Many Apache projects support interaction with a relational database. Creating a new connection for each user can be time consuming (often requiring multiple seconds of clock time), in order to perform a database transaction that might take milliseconds. Opening a connection per user can be unfeasible in a publicly-hosted Internet application where the number of simultaneous users can be very large. Accordingly, developers often wish to share a "pool" of open connections between all of the application's current users. The number of users actually performing a request at any given time is usually a very small percentage of the total number of active users, and during request processing is the only time that a database connection is required. The application itself logs into the DBMS, and handles any user account issues internally.
——摘自 Apache Commons DBCP
1.需要数据库交互
2.建立连接时间(s)远超事务执行时间(ms)。
3.对于大访问量web应用，很难为每个用户都建立连接。
基于以上，开发者想为应用下当前所有用户共享一个数据库连接池。
实际上在任意给定时间，实际执行请求的用户数所占活跃用户数的比例是很小的，只有当执行请求的时候才会用到数据库连接。