整体思路

1：DruidConnectionHolder[] connections 来表示还未使用连接池的连接的数量。假如程序中想获取一个连接的时候，首先先判断池子里面还是否有连接，假如有会直接获取。假如没有会重新创建物理连接，然后重新放到connections池子里面。

2：然后for(;;)这种方式抢占式获取连接，一旦拿到连接就会调用 decrementPoolingCount() 进行坐标转移。

3：回收过程是 connection.close();方法，最终调用result = putLast(holder, currentTimeMillis); 把用完的连接，放回到池子里面，同时调用  incrementPoolingCount 来增加下标。

private DruidConnectionHolder pollLast(long nanos) throws InterruptedException, SQLException {
        long estimate = nanos;

        for (;;) {
            // 下面语句表示，连接池里面无可用连接，这个时候通知创建的线程，进行创建一个物理连接。
            if (poolingCount == 0) {
                
                emptySignal(); // send signal to CreateThread create connection

                if (failFast && isFailContinuous()) {
                    throw new DataSourceNotAvailableException(createError);
                }

                if (estimate <= 0) {
                    waitNanosLocal.set(nanos - estimate);
                    return null;
                }

                notEmptyWaitThreadCount++;
                if (notEmptyWaitThreadCount > notEmptyWaitThreadPeak) {
                    notEmptyWaitThreadPeak = notEmptyWaitThreadCount;
                }

                try {
                    long startEstimate = estimate;
                    estimate = notEmpty.awaitNanos(estimate); // signal by
                                                              // recycle or
                                                              // creator
                    notEmptyWaitCount++;
                    notEmptyWaitNanos += (startEstimate - estimate);

                    if (!enable) {
                        connectErrorCountUpdater.incrementAndGet(this);

                        if (disableException != null) {
                            throw disableException;
                        }

                        throw new DataSourceDisableException();
                    }
                } catch (InterruptedException ie) {
                    notEmpty.signal(); // propagate to non-interrupted thread
                    notEmptySignalCount++;
                    throw ie;
                } finally {
                    notEmptyWaitThreadCount--;
                }

                if (poolingCount == 0) {
                    if (estimate > 0) {
                        continue;
                    }

                    waitNanosLocal.set(nanos - estimate);
                    return null;
                }
            }
            //这里会持续获取，直到超时。

            decrementPoolingCount();
            DruidConnectionHolder last = connections[poolingCount];
            connections[poolingCount] = null;

            long waitNanos = nanos - estimate;
            last.setLastNotEmptyWaitNanos(waitNanos);

            return last;
        }
    }

//新建的物理连接放到池子里面了
private boolean put(DruidConnectionHolder holder, long createTaskId) {
        lock.lock();
        try {
            if (poolingCount >= maxActive) {
                return false;
            }
            connections[poolingCount] = holder;
            incrementPoolingCount();

            if (poolingCount > poolingPeak) {
                poolingPeak = poolingCount;
                poolingPeakTime = System.currentTimeMillis();
            }

            notEmpty.signal();
            notEmptySignalCount++;

            if (createScheduler != null) {
                clearCreateTask(createTaskId);

                if (poolingCount + createTaskCount < notEmptyWaitThreadCount //
                    && activeCount + poolingCount + createTaskCount < maxActive) {
                    emptySignal();
                }
            }
        } finally {
            lock.unlock();
        }
        return true;
    }

public void close() throws SQLException {
        if (this.disable) {
            return;
        }

        DruidConnectionHolder holder = this.holder;
        if (holder == null) {
            if (dupCloseLogEnable) {
                LOG.error("dup close");
            }
            return;
        }

        DruidAbstractDataSource dataSource = holder.getDataSource();
        boolean isSameThread = this.getOwnerThread() == Thread.currentThread();

        if (!isSameThread) {
            dataSource.setAsyncCloseConnectionEnable(true);
        }

        if (dataSource.isAsyncCloseConnectionEnable()) {
            syncClose();
            return;
        }

        if (!CLOSING_UPDATER.compareAndSet(this, 0, 1)) {
            return;
        }

        try {
            for (ConnectionEventListener listener : holder.getConnectionEventListeners()) {
                listener.connectionClosed(new ConnectionEvent(this));
            }

            List<Filter> filters = dataSource.getProxyFilters();
            if (filters.size() > 0) {
                FilterChainImpl filterChain = new FilterChainImpl(dataSource);
                filterChain.dataSource_recycle(this);
            } else {
                //回收的方法
                recycle();
            }
        } finally {
            CLOSING_UPDATER.set(this, 0);
        }

        this.disable = true;
    }

protected void recycle(DruidPooledConnection pooledConnection) throws SQLException {
        final DruidConnectionHolder holder = pooledConnection.holder;

        if (holder == null) {
            LOG.warn("connectionHolder is null");
            return;
        }

        if (logDifferentThread //
            && (!isAsyncCloseConnectionEnable()) //
            && pooledConnection.ownerThread != Thread.currentThread()//
        ) {
            LOG.warn("get/close not same thread");
        }

        final Connection physicalConnection = holder.conn;

        if (pooledConnection.traceEnable) {
            Object oldInfo = null;
            activeConnectionLock.lock();
            try {
                if (pooledConnection.traceEnable) {
                    oldInfo = activeConnections.remove(pooledConnection);
                    pooledConnection.traceEnable = false;
                }
            } finally {
                activeConnectionLock.unlock();
            }
            if (oldInfo == null) {
                if (LOG.isWarnEnabled()) {
                    LOG.warn("remove abandonded failed. activeConnections.size " + activeConnections.size());
                }
            }
        }

        final boolean isAutoCommit = holder.underlyingAutoCommit;
        final boolean isReadOnly = holder.underlyingReadOnly;
        final boolean testOnReturn = this.testOnReturn;

        try {
            // check need to rollback?
            if ((!isAutoCommit) && (!isReadOnly)) {
                pooledConnection.rollback();
            }

            // reset holder, restore default settings, clear warnings
            boolean isSameThread = pooledConnection.ownerThread == Thread.currentThread();
            if (!isSameThread) {
                final ReentrantLock lock = pooledConnection.lock;
                lock.lock();
                try {
                    holder.reset();
                } finally {
                    lock.unlock();
                }
            } else {
                holder.reset();
            }

            if (holder.discard) {
                return;
            }

            if (phyMaxUseCount > 0 && holder.useCount >= phyMaxUseCount) {
                discardConnection(holder);
                return;
            }

            if (physicalConnection.isClosed()) {
                lock.lock();
                try {
                    if (holder.active) {
                        activeCount--;
                        holder.active = false;
                    }
                    closeCount++;
                } finally {
                    lock.unlock();
                }
                return;
            }

            if (testOnReturn) {
                boolean validate = testConnectionInternal(holder, physicalConnection);
                if (!validate) {
                    JdbcUtils.close(physicalConnection);

                    destroyCountUpdater.incrementAndGet(this);

                    lock.lock();
                    try {
                        if (holder.active) {
                            activeCount--;
                            holder.active = false;
                        }
                        closeCount++;
                    } finally {
                        lock.unlock();
                    }
                    return;
                }
            }
            if (holder.initSchema != null) {
                holder.conn.setSchema(holder.initSchema);
                holder.initSchema = null;
            }

            if (!enable) {
                discardConnection(holder);
                return;
            }

            boolean result;
            final long currentTimeMillis = System.currentTimeMillis();

            if (phyTimeoutMillis > 0) {
                long phyConnectTimeMillis = currentTimeMillis - holder.connectTimeMillis;
                if (phyConnectTimeMillis > phyTimeoutMillis) {
                    discardConnection(holder);
                    return;
                }
            }

            lock.lock();
            try {
                if (holder.active) {
                    activeCount--;
                    holder.active = false;
                }
                closeCount++;

                result = putLast(holder, currentTimeMillis);
                recycleCount++;
            } finally {
                lock.unlock();
            }

            if (!result) {
                JdbcUtils.close(holder.conn);
                LOG.info("connection recyle failed.");
            }
        } catch (Throwable e) {
            holder.clearStatementCache();

            if (!holder.discard) {
                discardConnection(holder);
                holder.discard = true;
            }

            LOG.error("recyle error", e);
            recycleErrorCountUpdater.incrementAndGet(this);
        }
    }