Using log4j 2.10.0 with Java 9 System.Logger

Question

How do I setup Java 9 so that System.Logger instances write to log4j 2.10.0, instead of writing to JUL?

Soria

As far as I know, there are no log4j bindings for System.Logger yet. You have to write a service provider manually. – Ellinger

@Ellinger The log4j 2.9 release notes mention that java.util.ServiceLoader can now locate log4j, and java.lang.System.LoggerFinder javadocs say that java.util.ServiceLoader loads the logging implementation… – Soria

Answer 1

This is the paragraph you refer to:

This release contains the first support of Java 9 as well as bugfixes and minor enhancements. The Log4j API was modified to use java.util.ServiceLoader to locate Log4j implementations, although the former binding mechanism is still supported. The Log4j jar is now a multi-release jar to provide implementations of the Java 9 specific classes.

It doesn't mention System.LoggerFinder and indeed a full-text search of the repo (of version 2.10) didn't turn up any mention of "LoggerFinder" and nothing promising for "java.lang.System". I'm hence sure that Log4J 2.10 does not yet integrate with JEP 264: Platform Logging API and Service.

What the release notes seem to refer to instead is that Log4J now uses the ServiceLoader API when looking for implementations of its own API.

Do it yourself

If you really need/want Log4J to log these messages, you can roll your own adapter as I did here for SLF4J. All you need are Log4J adapters for System.Logger and System.LoggerFinder (both straightforward to implement if you skimp on some of the details) and either a META-INF/services file or a module declaration like the following:

module org.slf4j.platform {
    // put the right module name here
    requires log4j;
    provides java.lang.System.LoggerFinder
        // put the right class name here
        with org.log4J.Log4JSystemLoggerFinder;
}

Launching your application with that artifact will then lead to Log4J getting platform log messages.

Phraseologist

Nicolai, your analysis is correct. Log4j2 doesn’t integrate yet with JEP 264. This should be added to the roadmap. Would you min raising this as a feature request on the Log4j2 JIRA issue tracker? – Sufficient

Turns out, there has been one since April 2016. 😊 – Phraseologist

@Nicolai Did you consider to push this upstream to SLF4J? – Pismire

I subscribed to the mailing list in January and then... did nothing. I took your comment as inspiration to finally send a proposal. Thanks. 👍 – Phraseologist

Answer 2

Log4j 2 supports JDK platform logging since version 2.13.2. If you want to use System.Logger with Log4j 2 as a backend, you need to use log4j-jpl adapter:

<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-api</artifactId>
    <version>${replace.with.right.version}</version>
    <scope>runtime</scope>
</dependency>
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>${replace.with.right.version}</version>
    <scope>runtime</scope>
</dependency>
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-jpl</artifactId>
    <version>${replace.with.right.version}</version>
    <scope>runtime</scope>
</dependency>

After that, your System.Logger will redirect logs to Log4j:

public final class Main {
    private static final Logger LOGGER = System.getLogger("");

    public static void main(String[] args) {
        LOGGER.log(Level.ERROR, "Hello, {}!", "user");
    }
}

Output:

11:02:44.736 [main] ERROR  - Hello, user!

Ellinger

This answer should be accepted by the question owner. This is the right way to do. – Gusella

Answer 3

Why not simply use Log4j2’s Java util logging adapter to route all JUL logging to Log4j2?

To enable this, set system property java.util.logging.manager to org.apache.logging.log4j.jul.LogManager and add log4j-jul to your dependencies.

Sufficient