How to log all active properties of a spring boot application before the beans instantiation?

Question

There is already a question asking for logging the active configuration, there is a correct answer but the problem is that the configuration is logged only if all beans are correctly instantiated. I would like to log all properties even (mainly) if the application crash at startup. My question is more specific:

How to log all active properties of a spring boot application before the beans instantiation?

Solution

For doing this you need to register an ApplicationListener. The event to catch is the ApplicationPreparedEvent, according to the documentation:

ApplicationPreparedEvent is an event published when a SpringApplication is starting up and the ApplicationContext is fully prepared but not refreshed. The bean definitions will be loaded and the Environment is ready for use at this stage.

The main method would look like this:

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

I've reused the code of the answer cited in the current question but I've modified it because the context you get is not already refreshed and the structure of the environment is not exactly the same as after the startup of the application. I've also printed the properties by property sources: one for the the system environment, one for the system properties, one for the application configuration properties, etc... Note also that the ApplicationPreparedEvent can be triggered multiple times, and that properties are printed only the first time. See Spring Boot issue #8899 for details.

package com.toto.myapp.util;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.context.event.ApplicationPreparedEvent;
import org.springframework.context.ApplicationListener;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.EnumerablePropertySource;
import org.springframework.core.env.PropertySource;

import java.util.LinkedList;
import java.util.List;

public class PropertiesLogger implements ApplicationListener<ApplicationPreparedEvent> {
  private static final Logger log = LoggerFactory.getLogger(PropertiesLogger.class);

  private ConfigurableEnvironment environment;
  private boolean isFirstRun = true;

  @Override
  public void onApplicationEvent(ApplicationPreparedEvent event) {
    if (isFirstRun) {
      environment = event.getApplicationContext().getEnvironment();
      printProperties();
    }
    isFirstRun = false;
  }

  public void printProperties() {
    for (EnumerablePropertySource propertySource : findPropertiesPropertySources()) {
      log.info("******* " + propertySource.getName() + " *******");
      String[] propertyNames = propertySource.getPropertyNames();
      Arrays.sort(propertyNames);
      for (String propertyName : propertyNames) {
        String resolvedProperty = environment.getProperty(propertyName);
        String sourceProperty = propertySource.getProperty(propertyName).toString();
        if(resolvedProperty.equals(sourceProperty)) {
          log.info("{}={}", propertyName, resolvedProperty);
        }else {
          log.info("{}={} OVERRIDDEN to {}", propertyName, sourceProperty, resolvedProperty);
        }
      }
    }
  }

  private List<EnumerablePropertySource> findPropertiesPropertySources() {
    List<EnumerablePropertySource> propertiesPropertySources = new LinkedList<>();
    for (PropertySource<?> propertySource : environment.getPropertySources()) {
      if (propertySource instanceof EnumerablePropertySource) {
        propertiesPropertySources.add((EnumerablePropertySource) propertySource);
      }
    }
    return propertiesPropertySources;
  }
}