Understanding System Logger

I recently learned that instead of using SFL4J's API and the wanted implementation, you'd use System.Logger which is available since Java 9. Let's see how.

Nicolas Fränkel

CORE ·

Feb. 14, 22 · Tutorial

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December was not a good time for Java developers and even less for Ops. The former had to repackage their apps with a fixed Log4J's version, and the latter had to redeploy them - several times. Yet, every cloud has a silver lining. In my case, I learned about System.Logger.

In short, System.Logger is a façade over your logging engine. Instead of using, say, SFL4J's API and the wanted implementation, you'd use System.Logger instead of SLF4J. It's available since Java 9, and it's a bummer that I learned about it only recently.

System.Logger API

The API is a bit different than other logging APIs: it avoids different logging methods such as debug(), info() in favor of a single log() one where you pass a logging Level parameter.

If you don't provide any corresponding implementation on the classpath, System.Logger defaults to JUL.

    Java
   
   public class LoggerExample {

  private static final System.Logger LOGGER = System.getLogger("c.f.b.DefaultLogger"); // 1

  public static void main(String[] args) {
      LOGGER.log(DEBUG, "A debug message");
      LOGGER.log(INFO, "Hello world!");
  }
}

Get the logger.

Running the above snippet outputs the following:

    Plain Text
   
   Dec 24, 2021 10:38:15 AM c.f.b.DefaultLogger main
INFO: Hello world!

Compatible Implementations

Most applications currently use Log4J2 or SLF4J. Both provide a compatible System.Logger implementation.

For Log4J, we need to add two dependencies:

    XML
   
 

   <dependencies>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>
        <artifactId>log4j-core</artifactId>            <!-- 1 -->
        <version>2.17.0</version>
    </dependency>
    <dependency>
        <groupId>org.apache.logging.log4j</groupId>    <!-- 2 -->
        <artifactId>log4j-jpl</artifactId>
        <version>2.17.0</version>
    </dependency>
</dependencies>
  

Log4J implementation.
Bridge from System.Logger to Log4J.

The same logging snippet as above now outputs the following:

    Plain Text
   
   11:00:07.373 [main] INFO  c.f.b.DefaultLogger - Hello world!

To use SLF4J instead, use the following dependencies:

    XML
   
 

   <dependencies>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-simple</artifactId>               <!-- 1 -->
        <version>2.0.0-alpha5</version>
    </dependency>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-jdk-platform-logging</artifactId> <!-- 2 -->
        <version>2.0.0-alpha5</version>
    </dependency>
</dependencies>
  

Basic SLF4J implementation. Any other implementation will do, e.g. Logback.
Bridge from System.Logger to Log4J.

The snippet outputs:

    Plain Text
   
   [main] INFO c.f.b.DefaultLogger - Hello world!

Your Own `System.Logger` Implementation

System.Logger relies on Java's ServiceLoader mechanism. Both log4j-jpl and slf4j-jdk-platform-logging contain a META-INF/services/java.lang.System$LoggerFinder file that points to a LoggerFinder implementation.

We can create our own based on System.out for educational purposes.
The first step is to implement the logger itself.

    Java
   
 

   public class ConsoleLogger implements System.Logger {

    private final String name;

    public ConsoleLogger(String name) {
        this.name = name;
    }

    @Override
    public String getName() {
        return name;
    }

    @Override
    public boolean isLoggable(Level level) {
        return level.getSeverity() >= Level.INFO.getSeverity();
    }

    @Override
    public void log(Level level, ResourceBundle bundle, String msg, Throwable thrown) {
        if (isLoggable(level)) {
            System.out.println(msg);
            thrown.printStackTrace();
        }
    }

    @Override
    public void log(Level level, ResourceBundle bundle, String format, Object... params) {
        if (isLoggable(level)) {
            System.out.println(MessageFormat.format(format, params));
        }
    }
}
  

Then, we need to code the System.LoggerFinder:

    Java
   
   public class ConsoleLoggerFinder extends System.LoggerFinder {

    private static final Map<String, ConsoleLogger> LOGGERS = new HashMap<>(); // 1

    @Override
    public System.Logger getLogger(String name, Module module) {
        return LOGGERS.computeIfAbsent(name, ConsoleLogger::new);              // 2
    }
}

Keep a map of all existing loggers:

Create a logger if it doesn't already exist and store it.

Finally, we create a service file:

    Properties files
   
   ch.frankel.blog.ConsoleLoggerFinder

And now, running the same code snippet outputs:

    Plain Text
   
   Hello world!

Conclusion

While the API is more limited than other more established logging APIs, System.Logger is a great idea. It offers a façade that's part of the JDK. Thus, it avoids using a third-party façade that needs to wire calls to another unrelated implementation, e.g. SLF4J to Log4J2.

For this reason, I think I'll be trying System.Logger if only to get some hands-on experience.

The complete source code for this post can be found in Maven format there.

Log4j Plain text Implementation

Published at DZone with permission of Nicolas Fränkel, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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