public DruidPooledConnection getConnectionDirect(long maxWaitMillis) throws SQLException {
// 超时重试次数,达到这个值就报错
int notFullTimeoutRetryCnt = 0;
for (; ; ) {
// handle notFullTimeoutRetry
DruidPooledConnection poolableConnection;
try {
// 核心逻辑
poolableConnection = getConnectionInternal(maxWaitMillis);
} catch (GetConnectionTimeoutException ex) {
if (notFullTimeoutRetryCnt <= this.notFullTimeoutRetryCount && !isFull()) {
notFullTimeoutRetryCnt++;
if (LOG.isWarnEnabled()) {
LOG.warn("get connection timeout retry : " + notFullTimeoutRetryCnt);
}
continue;
}
throw ex;
}

        // testOnBorrow =true 每次获取链接都会验证
        if (testOnBorrow) {
            // 发送一条validateSql SELECT 1
            boolean validate = testConnectionInternal(poolableConnection.holder, poolableConnection.conn);
            if (!validate) {
                if (LOG.isDebugEnabled()) {
                    LOG.debug("skip not validate connection.");
                }

                // 如果不可用就回收
                discardConnection(poolableConnection.holder);
                continue;
            }
        } else {
            if (poolableConnection.conn.isClosed()) {
                // 关闭链接,更新activeCount,discardCount数量增加
                discardConnection(poolableConnection.holder); // 传入null，避免重复关闭
                continue;
            }

            // 当空闲时测试下 发送一条validateSql SELECT 1,空闲是通过空闲时间与timeBetweenEvictionRunsMillis的值比较
            // 如果判断idleMillis > timeBetweenEvictionRunsMillis 那么发送一条 SELECT 1,如果是断开了则回收
            if (testWhileIdle) {
                final DruidConnectionHolder holder = poolableConnection.holder;
                long currentTimeMillis = System.currentTimeMillis();
                long lastActiveTimeMillis = holder.lastActiveTimeMillis;
                long lastExecTimeMillis = holder.lastExecTimeMillis;
                long lastKeepTimeMillis = holder.lastKeepTimeMillis;

                if (checkExecuteTime
                        && lastExecTimeMillis != lastActiveTimeMillis) {
                    lastActiveTimeMillis = lastExecTimeMillis;
                }

                if (lastKeepTimeMillis > lastActiveTimeMillis) {
                    lastActiveTimeMillis = lastKeepTimeMillis;
                }

                long idleMillis = currentTimeMillis - lastActiveTimeMillis;

                long timeBetweenEvictionRunsMillis = this.timeBetweenEvictionRunsMillis;

                if (timeBetweenEvictionRunsMillis <= 0) {
                    timeBetweenEvictionRunsMillis = DEFAULT_TIME_BETWEEN_EVICTION_RUNS_MILLIS;
                }

                // 确认是空闲线程
                if (idleMillis >= timeBetweenEvictionRunsMillis
                        || idleMillis < 0 // unexcepted branch
                ) {
                    // 发送测试语句 SELECT 1 确保回收的是有效的链接
                    boolean validate = testConnectionInternal(poolableConnection.holder, poolableConnection.conn);
                    if (!validate) {
                        if (LOG.isDebugEnabled()) {
                            LOG.debug("skip not validate connection.");
                        }

                        // 回收
                        discardConnection(poolableConnection.holder);
                        continue;
                    }
                }
            }
        }

        // 则在获取连接的时候会将当前的连接放到activeconnections集合中。然后注册一个定时任务循环遍历当前集合中的connection，发现无用的就移除。
        if (removeAbandoned) {
            StackTraceElement[] stackTrace = Thread.currentThread().getStackTrace();
            poolableConnection.connectStackTrace = stackTrace;
            poolableConnection.setConnectedTimeNano();
            poolableConnection.traceEnable = true;

            activeConnectionLock.lock();
            try {
                // IdentityHashMap key比较的是对象
                activeConnections.put(poolableConnection, PRESENT);
            } finally {
                activeConnectionLock.unlock();
            }
        }

        // 设置是否自动提交
        if (!this.defaultAutoCommit) {
            poolableConnection.setAutoCommit(false);
        }

        return poolableConnection;
    }
}