Spring中使用C3P0数据源需要导入2个——3个jar包
mchange-commons-java-0.2.19.jar这是c3p0数据库连接池的辅助包，如果没有这个包系统启动时会报classnotfoundexception，这是更新c3p0-0.9.2版本后分离出来的包，0.9.1的时候还是只是一个包。
如果没有导入这个辅助包，配置bean时，不报错，但是获取容器实例化bean时会报错；
xml配置c3p0：
<bean id="datasource" class="com.mchange.v2.c3p0.ComboPooledDataSource">
<property name="driverClass" value="com.mysql.jdbc.Driver"/>
<property name="user" value="root"/>
<property name="password" value=""/>
<property name="jdbcUrl" value="jdbc:mysql://localhost:3306/study"/>
</bean>
测试代码：
@Test
public void testDataSource() throws SQLException {
ConfigurableApplicationContext ac = new ClassPathXmlApplicationContext("ioc3.xml");
DataSource dataSource = ac.getBean("datasource",DataSource.class);
System.out.println(dataSource);
}
报错信息：
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'datasource' defined in class path resource [ioc3.xml]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [com.mchange.v2.c3p0.ComboPooledDataSource]: No default constructor found; nested exception is java.lang.NoClassDefFoundError: com/mchange/v2/ser/Indirector
//翻译大概意思就是：创建名为'datasource'的bean时出错，该bean在类路径资源[ioc3.xml]中定义：实例化bean失败，为找到默认构造函数
//但是ComboPooledDataSource是有无参构造的，所以可能无法找到准确的错误原因
导入辅助包后正常获取到DataSource:
结果：
信息: Initializing c3p0-0.9.5.5 [built 11-December-2019 22:07:46 -0800; debug? true; trace: 10]
com.mchange.v2.c3p0.ComboPooledDataSource[ identityToken -> 1b61iobahze2bpj1a5zcgw|7fc229ab, dataSourceName -> 1b61iobahze2bpj1a5zcgw|7fc229ab ]
Process finished with exit code 0
