Add Log4J2 PropertySource backed by the Spring Environment

Register a new `PropertySource` when initializing Log4j2 so that
properties may be resolved against Spring's Environment.

See gh-32733

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystem.java

@@ -234,6 +234,7 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 		}
 		Environment environment = initializationContext.getEnvironment();
 		getLoggerContext().putObjectIfAbsent(ENVIRONMENT_KEY, environment);
+		PropertiesUtil.getProperties().addPropertySource(new SpringPropertySource(environment));
 		loggerContext.getConfiguration().removeFilter(FILTER);
 		super.initialize(initializationContext, configLocation, logFile);
 		markAsInitialized(loggerContext);

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

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2012-2022 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.logging.log4j2;
+
+import org.apache.logging.log4j.util.PropertySource;
+
+import org.springframework.core.env.Environment;
+
+/**
+ * Returns properties from Spring.
+ *
+ * @author Ralph Goers
+ * @since 3.0.0
+ */
+public class SpringPropertySource implements PropertySource {
+
+	private static final int DEFAULT_PRIORITY = -100;
+
+	private final Environment environment;
+
+	public SpringPropertySource(Environment environment) {
+		this.environment = environment;
+	}
+
+	/**
+	 * System properties take precedence followed by properties in Log4j properties files.
+	 * @return this PropertySource's priority.
+	 */
+	@Override
+	public int getPriority() {
+		return DEFAULT_PRIORITY;
+	}
+
+	@Override
+	public String getProperty(String key) {
+		if (this.environment != null) {
+			return this.environment.getProperty(key);
+		}
+		return null;
+	}
+
+	@Override
+	public boolean containsProperty(String key) {
+		if (this.environment != null) {
+			return this.environment.containsProperty(key);
+		}
+		return false;
+	}
+
+}