Re: [D] Help with migrating from log4j to log4j2 (we also use a slf4j -> log4j) [logging-log4j2]

GitHub user ppkarwasz added a comment to the discussion: Help with migrating from log4j to log4j2 (we also use a slf4j -> log4j)

Hi @paladox,  

### On your current approach

In your current code you **stop** `async` before creating a new one. That means:  

- For a short time no appender is available at all.  
- Even worse, the `async` field is temporarily `null`.  

A safer pattern is to build the replacement first and only swap the reference at the very end. That way you never expose a “gap” where logging calls see no appender.  

Also note: since you’re not attaching the `AsyncAppender` to any logger, there’s no need to register it with the active `LoggerContext` configuration. The only reason you need a `Configuration` is because the `AsyncAppender` builder requires it in order to look up its referenced appenders. In this case you can use your own private `NullConfiguration` instance instead of modifying the shared context.  

### Programmatic configuration options

If the goal is to combine a file-based configuration with some programmatically defined components, there are two main approaches:  

1. **CompositeConfiguration + ConfigurationBuilder (recommended)**  
   - Use [CompositeConfiguration](https://logging.apache.org/log4j/2.x/manual/customconfig.html#CompositeConfiguration) to merge your static file with a dynamically built config.  
   - Build the dynamic part using [ConfigurationBuilder](https://logging.apache.org/log4j/2.x/manual/customconfig.html#ConfigurationBuilder).  
   - Advantage: this doesn’t require instantiating or managing any Core components yourself: you’re just describing what should exist, and Log4j Core builds and wires everything.  
   - Limitation: attributes are provided as strings, so it’s not 100% type-safe, though the builder offers type-safe helpers for the common ones.  

2. **Custom ConfigurationFactory**  
   - Implement your own `ConfigurationFactory` that loads a standard config (delegating to an existing factory) and then applies programmatic modifications.  
   - Advantage: fully type-safe, since you’re working directly with configuration objects. The lifecycle is handled for you because the configuration is only started after the factory returns it.  
   - Useful if you want strict control over configuration composition without relying on string-based attributes.  



GitHub link: https://github.com/apache/logging-log4j2/discussions/3914#discussioncomment-14348063

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