Why

JDK 8 support is ending in 2020, unless you purchase commercial license (1)
Improved Security : TLS 1.3 implementation, better SSL-validation, deprecated old ciphers,.. (2)
Faster, improved garbage collection
Default container-support, like with Docker (3)
Extra language features : like ‘var’ keyword, Optional-Stream, extra Stream-methods, handy collection-functions like Lists.of(..),…
Safer reflection-operations

How

Select your JDK version :
- Every 6 months there will be a new major version of Java. It’s best to choose a version with long time support like Java 11, 14, 17,…
- There are several commercial providers, but also free and good alternatives like : https://adoptopenjdk.net/ (4)
Update your tools :
- Maven 3.5+, Compiler 3.8+, surefire & failsafe plugins 2.22+
Change the version of Java in your project so in your pom-file
<properties><project.build.sourceEncoding>UTF-8</project.build.sourceEncoding><project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding><java.version>11</java.version><maven.compiler.source>${java.version}</maven.compiler.source><maven.compiler.target>${java.version}</maven.compiler.target></properties>
In your IDE change the version of your modules to JDK 11 and change the language level to Java 11. (In IntelliJ : short-cut F4)
The ultimate test is to use somewhere in your code the ‘var’-keyword : Recompile, run all the tests and start the application. If everythings works : the migration is a succes
As final step it is often needed to update your pipeline or CI/CD :
- The security-configuration file got moved to the config-folder of your JVM
- The command-line arguments are now easier, like -XX:+UseContainerSupport is now default

Known problems

Java EE modules got removed (6)
- Exceptions like ‘Exception in thread “main” java.lang.NoClassDefFoundError: javax/xml/bind/JAXBException’. Just include<dependency><groupId>com.sun.xml.bind</groupId><artifactId>jaxb-impl</artifactId><version>2.3.2</version></dependency>
Embedded Postges database gave some issues and this fixes it. Alternatively you can use testcontainers.<dependency><groupId>ru.yandex.qatools.embed</groupId><artifactId>postgresql-embedded</artifactId><scope>test</scope><exclusions><exclusion><groupId>de.flapdoodle.embed</groupId><artifactId>de.flapdoodle.embed.process</artifactId></exclusion></exclusions></dependency><dependency><groupId>de.flapdoodle.embed</groupId><artifactId>de.flapdoodle.embed.process</artifactId><version>2.0.5</version></dependency>
SSL-issues : they were due to more validation by Java 11. This is normally all fixed now
Exceptions where they complain about the byte code version mismatch. Just remember that byte code of Java 8 is version 52 and the byte code of Java 11 is version 55. You probably didn’t rebuild all modules or somewhere there is a wrong Java version declared. The exception is normally very verbose and easy to understand (7)

References