在类SpringbootdemoApplication上右键Run as选择Spring Boot App后Console输出报错日志如下：

com.mongodb.MongoSocketOpenException: Exception opening socket
at com.mongodb.connection.SocketStream.open(SocketStream.java:63) ~[mongodb-driver-core-3.4.2.jar:na]
at com.mongodb.connection.InternalStreamConnection.open(InternalStreamConnection.java:115) ~[mongodb-driver-core-3.4.2.jar:na]
at com.mongodb.connection.DefaultServerMonitor$ServerMonitorRunnable.run(DefaultServerMonitor.java:113) ~[mongodb-driver-core-3.4.2.jar:na]
at java.lang.Thread.run(Unknown Source) [na:1.8.0_91]
Caused by: java.net.ConnectException: Connection refused: connect
at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method) ~[na:1.8.0_91]
at java.net.DualStackPlainSocketImpl.socketConnect(Unknown Source) ~[na:1.8.0_91]
at java.net.AbstractPlainSocketImpl.doConnect(Unknown Source) ~[na:1.8.0_91]
at java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source) ~[na:1.8.0_91]
at java.net.AbstractPlainSocketImpl.connect(Unknown Source) ~[na:1.8.0_91]
at java.net.PlainSocketImpl.connect(Unknown Source) ~[na:1.8.0_91]
at java.net.SocksSocketImpl.connect(Unknown Source) ~[na:1.8.0_91]
at java.net.Socket.connect(Unknown Source) ~[na:1.8.0_91]
at com.mongodb.connection.SocketStreamHelper.initialize(SocketStreamHelper.java:57) ~[mongodb-driver-core-3.4.2.jar:na]
at com.mongodb.connection.SocketStream.open(SocketStream.java:58) ~[mongodb-driver-core-3.4.2.jar:na]
… 3 common frames omitted 

上网百度，找到一篇文章说原因是springboot自动配置了支持mongodb。在启动springboot时会自动实例化一个mongo实例，需要禁用自动配置 ，

增加@SpringBootApplication(exclude = MongoAutoConfiguration.class)这个注解即可，原文链接为http://blog.csdn.net/xuyw10000/article/details/72918360。

我增加在类上增加注解后，这个错误解决了，但是输出了新的错误日志，如下：

java.lang.ClassCastException: org.springframework.boot.context.event.ApplicationFailedEvent cannot be cast to org.springframework.boot.web.context.WebServerInitializedEvent
at org.springframework.context.event.SimpleApplicationEventMulticaster.invokeListener(SimpleApplicationEventMulticaster.java:159) [spring-context-5.0.0.RC3.jar:5.0.0.RC3]
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:139) [spring-context-5.0.0.RC3.jar:5.0.0.RC3]
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:127) [spring-context-5.0.0.RC3.jar:5.0.0.RC3]
at org.springframework.boot.context.event.EventPublishingRunListener.finished(EventPublishingRunListener.java:114) [spring-boot-2.0.0.M3.jar:2.0.0.M3]
at org.springframework.boot.SpringApplicationRunListeners.callFinishedListener(SpringApplicationRunListeners.java:79) [spring-boot-2.0.0.M3.jar:2.0.0.M3]
at org.springframework.boot.SpringApplicationRunListeners.finished(SpringApplicationRunListeners.java:72) [spring-boot-2.0.0.M3.jar:2.0.0.M3]
at org.springframework.boot.SpringApplication.handleRunFailure(SpringApplication.java:803) [spring-boot-2.0.0.M3.jar:2.0.0.M3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:338) [spring-boot-2.0.0.M3.jar:2.0.0.M3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1245) [spring-boot-2.0.0.M3.jar:2.0.0.M3]
at org.springframework.boot.SpringApplication.run(SpringApplication.java:1233) [spring-boot-2.0.0.M3.jar:2.0.0.M3]
at com.example.demo.SpringbootdemoApplication.main(SpringbootdemoApplication.java:22) [classes/:na]，

复制第一行错误日志，在谷歌上搜索答案，需要该项目的pom.xml文件注释Mongo相关的启动语句，如下：

<!– <dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-data-mongodb</artifactId>
</dependency> –>

保存后重新启动，不会再报错。在github原文链接为：https://github.com/spring-projects/spring-boot/issues/10047。