求助：SSM用c3p0连接mysql时Could not get JDBC Connection

【控制台log】
严重: Servlet.service() for servlet [spring_mvc] in context with path [/P_AI] threw exception [Request processing failed; nested exception is org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:

Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database!

The error may exist in config/mybatis/mapper/CommAdminMapper.xml

The error may involve cn.java.demo.mapper.CommAdminMapper.selectAdminByInfo

The error occurred while executing a query

Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: Connections could not be acquired from the underlying database!] with root cause

com.mchange.v2.resourcepool.CannotAcquireResourceException: A ResourcePool could not acquire a resource from its primary factory or source.
at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1002)
at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:293)
at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:348)
at com.mchange.v2.c3p0.PoolBackedDataSource.getConnection(PoolBackedDataSource.java:120)
at com.mchange.v2.c3p0.ComboPooledDataSource.getConnection(ComboPooledDataSource.java:601)
at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:111)
at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:77)
at org.mybatis.spring.transaction.SpringManagedTransaction.openConnection(SpringManagedTransaction.java:82)
at org.mybatis.spring.transaction.SpringManagedTransaction.getConnection(SpringManagedTransaction.java:68)
at org.apache.ibatis.executor.BaseExecutor.getConnection(BaseExecutor.java:336)
at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:84)
at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:62)
at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:324)
at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:156)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:109)
at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:83)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:148)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:141)
at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:77)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:433)
at com.sun.proxy.$Proxy13.selectOne(Unknown Source)
at org.mybatis.spring.SqlSessionTemplate.selectOne(SqlSessionTemplate.java:166)
at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:82)
at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:59)
at com.sun.proxy.$Proxy14.selectAdminByInfo(Unknown Source)
at cn.java.demo.dao.impl.CommAdminDaoImpl.selectAdminByInfo(CommAdminDaoImpl.java:21)
at cn.java.demo.logic.impl.CommAdminLogicImpl.selectAdminByInfo(CommAdminLogicImpl.java:27)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:317)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:190)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:157)
at org.springframework.transaction.interceptor.TransactionInterceptor$1.proceedWithInvocation(TransactionInterceptor.java:98)
at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:262)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:95)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:92)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:179)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:207)
at com.sun.proxy.$Proxy18.selectAdminByInfo(Unknown Source)
at cn.java.demo.controller.CommAdminController.admin(CommAdminController.java:33)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:175)
at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:446)
at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:434)
at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:938)
at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:870)
at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961)
at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:852)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81)
at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:650)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342)
at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803)
at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790)
at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459)
at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:748)

错误原因：

　　在系统中也有个username属性，这时系统变量覆盖了Properties中的值，这时取得username的值为系统的用户名Administrator，密码为properties中的password去查询数据库，此时用户名名和密码并不匹配就会报错。在Spring完成注入时是用 “${..}” 方式获取值完成注入的。而通过这种表达式也能直接获取到JVM系统属性……….

解决方案：

　　方案一：将properties文件中的username换成user或其他就字符串就可以成功获取连接访问数据库。建议：username时敏感词汇，为了安全起见还是尽量不要使用username。

　　方案二：在Spring配置文件中修改成：

　　　　　　添加一个system-properties-mode属性

　　　　　　该属性有三个值：FALLBACK　　— 默认值，不存在时覆盖

　　　　　　　　　　　　　　NEVER　　　 — 不覆盖

　　　　　　　　　　　　　　OVERRIDE　　— 覆盖

传送门：https://blog.csdn.net/weixin_41558061/article/details/78958655

jdk11/jdk8也试过
mysql-connector-8.12.jar
spring4.0
c3p0-0.9.5.2.jar
mysql 8.0.12

【applicationContext】
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx"
xmlns:jdbc="http://www.springframework.org/schema/jdbc"
xmlns:mybatis-spring="http://mybatis.org/schema/mybatis-spring"
xmlns:jee="http://www.springframework.org/schema/jee"
xsi:schemaLocation="http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee-4.0.xsd
http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc-4.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.0.xsd
http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-4.0.xsd
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd
http://mybatis.org/schema/mybatis-spring http://mybatis.org/schema/mybatis-spring-1.2.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.0.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.0.xsd">

<!-- 扫描 -->
<context:component-scan base-package="cn.java.demo.logic.impl,cn.java.demo.dao.impl" />

<!-- 加载db.properties文件中的内容，db.properties文件中key命名要有一定的特殊规则 -->
<context:property-placeholder location="classpath:config/spring/mysql_db.properties"/>
 <!-- 创建连接池  c3p0 -->
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
    <property name="driverClass" value="${jdbc.driver}"/>
    <property name="jdbcUrl" value="${jdbc.url}"/>
    <property name="user" value="${jdbc.user}"/>
    <property name="password" value="${jdbc.password}"/>
    <property name="maxStatements" value="0" />
    <property name="initialPoolSize" value="3" />
    <property name="maxIdleTime" value="7200" />
    <!-- 一次性获取连接数，Default:3  -->  
    <property name="acquireIncrement" value="5"/>  
    <!-- 获取连接失败后，重复尝试的次数，Default:3 -->  
    <property name="acquireRetryAttempts" value="3"/>  
    <!-- 获取连接超时时间，超时后将抛出SQLException，Default:0，单位：毫秒 -->  
    <property name="checkoutTimeout" value="18000"/>  
    <!-- 多长时间检查所有连接池中的空闲连接，Default:0，单位：秒 -->  
    <property name="idleConnectionTestPeriod" value="1200"/>  
    <!-- 取得连接的同时是否校验连接的有效性，Default:false -->  
    <property name="testConnectionOnCheckin" value="true"/> 
    <property name="maxPoolSize" value="30" />
    <property name="minPoolSize" value="10" />
    <property name="autoCommitOnClose" value="false" />
</bean>

<!-- sqlSessionFactory -->
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <!-- 配置事务管理器 --> 
    <property name="dataSource" ref="dataSource" />

     <!-- 加载mybatis的全局配置文件 -->
     <property name="configLocation" value="classpath:config/mybatis/mybatis-config.xml" />
     <!-- mapper 扫描器 -->

</bean>

<!-- 事务管理器对mybatis操作数据库进行事务控制，spring使用jdbc的事务控制类 -->    
<bean id="transactionManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager">
    <!-- 数据源dataSource-->
    <property name="dataSource" ref="dataSource" />
</bean>

<!-- 通知 -->
<tx:advice id="txAdvice" transaction-manager="transactionManager">
    <tx:attributes>
        <!-- 传播行为 -->
        <tx:method name="delete*" propagation="REQUIRED"/>
        <tx:method name="insert*" propagation="REQUIRED"/>
        <tx:method name="update*" propagation="REQUIRED"/>
        <tx:method name="select*" propagation="SUPPORTS"/>
    </tx:attributes>
</tx:advice>

<!-- aop -->
<aop:config>
    <aop:advisor advice-ref="txAdvice" pointcut="execution(* cn.java.demo.logic.impl.*.*(..))"/>
</aop:config>

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
    <property name="basePackage" value="cn.java.demo.mapper" />
    <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
</bean>

【mysql_db.properties】
jdbc.driver=com.mysql.cj.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/db_ti?useUnicode=true&characterEncoding=utf8&useSSL=false&serverTimezone=UTC&autoReconnect=true
jdbc.user=root
jdbc.password=123456

db.properties中的数据库连接参数与调用若没有错误后，检查一下数据库的服务是否打开，然后确认数据库处于可用状态。若以上问题还没有解决，看这一句：Connections could not be acquired from the underlying database!（无法从数据库获取连接），数据库的问题，好好检查一下数据库是否宕机，检查一下数据库的状态，还不行的话就清理一下项目的缓存。然后重启一下主机。