Refine 'No log4j2 configuration file found' hiding

Refine commit 1e9e1b04 since it unfortunately had the side-effect of
disabling logging when a valid `log4j2.xml` file was found.

The updated code uses a similar technique but instead of accepting
'*' as a file type it now looks for `.springboot` (which it finds
because we ship a `log4j2.springboot` file in out JAR.

Rather than exiting Log4J2's `ConfigurationFactory.Factory`
`getConfiguration` method early, our `SpringBootConfigurationFactory`
is now only triggered on the last call to `getConfiguration`.

Closes gh-4809

spring-boot/src/main/java/org/springframework/boot/logging/log4j2/SpringBootConfigurationFactory.java

@@ -16,8 +16,6 @@
 
 package org.springframework.boot.logging.log4j2;
 
-import java.net.URI;
-
 import org.apache.logging.log4j.core.LoggerContext;
 import org.apache.logging.log4j.core.config.Configuration;
 import org.apache.logging.log4j.core.config.ConfigurationFactory;
@@ -26,42 +24,38 @@ import org.apache.logging.log4j.core.config.DefaultConfiguration;
 import org.apache.logging.log4j.core.config.Order;
 import org.apache.logging.log4j.core.config.plugins.Plugin;
 
+import org.springframework.boot.logging.LoggingSystem;
+
 /**
  * Spring Boot {@link ConfigurationFactory} that prevents logger warnings from being
- * printed when the application first starts.
+ * printed when the application first starts. This factory is ordered last and is
+ * triggered by a {@code log4j2.springboot} classpath resource (which is bundled in this
+ * jar). If the {@link Log4J2LoggingSystem} is active, a {@link DefaultConfiguration} is
+ * returned with the expectation that the system will later re-initialize Log4J2 with the
+ * correct configuration file.
  *
  * @author Phillip Webb
  * @since 1.5.0
  */
 @Plugin(name = "SpringBootConfigurationFactory", category = ConfigurationFactory.CATEGORY)
-@Order(4) // Order behind XmlConfigurationFactory
+@Order(0)
 public class SpringBootConfigurationFactory extends ConfigurationFactory {
 
-	private static final String[] ALL_TYPES = { "*" };
+	private static final String[] TYPES = { ".springboot" };
 
 	@Override
 	protected String[] getSupportedTypes() {
-		return ALL_TYPES;
+		return TYPES;
 	}
 
 	@Override
-	public Configuration getConfiguration(LoggerContext loggerContext, String name,
-			URI configLocation) {
-		if (configLocation == null) {
+	public Configuration getConfiguration(LoggerContext loggerContext,
+			ConfigurationSource source) {
+		if (source != null && source != ConfigurationSource.NULL_SOURCE) {
+			if (LoggingSystem.get(loggerContext.getClass().getClassLoader()) != null) {
 				return new DefaultConfiguration();
 			}
-		return null;
 		}
-
-	@Override
-	public Configuration getConfiguration(LoggerContext loggerContext, String name,
-			URI configLocation, ClassLoader loader) {
-		return null;
-	}
-
-	@Override
-	public Configuration getConfiguration(LoggerContext loggerContext,
-			ConfigurationSource source) {
 		return null;
 	}
 

spring-boot/src/main/resources/log4j2.springboot

+See SpringBootConfigurationFactory