Minimize pollution of Log4j2's environment

Closes gh-40178

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

@@ -88,6 +88,8 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 
 	private static final String LOG4J_LOG_MANAGER = "org.apache.logging.log4j.jul.LogManager";
 
+	private static final SpringEnvironmentPropertySource propertySource = new SpringEnvironmentPropertySource();
+
 	static final String ENVIRONMENT_KEY = Conventions.getQualifiedAttributeName(Log4J2LoggingSystem.class,
 			"environment");
 
@@ -214,8 +216,9 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 		}
 		Environment environment = initializationContext.getEnvironment();
 		if (environment != null) {
-			getLoggerContext().putObjectIfAbsent(ENVIRONMENT_KEY, environment);
+			getLoggerContext().putObject(ENVIRONMENT_KEY, environment);
-			PropertiesUtil.getProperties().addPropertySource(new SpringEnvironmentPropertySource(environment));
+			Log4J2LoggingSystem.propertySource.setEnvironment(environment);
+			PropertiesUtil.getProperties().addPropertySource(Log4J2LoggingSystem.propertySource);
 		}
 		loggerContext.getConfiguration().removeFilter(FILTER);
 		super.initialize(initializationContext, configLocation, logFile);
@@ -436,6 +439,8 @@ public class Log4J2LoggingSystem extends AbstractLoggingSystem {
 		LoggerContext loggerContext = getLoggerContext();
 		markAsUninitialized(loggerContext);
 		loggerContext.getConfiguration().removeFilter(FILTER);
+		Log4J2LoggingSystem.propertySource.setEnvironment(null);
+		getLoggerContext().removeObject(ENVIRONMENT_KEY);
 	}
 
 	private LoggerConfig getLogger(String name) {

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

@@ -19,7 +19,6 @@ package org.springframework.boot.logging.log4j2;
 import org.apache.logging.log4j.util.PropertySource;
 
 import org.springframework.core.env.Environment;
-import org.springframework.util.Assert;
 
 /**
  * Returns properties from Spring.
@@ -33,12 +32,7 @@ class SpringEnvironmentPropertySource implements PropertySource {
 	 */
 	private static final int PRIORITY = -100;
 
-	private final Environment environment;
+	private volatile Environment environment;
-
-	SpringEnvironmentPropertySource(Environment environment) {
-		Assert.notNull(environment, "Environment must not be null");
-		this.environment = environment;
-	}
 
 	@Override
 	public int getPriority() {
@@ -47,12 +41,18 @@ class SpringEnvironmentPropertySource implements PropertySource {
 
 	@Override
 	public String getProperty(String key) {
-		return this.environment.getProperty(key);
+		Environment environment = this.environment;
+		return (environment != null) ? environment.getProperty(key) : null;
 	}
 
 	@Override
 	public boolean containsProperty(String key) {
-		return this.environment.containsProperty(key);
+		Environment environment = this.environment;
+		return (environment != null) ? environment.containsProperty(key) : false;
+	}
+
+	void setEnvironment(Environment environment) {
+		this.environment = environment;
 	}
 
 }

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/Log4J2LoggingSystemTests.java

@@ -484,6 +484,20 @@ class Log4J2LoggingSystemTests extends AbstractLoggingSystemTests {
 		assertThat(properties.getStringProperty("spring")).isEqualTo("boot");
 	}
 
+	@Test
+	void environmentIsUpdatedUponReinitialization() {
+		MockEnvironment environment = new MockEnvironment();
+		environment.setProperty("spring", "boot: one");
+		this.loggingSystem.beforeInitialize();
+		this.loggingSystem.initialize(new LoggingInitializationContext(environment), null, null);
+		assertThat(PropertiesUtil.getProperties().getStringProperty("spring")).isEqualTo("boot: one");
+		this.loggingSystem.cleanUp();
+		this.environment.setProperty("spring", "boot: two");
+		this.loggingSystem.beforeInitialize();
+		this.loggingSystem.initialize(this.initializationContext, null, null);
+		assertThat(PropertiesUtil.getProperties().getStringProperty("spring")).isEqualTo("boot: two");
+	}
+
 	@Test
 	void nonFileUrlsAreResolvedUsingLog4J2UrlConnectionFactory() {
 		this.loggingSystem.beforeInitialize();

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/logging/log4j2/SpringEnvironmentPropertySourceTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2023 the original author or authors.
+ * Copyright 2012-2024 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.
@@ -26,7 +26,6 @@ import org.junit.jupiter.api.Test;
 import org.springframework.mock.env.MockEnvironment;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Tests for {@link SpringEnvironmentPropertySource}.
@@ -43,13 +42,8 @@ class SpringEnvironmentPropertySourceTests {
 	void setup() {
 		this.environment = new MockEnvironment();
 		this.environment.setProperty("spring", "boot");
-		this.propertySource = new SpringEnvironmentPropertySource(this.environment);
+		this.propertySource = new SpringEnvironmentPropertySource();
-	}
+		this.propertySource.setEnvironment(this.environment);
-
-	@Test
-	void createWhenEnvironmentIsNullThrowsException() {
-		assertThatIllegalArgumentException().isThrownBy(() -> new SpringEnvironmentPropertySource(null))
-			.withMessage("Environment must not be null");
 	}
 
 	@Test
@@ -65,6 +59,12 @@ class SpringEnvironmentPropertySourceTests {
 		assertThat(this.propertySource.getProperty("spring")).isEqualTo("boot");
 	}
 
+	@Test
+	void getPropertyWhenEnvironmentIsNullReturnsNull() {
+		this.propertySource.setEnvironment(null);
+		assertThat(this.propertySource.getProperty("spring")).isNull();
+	}
+
 	@Test
 	void getPropertyWhenNotInEnvironmentReturnsNull() {
 		assertThat(this.propertySource.getProperty("nope")).isNull();
@@ -75,6 +75,12 @@ class SpringEnvironmentPropertySourceTests {
 		assertThat(this.propertySource.containsProperty("spring")).isTrue();
 	}
 
+	@Test
+	void containsPropertyWhenEnvironmentIsNullReturnsFalse() {
+		this.propertySource.setEnvironment(null);
+		assertThat(this.propertySource.containsProperty("spring")).isFalse();
+	}
+
 	@Test
 	void containsPropertyWhenNotInEnvironmentReturnsFalse() {
 		assertThat(this.propertySource.containsProperty("nope")).isFalse();