Use Spring boot application properties in log4j2.xml

Question

I am working on a web application based on spring boot and want to use log4j2 as the logger implementation.
Everything works fine with the logging configuration defined in a log4j2-spring.xml file.

What is not working: I want to use property placeholders in the log4j2-spring.xml file that should be resolved from properties defined in the application.yml file used for configuring spring boot.

Is this possible? If yes, how?

Answer 1

Direct substitution of properties in log4j2-spring.xml via property placeholder is not possible as the log4j2-spring.xml is outside the ambit of Spring, and used purely for configuration purpose.

However, you can leverage the Log4j2 out-of-box feature of property substitution as outlined here.

Step 1 - Specify the property name and its variable in log4j2-spring.xml as below

<Configuration status="warn">
    <Properties>
        <Property name="someProp">${bundle:test:someKey}</Property>
    </Properties> 
    <!--other configs -->
</Configuration>

Step 2 - Use the above defined property in the log configuration e.g. suffix to log file name

<Appenders>
    <File name="file" fileName="/path/to/logs/app-${someProp}.log">
        <PatternLayout pattern="%d{yyyy-MM-dd HH:mm:ss.SSS} %-5p %-40c{1.} - %m%n"/>
    </File>
</Appenders>

Step 3 - Create a bundle (viz. properties file) to hold the properties value e.g. test.properties

# properties for log4j2
someKey=someValue
someKey1=someValue1

In your case this file will contain the values in yaml which you seek to use in log4j2 configuration. In case those properties are used in application as well, they will be duplicated in yaml and the bundle (i.e. properties file) which should be acceptable compromise given spring can not inject them in log4j2 configuration.

Let know in comments in case of any more information is required.

Answer 2

I've faced similiar problem with injecting Spring Boot YAML properties into log4j xml configuration, and I found a solution for Spring Boot 1.5.X (and probably 2.0, I didn't test it) which is a little bit hacky and operates on system properties lookup but it certainly works.

Let say you have profile "dev" in your application and some property to inject, then your application-dev.yml looks like this:

property:
    toInject: someValue

In your xml configuration log4j2-spring-dev.xml you put something like this:

<Properties>
    <property name="someProp">${sys:property.toInject}</property>
</Properties>

Now you have to somehow transfer this spring property to system property. You have to do that after application environment will be prepared and before logging system will initialize. In Spring Boot there is a listener LoggingApplicationListener, which initialize whole logging system and it's triggered by event ApplicationEnvironmentPreparedEvent, so let's create listener with order with higher precedence than LoggingApplicationListener:

public class LoggingListener implements ApplicationListener, Ordered {

@Override
public int getOrder() {
    return LoggingApplicationListener.DEFAULT_ORDER - 1;
}

@Override
public void onApplicationEvent(ApplicationEvent event) {
    if (event instanceof ApplicationEnvironmentPreparedEvent) {
        ConfigurableEnvironment environment = ((ApplicationEnvironmentPreparedEvent) event).getEnvironment();
        List<String> activeProfiles = Arrays.asList(environment.getActiveProfiles());
        if (!activeProfiles.contains("dev")) {
            return;
        }

        String someProp = environment.getProperty("property.toInject")
        validateProperty(someProp);

        System.setProperty("property.toInject", someProp);
    }
}

Now register this listener in your application:

public static void main(String[] args) {
    SpringApplication application = new SpringApplication(MyApplication.class);
    application.addListeners(new LoggingListener());
    application.run(args);
}

And that's it. Your Spring Boot properties should be "injected" in your log4j2 configuration file. This solution works with classpath properties and --spring.config.location properties. Note, it would not work with with some external configuration system like Spring Cloud Config.

Hope it helps