整体思路：
1：druid首先是连接池，DruidConnectionHolder[] 是通过对象数组来实现连接池的池子的，然后动态的维护下表，所以就要求并发性需要比较高，不然很容导致下表不准的现象。所以池子里面动态床架与维护分别通过信号量 empty和notempty来作为信使动态维护。基本上相当于自己实现了一个阻塞队列，我看jedis源码的时候，这部分直接用了阻塞队列。
2：由于druid里面有大量的监控数据，所以代码里面有很多变量来统计数据，感觉这块代码比较乱哈，分离的不够好。
3：再者就是由于druid支持各种配置参数，所以中间有很多代码也是来判断配置条件的。
4：还有代码里面有责任链模式，来实现各种层面的监控，同时这样也支持了自定义，监控也是druid主要卖点之一
5：获取链接的时候分真实的物理连接和连接池连接。也是按照规则来动态创建，假如连接池拿不到，那就根据条件来创新新的链接。
//当连接池连接够用时，利用empty阻塞添加连接的守护线程（主流程3），当连接池连接不够用时，获取连接的那个线程（这里记为业务线程A）就会阻塞在notEmpty上，// 且唤起阻塞在empty上的添加连接的守护线程，走完添加连接的流程，走完后会重新唤起阻塞在notEmpty上的业务线程A，业务线程A就会继续尝试获取连接//连接池有初始化大小，有最大大小，还有回收策略，所以再取和创建的时候，两者都需要动态通知。主要通过empty和 notempty来通信实现，就像两个小兵来负责拿着收集通信来调度双方的需求，protected ReentrantLock lock;protected Condition notEmpty; //连接池不空的信号量，假如空来的话 会阻塞protected Condition empty;
private DruidPooledConnection getConnectionInternal(long maxWait) throws SQLException {if (closed) {connectErrorCountUpdater.incrementAndGet(this);throw new DataSourceClosedException("dataSource already closed at " + new Date(closeTimeMillis));}if (!enable) {connectErrorCountUpdater.incrementAndGet(this);if (disableException != null) {throw disableException;}throw new DataSourceDisableException();} //上面先进行状态判断final long nanos = TimeUnit.MILLISECONDS.toNanos(maxWait);final int maxWaitThreadCount = this.maxWaitThreadCount;DruidConnectionHolder holder;for (boolean createDirect = false;;) {if (createDirect) {createStartNanosUpdater.set(this, System.nanoTime());//当前纳秒和对象绑定 将此更新器管理的给定对象的字段设置为给定更新值。if (pareAndSet(this, 0, 1)) { //如果当前值 == 预期值，则以原子方式将此更新器所管理的给定对象的字段设置为给定的更新值PhysicalConnectionInfo pyConnInfo = DruidDataSource.this.createPhysicalConnection();holder = new DruidConnectionHolder(this, pyConnInfo);holder.lastActiveTimeMillis = System.currentTimeMillis();creatingCountUpdater.decrementAndGet(this); //假如直接写成 普通的自增对象是不是可以避免误写操作directCreateCountUpdater.incrementAndGet(this);if (LOG.isDebugEnabled()) {LOG.debug("conn-direct_create ");}boolean discard = false;lock.lock();try {if (activeCount < maxActive) {activeCount++;holder.active = true;if (activeCount > activePeak) {activePeak = activeCount;activePeakTime = System.currentTimeMillis();}break;} else {discard = true;}} finally {lock.unlock();}if (discard) {JdbcUtils.close(pyConnInfo.getPhysicalConnection());}}}try {lock.lockInterruptibly();} catch (InterruptedException e) {connectErrorCountUpdater.incrementAndGet(this);throw new SQLException("interrupt", e);}//druid其实支持这种策略的，在maxWaitThreadCount属性为默认值（-1）的情况下不启用，// 如果maxWaitThreadCount配置大于0，表示启用，这是druid做的一种丢弃措施，如果你不希望在池子里的连接完全不够用导阻塞的业务线程过多，// 就可以考虑配置该项，这个属性的意思是说在连接不够用时最多让多少个业务线程发生阻塞，流程1.2的图里没有体现这个开关的用途，可以在代码里查看，// 每次在pollLast方法里陷入等待前会把属性notEmptyWaitThreadCount进行累加，阻塞结束后会递减，由此可见notEmptyWaitThreadCount就是表示当前等待可用连接时阻塞的业务线程的总个数，// 而getConnectionInternal在每次调用pollLast前都会判断这样一段代码：try {if (maxWaitThreadCount > 0&& notEmptyWaitThreadCount >= maxWaitThreadCount) {connectErrorCountUpdater.incrementAndGet(this);throw new SQLException("maxWaitThreadCount " + maxWaitThreadCount + ", current wait Thread count "+ lock.getQueueLength());}if (onFatalError&& onFatalErrorMaxActive > 0&& activeCount >= onFatalErrorMaxActive) {connectErrorCountUpdater.incrementAndGet(this);StringBuilder errorMsg = new StringBuilder();errorMsg.append("onFatalError, activeCount ").append(activeCount).append(", onFatalErrorMaxActive ").append(onFatalErrorMaxActive);if (lastFatalErrorTimeMillis > 0) {errorMsg.append(", time '").append(StringUtils.formatDateTime19(lastFatalErrorTimeMillis, TimeZone.getDefault())).append("'");}if (lastFatalErrorSql != null) {errorMsg.append(", sql \n").append(lastFatalErrorSql);}throw new SQLException(errorMsg.toString(), lastFatalError);}connectCount++;if (createScheduler != null&& poolingCount == 0&& activeCount < maxActive&& creatingCountUpdater.get(this) == 0&& createScheduler instanceof ScheduledThreadPoolExecutor) {ScheduledThreadPoolExecutor executor = (ScheduledThreadPoolExecutor) createScheduler;if (executor.getQueue().size() > 0) {createDirect = true;continue;}}if (maxWait > 0) {holder = pollLast(nanos);} else {holder = takeLast();}if (holder != null) {if (holder.discard) {continue;}activeCount++;holder.active = true;if (activeCount > activePeak) {activePeak = activeCount;activePeakTime = System.currentTimeMillis();}}} catch (InterruptedException e) {connectErrorCountUpdater.incrementAndGet(this);throw new SQLException(e.getMessage(), e);} catch (SQLException e) {connectErrorCountUpdater.incrementAndGet(this);throw e;} finally {lock.unlock();}break;}if (holder == null) {long waitNanos = waitNanosLocal.get();final long activeCount;final long maxActive;final long creatingCount;final long createStartNanos;final long createErrorCount;final Throwable createError;try {lock.lock();activeCount = this.activeCount;maxActive = this.maxActive;creatingCount = this.creatingCount;createStartNanos = this.createStartNanos;createErrorCount = this.createErrorCount;createError = this.createError;} finally {lock.unlock();}StringBuilder buf = new StringBuilder(128);buf.append("wait millis ")//.append(waitNanos / (1000 * 1000))//.append(", active ").append(activeCount)//.append(", maxActive ").append(maxActive)//.append(", creating ").append(creatingCount)//;if (creatingCount > 0 && createStartNanos > 0) {long createElapseMillis = (System.nanoTime() - createStartNanos) / (1000 * 1000);if (createElapseMillis > 0) {buf.append(", createElapseMillis ").append(createElapseMillis);}}if (createErrorCount > 0) {buf.append(", createErrorCount ").append(createErrorCount);}List<JdbcSqlStatValue> sqlList = this.getDataSourceStat().getRuningSqlList();for (int i = 0; i < sqlList.size(); ++i) {if (i != 0) {buf.append('\n');} else {buf.append(", ");}JdbcSqlStatValue sql = sqlList.get(i);buf.append("runningSqlCount ").append(sql.getRunningCount());buf.append(" : ");buf.append(sql.getSql());}String errorMessage = buf.toString();if (createError != null) {throw new GetConnectionTimeoutException(errorMessage, createError);} else {throw new GetConnectionTimeoutException(errorMessage);}}holder.incrementUseCount();DruidPooledConnection poolalbeConnection = new DruidPooledConnection(holder);return poolalbeConnection;}
publicDruidPooledConnection getConnection(long maxWaitMillis) throws SQLException {init(); //首先初始化if (filters.size() > 0) {FilterChainImpl filterChain = new FilterChainImpl(this);return filterChain.dataSource_connect(this, maxWaitMillis);} else {return getConnectionDirect(maxWaitMillis);}}
