Question
I have a Java application using Logback as implementation for logging. I want to change log level at runtime for a given Java package or a given Java class without restarting my Java application. Is it possible? In particular, I am interested in enable the DEBUG level logs for a given package a short period of time so that I can troubleshoot issues in production.
Answer
One possible solution is to retrieve the logger from logger context and modify its log level. Let’s say that you are uring SLF4J as interface and Logback as implementation, you can do that as follows:
import ch.qos.logback.classic.Level;
import ch.qos.logback.classic.Logger;
import ch.qos.logback.classic.LoggerContext;
public class LogbackLevelChanger {
public static void setLevel(String name, Level level) {
var context = (LoggerContext) LoggerFactory.getILoggerFactory();
var logger = context.getLogger(name);
if (logger != null) {
logger.setLevel(level);
} else {
// handle missing logger here
}
}
}
Then you can expose this method to your RESTful API so that the log level can be changed remotely at runtime:
@PUT
@Path("/log-level/{name}/{level}")
public void setLogLevel(
@PathParam("name") String name,
@PathParam("level") String level) {
...
}
If you are using this technique for troubleshooting, you may consider adding a duration for the expiry so that the log level can be reset after the given duration. This avoid creating additional logs and potentially increase the fee for logging.
Going Further
The solution described above is not the only solution. Milind Deobhankar described 5 ways to change log levels at runtime in his post, including:
- Using the Spring Boot Admin
- Scan the logback.xml
- Access logback.xml via URL
- Creating API To Change the Log Levels (what we discussed in this post)
- Pub / Sub – the application instance needs to subscribe to the topic where log level changes will get published.
It’s worth reading if you need more options.
Hope you enjoy this article, see you the next time!
References
- “Logback Project”, Logback, 2021.
- Milind Deobhankar, “5 ways to change log levels at runtime”, DZone, 2020.