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As you pointed out this doesn't happen in production, where a (re)deployed war file has a separate class loader - and ZK can initialize it's static members correctly.

What you didn't mention is the way/method you are starting your spring-boot application (so I am guessing you are using spring-boot-devtools as well). I could reproduce your exception adding spring-boot-devtools to the zkspringboot-demo-jar project

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <version>${springboot.version}</version>
    <optional>true</optional>
</dependency>

Then start it the Main class from the IDE, open http://localhost:8080/mvvm (which uses i18n labels).

Then change a java class -> recompile -> which triggers spring boot to reload the application, causing the error next time you access the same url.

During startup ZK assumes to be on a clean class loader (also in other places) so that I'd not try to change the ZK code.

Instead you can configure spring-boot-devtools accordingly to include the zk-jars during an application restart/reload.

Spring docs go quite into detail on how to use the devtools

In the restart-vs-reload section they mention 2 different class loaders which can be customized to include/exclude certain jar files (8.2.6).

Following the docs adding a spring-devtools.properties to include zk jars (brute force the ones starting with z*.jar) into the "restart"-classloader fixes the problem (might be fine tuned to only certain jars)

As you pointed out this doesn't happen in production, where a (re)deployed war file has a separate class loader - and ZK can initialize it's static members correctly.

What you didn't mention is the way/method you are starting your spring-boot application (so I am guessing you are using spring-boot-devtools as well). I could reproduce your exception adding spring-boot-devtools to the zkspringboot-demo-jar project

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <version>${springboot.version}</version>
    <optional>true</optional>
</dependency>

Then start it the Main class from the IDE, open http://localhost:8080/mvvm (which uses i18n labels).

Then change a java class -> recompile -> which triggers spring boot to reload the application, causing the error next time you access the same url.

During startup ZK assumes to be on a clean class loader (also in other places) so that I'd not try to change the ZK code.

Instead you can configure spring-boot-devtools accordingly to include the zk-jars during an application restart/reload.

Spring docs go quite into detail on how to use the devtools

In the restart-vs-reload section they mention 2 different class loaders which can be customized to include/exclude certain jar files (8.2.6).

Following the docs adding a spring-devtools.properties to include zk jars (brute force the ones starting with z*.jar) into the "restart"-classloader fixes the problem (might be fine tuned to only certain jars)