Failed to instantiate [java.util.List]: Specified class is an interface

[Desmond Kirrane]

I'm getting the following warning

2024-02-02 | 10:46:54.394 |  WARN | main | picocli.spring.PicocliSpringFactory | 
Unable to get bean of class interface java.util.List, using fallback factory picocli.CommandLine$DefaultFactory
(org.springframework.beans.factory.BeanCreationException: 
Error creating bean with name 'java.util.List': Failed to instantiate [java.util.List]: 
Specified class is an interface)

my guess is it's coming from the following but I'm not exactly sure.

    @Option(
            names = "-topics",
            required = true,
            split = ",",
            arity = "1..*",
            description = "List of topic names")
    List<String> topics;

I'm using

        <dependency>
            <groupId>info.picocli</groupId>
            <artifactId>picocli-spring-boot-starter</artifactId>
            <version>4.7.5</version>
        </dependency>

[Remko Popma]

Thank you for raising this.

This is strange and should not be happening. I thought I had a test for this but I'll double check. I may need more details or a minimum example to reproduce the issue.

[Remko Popma]

Sorry for the late response.

From the code, it looks like there is no problem.
The log simply reports that it was unable to create a Bean java.util.List, and after logging that warning, the call to the default factory succeeds in creating a concrete List instance.

Potential improvements:

• The log message is confusing; the message should be clearer that while Spring was unable to instantiate a List, the library will now delegate to the Picocli factory and try again. The current message suggests that the Picocli default factory was also unable to instantiate a List, which is misleading.
• Should the log level be WARN? Multi-value List and Map options will always result in these log messages being generated, so this is not a cause for concern, but business as usual. Perhaps INFO is a more appropriate log level here.