今天在跑项目的时候，登入系统遇到这个异常，于是就想着跟大家分享一下，如何解决此问题？****
如下：是抛出的异常源码：
Communications link failure due to underlying exception: ** BEGIN NESTED EXCEPTION **
java.net.ConnectException MESSAGE: Connection refused: connect STACKTRACE: java.net.ConnectException: Connection refused: connect at java.net.DualStackPlainSocketImpl.connect0(Native Method) at
java.net.DualStackPlainSocketImpl.socketConnect(Unknown Source) at
java.net.AbstractPlainSocketImpl.doConnect(Unknown Source) at
java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source) at
java.net.AbstractPlainSocketImpl.connect(Unknown Source) at java.net.PlainSocketImpl.connect(Unknown Source) at java.net.SocksSocketImpl.connect(Unknown Source) at java.net.Socket.connect(Unknown Source) at
java.net.Socket.connect(Unknown Source) at java.net.Socket.(Unknown Source) at java.net.Socket.(Unknown
Source) at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:256) at
com.mysql.jdbc.MysqlIO.(MysqlIO.java:271) at com.mysql.jdbc.Connection.createNewIO(Connection.java:2771) at
com.mysql.jdbc.Connection.(Connection.java:1555) at
com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:285) at
java.sql.DriverManager.getConnection(Unknown Source) at java.sql.DriverManager.getConnection(Unknown Source) at org.logicalcobwebs.proxool.DefaultConnectionBuilder.buildConnection(DefaultConnectionBuilder.java:39)
at org.logicalcobwebs.proxool.Prototyper.buildConnection(Prototyper.java:159) at
org.logicalcobwebs.proxool.ConnectionPool.getConnection(ConnectionPool.java:211) at org.logicalcobwebs.proxool.ProxoolDriver.connect(ProxoolDriver.java:89) at
java.sql.DriverManager.getConnection(Unknown Source) at java.sql.DriverManager.getConnection(Unknown Source) at org.hibernate.connection.ProxoolConnectionProvider.getConnection(ProxoolConnectionProvider.java:75)
at org.hibernate.jdbc.ConnectionManager.openConnection(ConnectionManager.java:446) at
org.hibernate.jdbc.ConnectionManager.getConnection(ConnectionManager.java:167) at
org.hibernate.jdbc.JDBCContext.connection(JDBCContext.java:142) at
org.hibernate.transaction.JDBCTransaction.begin(JDBCTransaction.java:85) at
org.hibernate.impl.SessionImpl.beginTransaction(SessionImpl.java:1463) at org.springframework.orm.hibernate3.HibernateTransactionManager.doBegin(HibernateTransactionManager.java:555) at
org.springframework.transaction.support.AbstractPlatformTransactionManager.getTransaction(AbstractPlatformTransactionManager.java:371) at
org.springframework.transaction.interceptor.TransactionAspectSupport.createTransactionIfNecessary(TransactionAspectSupport.java:335) at
org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:105) at
org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172) at
org.springframework.aop.interceptor.ExposeInvocationInterceptor.invoke(ExposeInvocationInterceptor.java:89) at
org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:172) at
org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:202) at com.sun.proxy.$Proxy11.selectUserbByusernameByPassword(Unknown Source) at
manage.action.ManageAction.login(ManageAction.java:114) at
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) at
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) at java.lang.reflect.Method.invoke(Unknown
Source) at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:452) at
com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:291) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:254) at
com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:263) at
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68) at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:133) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207) at
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207) at
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java
:141) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:142) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:166) at
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:190) at
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:18
7) at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248) at
org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52) at
org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:485) at
org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77) at
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:9
1) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) at
org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) at
org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java
:198) at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76) 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:478) at
org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140) at
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:80) at
org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624) 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:799) at
org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) at
org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868) at
org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1455) at
org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) at
java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) at
java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) at
org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) at
java.lang.Thread.run(Unknown Source) ** END NESTED EXCEPTION ** Last packet sent to the server was 1 ms ago.
Cannot open connection
Could not open Hibernate Session for transaction; nested exception is org.hibernate.exception.JDBCConnectionException: Cannot open connection
File: com/mysql/jdbc/Connection.java
Line number: 2,847
###要在异常中能准确查找到关键问题所在位置，而不是全篇从头看到尾！
解决 ：
1、检查mysql服务是否启动；
win+R 输入：services.msc将mysql的服务启动后问题解决。
或者：右键 <计算机>---->>>管理—>>>服务和应用程序---->>>服务，在右侧的栏目中找到名称为MYSQL的服务。这个时候他的状态应该是空，即未启动，启动此服务就OK了。
如图：
2、检查mysql的登陆用户名和密码是否正确；
小伙伴，你们学会了吗？
