ppkarwasz commented on issue #2720:
URL:
https://github.com/apache/logging-log4j2/issues/2720#issuecomment-2217954069
@blaghed,
> After receiving your feedback and going again through the mentioned code,
I can't really find a nice way to "cache" the `LoggerContext` in order to
prevent the expensive `StackWalker` to always be called, at least not without
some hacky approach. Indeed, logback doesn't have this issue, so I should
simply use that one instead.
As @vy explained, the expensive `StackWalker` call is necessary to return a
different `Logger` instance depending on the caller of
`LoggerFactory#getLogger()`. If you eliminate the call (e.g. by switching to
Logback) you lose **all advantages** that instance loggers have over static
loggers.
The trick of using instance loggers is to manage the cost of `Logger`
creation by assigning the logger to a field:
```java
public class MyService {
private final org.slf4j.Logger log =
org.slf4j.LoggerFactory.getLogger(MyApp.class);
public void doSomething() {
log.info("Hello World!");
}
}
```
This way if both application `A` and application `B` instantiate
`MyService`, they could get different `Logger` instances, configured to log to
different appenders.
The choice of using instance loggers implies however other limitations to
your library. You can not use logging in classical "utility" classes:
```java
public final class MyUtil {
private static final Logger log = LoggerFactory.getLogger(MyUtil.class);
public static String transform(String value) {
log.debug("Transforming {}.", value);
}
```
You need to either to pass `Logger` as parameter to the utility class:
```java
public final class MyUtil {
public static String transform(String value, Logger log) {
log.debug("Transforming {}.", value);
}
```
or replace static methods with instance methods:
```java
public final class MyUtil {
private final Logger log = LoggerFactory.getLogger(MyUtil.class);
public String transform(String value) {
log.debug("Transforming {}.", value);
}
```
**Remark**: If you want to emulate Logback's behavior, you can also set the
[`log4j2.contextSelector` configuration
property](https://logging.staged.apache.org/log4j/2.x/manual/systemproperties.html#log4j2.contextSelector)
to `org.apache.logging.log4j.core.selector.BasicContextSelector`. There will
be a single logger context for the entire application, so the caller lookup
will not be necessary.
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