项目启动报错：
1 [2016-07-13 10:04:15,074] ERROR org.apache.ibatis.executor.BaseExecutor Could not get a databaseId from dataSource 2 java.sql.SQLException: Connections could not be acquired from the underlying database! 3 at com.mchange.v2.sql.SqlUtils.toSQLException(SqlUtils.java:104) 4 at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:264) 5 at com.mchange.v2.c3p0.PoolBackedDataSource.getConnection(PoolBackedDataSource.java:94) 6 at com.mchange.v2.c3p0.ComboPooledDataSource.getConnection(ComboPooledDataSource.java:521) 7 at org.apache.ibatis.mapping.VendorDatabaseIdProvider.getDatabaseProductName(VendorDatabaseIdProvider.java:77) 8 at org.apache.ibatis.mapping.VendorDatabaseIdProvider.getDatabaseName(VendorDatabaseIdProvider.java:62) 9 at org.apache.ibatis.mapping.VendorDatabaseIdProvider.getDatabaseId(VendorDatabaseIdProvider.java:50) 10 at org.mybatis.spring.SqlSessionFactoryBean.buildSqlSessionFactory(SqlSessionFactoryBean.java:445) 11 at org.mybatis.spring.SqlSessionFactoryBean.afterPropertiesSet(SqlSessionFactoryBean.java:336) 12 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1571) 13 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1509) 14 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:521) 15 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) 16 at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:296) 17 at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223) 18 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:293) 19 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194) 20 at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:610) 21 at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:932) 22 at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:479) 23 at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:410) 24 at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:306) 25 at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:112) 26 at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4210) 27 at org.apache.catalina.core.StandardContext.start(StandardContext.java:4709) 28 at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:802) 29 at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:779) 30 at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:583) 31 at org.apache.catalina.startup.HostConfig.deployDirectory(HostConfig.java:1080) 32 at org.apache.catalina.startup.HostConfig.deployDirectories(HostConfig.java:1003) 33 at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:507) 34 at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1322) 35 at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:325) 36 at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:142) 37 at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1068) 38 at org.apache.catalina.core.StandardHost.start(StandardHost.java:822) 39 at org.apache.catalina.core.ContainerBase.start(ContainerBase.java:1060) 40 at org.apache.catalina.core.StandardEngine.start(StandardEngine.java:463) 41 at org.apache.catalina.core.StandardService.start(StandardService.java:525) 42 at org.apache.catalina.core.StandardServer.start(StandardServer.java:759) 43 at org.apache.catalina.startup.Catalina.start(Catalina.java:595) 44 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 45 at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) 46 at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 47 at java.lang.reflect.Method.invoke(Unknown Source) 48 at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:289) 49 at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:414) 50 Caused by: com.mchange.v2.resourcepool.CannotAcquireResourceException: A ResourcePool could not acquire a resource from its primary factory or source. 51 at com.mchange.v2.resourcepool.BasicResourcePool.awaitAcquire(BasicResourcePool.java:972) 52 at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:208) 53 at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:260) 54 ... 45 more
解决办法：
1、驱动配置有误：driver=com.mysql.jdbc.Driver
2、数据库连接地址有误：url=jdbc:mysql://localhost:3306/test?3useUnicode=true&characterEncoding=utf8
3、密码或帐号有误：username=root
password=root
4、数据库未启动或无权访问
5、项目未引入对应的驱动jar包mysql-connector-java-5.1.6-bin.jar
6、mysql root没有远程访问的权限，需要增加权限，增加权限的步骤如下：
进入mysql数据库：
grant all privileges on *.* to 'root'@'%' identified by 'root' with grant option;
flush privileges;
延伸：
1、在普通java项目中使用 mybatis+C3p0+spring时也遇见上述问题。
问题原因是：在xml配置文件中显示声明了自动装载 <beans default-autowire="byName">
解决方法： 把自动装载设置去掉，就没有问题了，应该使用默认的 byType
2、同一个ip在短时间内产生太多（超过mysql数据库max_connection_errors的最大值）中断的数据库连接而导致的阻塞，引发的。
解决方法： 重启数据库服务或者使用mysqladmin flush-hosts 命令清理一下hosts文件。
3、maven+spring 项目，用了org.apache.tomcat.jdbc.pool.DataSource； 报错：Could not get JDBC Connection; nested exception is java.sql.SQLException: com.mysql.jdbc.Driver
解决方法： 需要加入 tomcat-jdbc的引用