Tomcat JDBC DataSource組態注意

在Tomcat中設定server.xml來組態設定一或多組DataSource，除了可以從JNDI位置取得該資料來源外，還有Connection Pooling的優點。
但關於Connection Pool設定若是不當，輕則效能不彰、重則導致無法取得資料庫連線而造成程式錯誤。
下面是Tomcat 官方文件關於在server.xml設定DataSource的範例：

---

<Context path="/DBTest" docBase="DBTest"
debug="5" reloadable="true" crossContext="true">
<!-- maxActive: Maximum number of dB connections in pool. Make sure you
configure your mysqld max_connections large enough to handle
all of your db connections. Set to 0 for no limit.
-->
<!-- maxIdle: Maximum number of idle dB connections to retain in pool.
Set to -1 for no limit. See also the DBCP documentation on this
and the minEvictableIdleTimeMillis configuration parameter.
-->
<!-- maxWait: Maximum time to wait for a dB connection to become available
in ms, in this example 10 seconds. An Exception is thrown if
this timeout is exceeded. Set to -1 to wait indefinitely.
-->
<!-- username and password: MySQL dB username and password for dB connections -->
<!-- driverClassName: Class name for the old mm.mysql JDBC driver is
org.gjt.mm.mysql.Driver - we recommend using Connector/J though.
Class name for the official MySQL Connector/J driver is com.mysql.jdbc.Driver.
-->

<!-- url: The JDBC connection url for connecting to your MySQL dB.
The autoReconnect=true argument to the url makes sure that the
mm.mysql JDBC Driver will automatically reconnect if mysqld closed the
connection. mysqld by default closes idle connections after 8 hours.
-->
<Resource name="jdbc/TestDB" auth="Container" type="javax.sql.DataSource"
maxActive="100" maxIdle="30" maxWait="10000"
username="javauser" password="javadude" driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost:3306/javatest?autoReconnect=true"/>
</context>

---

其中注意兩個參數：

• maxActive: 代表Pool可容納Connection的最大數量
• maxIdlel：代表閒置時保留在Pool的Connection數量

親身經歷的事--在連線數使用超過maxActive時，後面取得Connection都為null，導致無法取得資料，造成程式錯誤！