ConnectionPool
                              |---------------------------| 
                              |                           |
       getConnection          |                           |    
      (notEmpty.await)        |                           |   
      (lowWater.signal)       |                           |
      (maxActive.await)       |                           |
  <-------------------------- |                           |
  <-------------------------- |                           |
  <-------------------------- |                           |
                              |                           |
                              |                           |
  --------------------------> |                           |
  --------------------------> |                           |    销毁多余连接的线程
  --------------------------> |                           |    (highWater.await, idleTimeout.await)
    close                     |                           | --------------------------------------> 
    (highWater.signal)        |                           |
    (maxActive.signal)        |                           |  
                              |                           |
                              |                           |
              产生连接的线程     |                           |
     (lowWater.await)         |                           |
     (notEmpty.signal)        |                           |
  --------------------------> |                           |
                              |                           |
                              |---------------------------|
                              
五个Condition：notEmpty、maxActive、lowWater、highWater, idleTime