关于c3p0报错：An attempt by a client to checkout a Connection has timed out

 An attempt by a client to checkout a Connection has timed out：

   网上关于这个异常的处理方式有多种，最常见的是关于连接池的问题，但是并不是所有的问题都是关于连接池，像已经正常使用过的项目抛出这个异常很大可能是连接池的问题，我的错误并不属于连接池的问题，是因为没有加一个关键的依赖：

mysql:mysql-connector-java:6.0.3

 版本随意，

 结果：

错误代码如下： 

2018-09-03 11:24:11.984 ERROR 27564 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] 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: An attempt by a client to checkout a Connection has timed out.
### The error may exist in com/hsm/ls/collection/web/mapper/engine/UserSimpleSessionLoggerMapper.java (best guess)
### The error may involve com.hsm.ls.collection.web.mapper.engine.UserSimpleSessionLoggerMapper.getString
### The error occurred while executing a query
### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.] with root cause

com.mchange.v2.resourcepool.TimeoutException: A client timed out while waiting to acquire a resource from com.mchange.v2.resourcepool.BasicResourcePool@70387997 -- timeout at awaitAvailable()
	at com.mchange.v2.resourcepool.BasicResourcePool.awaitAvailable(BasicResourcePool.java:1317) ~[c3p0-0.9.1.2.jar:0.9.1.2]
	at com.mchange.v2.resourcepool.BasicResourcePool.prelimCheckoutResource(BasicResourcePool.java:557) ~[c3p0-0.9.1.2.jar:0.9.1.2]
	at com.mchange.v2.resourcepool.BasicResourcePool.checkoutResource(BasicResourcePool.java:477) ~[c3p0-0.9.1.2.jar:0.9.1.2]
	at com.mchange.v2.c3p0.impl.C3P0PooledConnectionPool.checkoutPooledConnection(C3P0PooledConnectionPool.java:525) ~[c3p0-0.9.1.2.jar:0.9.1.2]
	at com.mchange.v2.c3p0.impl.AbstractPoolBackedDataSource.getConnection(AbstractPoolBackedDataSource.java:128) ~[c3p0-0.9.1.2.jar:0.9.1.2]
	at org.springframework.jdbc.datasource.DataSourceUtils.doGetConnection(DataSourceUtils.java:111) ~[spring-jdbc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
	at org.springframework.jdbc.datasource.DataSourceUtils.getConnection(DataSourceUtils.java:77) ~[spring-jdbc-4.3.12.RELEASE.jar:4.3.12.RELEASE]
	at org.mybatis.spring.transaction.SpringManagedTransaction.openConnection(SpringManagedTransaction.java:82) ~[mybatis-spring-1.2.3.jar:1.2.3]
	at org.mybatis.spring.transaction.SpringManagedTransaction.getConnection(SpringManagedTransaction.java:68) ~[mybatis-spring-1.2.3.jar:1.2.3]
	at org.apache.ibatis.executor.BaseExecutor.getConnection(BaseExecutor.java:271) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.executor.SimpleExecutor.prepareStatement(SimpleExecutor.java:69) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.executor.SimpleExecutor.doQuery(SimpleExecutor.java:56) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(BaseExecutor.java:259) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.executor.BaseExecutor.query(BaseExecutor.java:132) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:105) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.executor.CachingExecutor.query(CachingExecutor.java:81) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:104) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:98) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:62) ~[mybatis-3.2.3.jar:3.2.3]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_161]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_161]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_161]
	at org.mybatis.spring.SqlSessionTemplate$SqlSessionInterceptor.invoke(SqlSessionTemplate.java:386) ~[mybatis-spring-1.2.3.jar:1.2.3]
	at com.sun.proxy.$Proxy47.selectOne(Unknown Source) ~[na:na]
	at org.mybatis.spring.SqlSessionTemplate.selectOne(SqlSessionTemplate.java:165) ~[mybatis-spring-1.2.3.jar:1.2.3]
	at org.apache.ibatis.binding.MapperMethod.execute(MapperMethod.java:63) ~[mybatis-3.2.3.jar:3.2.3]
	at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:43) ~[mybatis-3.2.3.jar:3.2.3]
	at com.sun.proxy.$Proxy61.getString(Unknown Source) ~[na:na]
	at com.hsm.ls.collection.web.controller.IndexController.getResult(IndexController.java:42) ~[classes/:na]
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_161]
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_161]
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_161]
	at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_161]
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:209) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:136) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:102) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:870) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:776) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:991) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:925) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:978) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:870) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:635) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:855) ~[spring-webmvc-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:742) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52) ~[tomcat-embed-websocket-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.springframework.web.filter.HttpPutFormContentFilter.doFilterInternal(HttpPutFormContentFilter.java:109) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:81) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107) ~[spring-web-5.0.4.RELEASE.jar:5.0.4.RELEASE]
	at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:199) ~[tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:496) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:81) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:342) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:803) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:790) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1459) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) [na:1.8.0_161]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) [na:1.8.0_161]
	at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-8.5.28.jar:8.5.28]
	at java.lang.Thread.run(Thread.java:748) [na:1.8.0_161]