How to know which tomcat version embedded in spring boot

You can also check the version without leaving your IDE by seeing the effective pom.

For example, if you are using IntelliJ you can view effective pom by right clicking pom.xml > Maven > Show effective POM.

...or from the command line by issuing mvn help:effective-pom

You can look at http://mvnrepository.com/:

http://mvnrepository.com/artifact/org.springframework.boot/spring-boot-starter-tomcat/1.2.5.RELEASE

Below you have section Compile Dependencies and you can see that it uses Tomcat 8.0.23.

Via http://search.maven.org/, in https://repo1.maven.org/maven2/org/springframework/boot/spring-boot-dependencies/1.2.5.RELEASE/spring-boot-dependencies-1.2.5.RELEASE.pom:

<tomcat.version>8.0.23</tomcat.version>

or For Gradle print the dependepency tree via the console with

./gradlew dependencies

Example snippet from output:

...
|    +--- org.springframework.boot:spring-boot-starter-tomcat:2.1.0.RELEASE
|    |    +--- javax.annotation:javax.annotation-api:1.3.2
|    |    +--- org.apache.tomcat.embed:tomcat-embed-core:9.0.12
|    |    +--- org.apache.tomcat.embed:tomcat-embed-el:9.0.12
|    |    \--- org.apache.tomcat.embed:tomcat-embed-websocket:9.0.12
|    |         \--- org.apache.tomcat.embed:tomcat-embed-core:9.0.12
...

In my example above it is tomcat version 9.0.12