Spring Beans should not be resolved from application context in scripts if custom beans have been provided in the engine configuration

Environment

• A Camunda project with version >= 7.12.0 that uses the engine-spring module (including all Spring Boot setups)

Description

• According to https://docs.camunda.org/manual/7.14/user-guide/spring-framework-integration/expressions/#limit-the-exposing-spring-beans-in-expressions, if any custom beans are defined in a process engine configuraiton, no beans should be resolved from the application context
• This documentation talks about JUEL expressions, but the same should apply to scripts to be consistent

Steps to reproduce

1. Define a custom bean with name fooBean in the engine configuration (like described in https://docs.camunda.org/manual/7.14/user-guide/spring-framework-integration/expressions/#limit-the-exposing-spring-beans-in-expressions for example)
2. Create a bean called fooBean in the Spring application context (i.e. a different object than the one in step 1)
3. Create a script task that references fooBean, e.g. with Groovy calling a method fooBean.doSomething()

Observed Behavior

• fooBean resolves to the bean from the application context (step 2)

Expected behavior

• fooBean resolves to the custom bean from the engine configuration (step 1)

Root Cause

• When using a Spring engine configuration, an additional script resolver is added to resolve script variables from the Spring application context, see https://github.com/camunda/camunda-bpm-platform/blob/7.14.0/engine-spring/core/src/main/java/org/camunda/bpm/engine/spring/SpringProcessEngineConfiguration.java#L45-L47
• In super.initScripting(), another resolver for the custom beans is registered
• If two resolvers can resolve an object with the same name, then the last resolver wins; in our case, this means that the application context resolver always wins over the custom beans resolver

Solution Ideas

• The application context resolver is registered only if no custom beans are set
• The behavior should be consistent with the behavior for expressions as defined in https://docs.camunda.org/manual/7.14/user-guide/spring-framework-integration/expressions/#limit-the-exposing-spring-beans-in-expressions, i.e.
  • If no custom beans are set, then the application context beans are exposed
  • If custom beans are set, no application context beans are exposed
  • If an empty map of custom beans is set, then also no application context beans are exposed

Hints

• See https://github.com/camunda/camunda-bpm-platform/blob/7.14.0/engine-spring/core/src/main/java/org/camunda/bpm/engine/spring/SpringExpressionManager.java#L63-L69 for where this is implemented in the case of JUEL expressions
• Note that in ProcessEngineConfigurationImpl#initBeans, the beans map is instantiated if no the user has not supplied anything => we need to distinguish this case from the case that the user has provided an empty map to get all of the cases mentioned under "Solution Ideas" right