Short Answer 
The components log4j-slf4j-impl and log4j-to-slf4j serve different roles in Java logging; the former routes SLF4J API calls to Log4j, while the latter directs Log4j calls to the SLF4J API. Using both together creates a conflict with circular dependencies and logging confusion, so it’s important to select only one based on the project’s logging needs.
Step 1: Understand the Purpose of Each Component
The terms log4j-slf4j-impl and log4j-to-slf4j represent two different types of logging components within Java projects. It’s crucial to recognize that:
- log4j-slf4j-impl acts as an adapter to redirect SLF4J API calls to Log4j.
- log4j-to-slf4j serves as a bridge to route Log4j logging calls to the SLF4J API.
Understanding their individual roles helps clarify why they should not be used together.
Step 2: Identify the Conflict
Using both log4j-slf4j-impl and log4j-to-slf4j together creates a circular dependency problem. This occurs because:
- Both components redirect logging calls in opposite directions.
- Including both can lead to confusion in logging behavior.
- It complicates the logging configuration, potentially causing runtime errors.
Recognizing this conflict is key to maintaining a functional logging system.
Step 3: Choose the Correct Tool for Your Project
To avoid issues, you must choose only one component based on your needs:
- If you want to handle Log4j calls with SLF4J, include log4j-to-slf4j.
- If you prefer SLF4J calls to be handled by Log4j, opt for log4j-slf4j-impl.
This careful selection based on project requirements will help maintain an efficient logging structure without conflicts.