      super(config);

      this.connectionBag = new ConcurrentBag<>(this);
      this.suspendResumeLock = config.isAllowPoolSuspension() ? new SuspendResumeLock() : SuspendResumeLock.FAUX_LOCK;

      this.houseKeepingExecutorService = initializeHouseKeepingExecutorService();

      checkFailFast();

      if (config.getMetricsTrackerFactory() != null) {
         setMetricsTrackerFactory(config.getMetricsTrackerFactory());
      }
      else {
         setMetricRegistry(config.getMetricRegistry());
      }

      setHealthCheckRegistry(config.getHealthCheckRegistry());

      handleMBeans(this, true);

      ThreadFactory threadFactory = config.getThreadFactory();

      final int maxPoolSize = config.getMaximumPoolSize();
      LinkedBlockingQueue<Runnable> addConnectionQueue = new LinkedBlockingQueue<>(maxPoolSize);
      this.addConnectionQueueReadOnlyView = unmodifiableCollection(addConnectionQueue);
      this.addConnectionExecutor = createThreadPoolExecutor(addConnectionQueue, poolName + " connection adder", threadFactory, new ThreadPoolExecutor.DiscardOldestPolicy());
      this.closeConnectionExecutor = createThreadPoolExecutor(maxPoolSize, poolName + " connection closer", threadFactory, new ThreadPoolExecutor.CallerRunsPolicy());

      this.leakTaskFactory = new ProxyLeakTaskFactory(config.getLeakDetectionThreshold(), houseKeepingExecutorService);

      this.houseKeeperTask = houseKeepingExecutorService.scheduleWithFixedDelay(new HouseKeeper(), 100L, housekeepingPeriodMs, MILLISECONDS);

      if (Boolean.getBoolean("com.zaxxer.hikari.blockUntilFilled") && config.getInitializationFailTimeout() > 1) {
         addConnectionExecutor.setCorePoolSize(Math.min(16, Runtime.getRuntime().availableProcessors()));
         addConnectionExecutor.setMaximumPoolSize(Math.min(16, Runtime.getRuntime().availableProcessors()));

         final long startTime = currentTime();
         while (elapsedMillis(startTime) < config.getInitializationFailTimeout() && getTotalConnections() < config.getMinimumIdle()) {
            quietlySleep(MILLISECONDS.toMillis(100));
         }

         addConnectionExecutor.setCorePoolSize(1);
         addConnectionExecutor.setMaximumPoolSize(1);
      }
   }

HikariDataSource#getConnection

使用双层检查锁初始化数据库连接池。

@Override
   public Connection getConnection() throws SQLException
   {
      if (isClosed()) {
         throw new SQLException("HikariDataSource " + this + " has been closed.");
      }

      if (fastPathPool != null) {
         return fastPathPool.getConnection();
      }

      // See http://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java
      HikariPool result = pool;
      if (result == null) {
         synchronized (this) {
            result = pool;
            if (result == null) {
               validate();
               LOGGER.info("{} - Starting...", getPoolName());
               try {
                  pool = result = new HikariPool(this);
                  this.seal();
               }
               catch (PoolInitializationException pie) {
                  if (pie.getCause() instanceof SQLException) {
                     throw (SQLException) pie.getCause();
                  }
                  else {
                     throw pie;
                  }
               }
               LOGGER.info("{} - Start completed.", getPoolName());
            }
         }
      }

      return result.getConnection();
   }

HikariPool#getConnection()

调用ConnectionBag#connect方法获取一个数据库连接，对连接的有效性进行检查，通过PoolEntry是否被标记为清除状态、当前PoolEntry的存活时间是否超时和当前连接是否是活跃有效的进行判断，如果存在一项检查不通过，就关闭这个链接，再重新从资源池获取连接，直到获取到有效连接为止。

获取到有效连接后，会通过ProxyFactory.getProxyConnection返回代理过的连接。HikariCp利用Java字节码修改类库 Javassist实现动态代理。

HikariPool实现了HikariPoolMXBean接口，用于对外暴露HikariCP连接池相关的监控管理功能；
HikariConfig实现了HikariConfigMXBean接口，用于暴露配置相关的监控管理功能。

 public Connection getConnection() throws SQLException{
     return getConnection(connectionTimeout);
 }

public Connection getConnection(final long hardTimeout) throws SQLException
   {
      suspendResumeLock.acquire();
      final long startTime = currentTime();

      try {
         long timeout = hardTimeout;
         do {
            PoolEntry poolEntry = connectionBag.borrow(timeout, MILLISECONDS);
            if (poolEntry == null) {
               break; // We timed out... break and throw exception
            }

            final long now = currentTime();
            if (poolEntry.isMarkedEvicted() || (elapsedMillis(poolEntry.lastAccessed, now) > aliveBypassWindowMs && !isConnectionAlive(poolEntry.connection))) {
               closeConnection(poolEntry, poolEntry.isMarkedEvicted() ? EVICTED_CONNECTION_MESSAGE : DEAD_CONNECTION_MESSAGE);
               timeout = hardTimeout - elapsedMillis(startTime);
            }
            else {
               metricsTracker.recordBorrowStats(poolEntry, startTime);
               return poolEntry.createProxyConnection(leakTaskFactory.schedule(poolEntry), now);
            }
         } while (timeout > 0L);

         metricsTracker.recordBorrowTimeoutStats(startTime);
         throw createTimeoutException(startTime);
      }
      catch (InterruptedException e) {
         Thread.currentThread().interrupt();
         throw new SQLException(poolName + " - Interrupted during connection acquisition", e);
      }
      finally {
         suspendResumeLock.release();
      }
   }

PoolEntry#createProxyConnection

Connection createProxyConnection(final ProxyLeakTask leakTask, final long now)
   {
      return ProxyFactory.getProxyConnection(this, connection, openStatements, leakTask, now, isReadOnly, isAutoCommit);
   }

ProxyFactory#getProxyConnection

static ProxyConnection getProxyConnection(final PoolEntry poolEntry, final Connection connection, final FastList<Statement> openStatements, final ProxyLeakTask leakTask, final long now, final boolean isReadOnly, final boolean isAutoCommit)
   {
      // Body is replaced (injected) by JavassistProxyFactory
      throw new IllegalStateException("You need to run the CLI build and you need target/classes in your classpath to run.");
   }

ConcurrentBag#borrow

ConcurentBag采用queue-stealing的机制获取元素：首先从ThreadLocal中获取属于当前线程的元素来避免锁竞争。如果没有可用的元素再从CopyOnWriteArrayList中获取。

private final ThreadLocal<List<Object>> threadList;
private final boolean weakThreadLocals;
private final AtomicInteger waiters;
private final CopyOnWriteArrayList<T> sharedList;
private final SynchronousQueue<T> handoffQueue;

public T borrow(long timeout, final TimeUnit timeUnit) throws InterruptedException
   {
      // Try the thread-local list first
      final List<Object> list = threadList.get();
      for (int i = list.size() - 1; i >= 0; i--) {
         final Object entry = list.remove(i);
         @SuppressWarnings("unchecked")
         final T bagEntry = weakThreadLocals ? ((WeakReference<T>) entry).get() : (T) entry;
         if (bagEntry != null && bagEntry.compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
            return bagEntry;
         }
      }

      // Otherwise, scan the shared list ... then poll the handoff queue
      final int waiting = waiters.incrementAndGet();
      try {
         for (T bagEntry : sharedList) {
            if (bagEntry.compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
               // If we may have stolen another waiter's connection, request another bag add.
               if (waiting > 1) {
                  listener.addBagItem(waiting - 1);
               }
               return bagEntry;
            }
         }

         listener.addBagItem(waiting);

         timeout = timeUnit.toNanos(timeout);
         do {
            final long start = currentTime();
            final T bagEntry = handoffQueue.poll(timeout, NANOSECONDS);
            if (bagEntry == null || bagEntry.compareAndSet(STATE_NOT_IN_USE, STATE_IN_USE)) {
               return bagEntry;
            }

            timeout -= elapsedNanos(start);
         } while (timeout > 10_000);

         return null;
      }
      finally {
         waiters.decrementAndGet();
      }
   }