Support Log4J2 string lookups from the Spring Environment

Add a Log4j2 `SpringLookup` plugin which can be used to resolve strings
from the Spring Environment.

See gh-32732

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

@@ -0,0 +1,112 @@
+/*
+ * 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 java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.core.LogEvent;
+import org.apache.logging.log4j.core.LoggerContext;
+import org.apache.logging.log4j.core.config.LoggerContextAware;
+import org.apache.logging.log4j.core.config.plugins.Plugin;
+import org.apache.logging.log4j.core.lookup.StrLookup;
+import org.apache.logging.log4j.status.StatusLogger;
+
+import org.springframework.core.env.Environment;
+
+/**
+ * Lookup for Spring properties.
+ *
+ * @author Ralph Goers
+ * @since 3.0.0
+ */
+@Plugin(name = "spring", category = StrLookup.CATEGORY)
+public class SpringLookup implements LoggerContextAware, StrLookup {
+
+	private static final Logger LOGGER = StatusLogger.getLogger();
+
+	private static final String ACTIVE = "profiles.active";
+
+	private static final String DEFAULT = "profiles.default";
+
+	private static final String PATTERN = "\\[(\\d+?)\\]";
+
+	private static final Pattern ACTIVE_PATTERN = Pattern.compile(ACTIVE + PATTERN);
+
+	private static final Pattern DEFAULT_PATTERN = Pattern.compile(DEFAULT + PATTERN);
+
+	private volatile Environment environment;
+
+	@Override
+	public String lookup(String key) {
+		if (this.environment == null) {
+			return null;
+		}
+		String lowerKey = key.toLowerCase();
+		if (lowerKey.startsWith(ACTIVE)) {
+			return doMatch(ACTIVE_PATTERN, key, this.environment.getActiveProfiles());
+		}
+		else if (lowerKey.startsWith(DEFAULT)) {
+			return doMatch(DEFAULT_PATTERN, key, this.environment.getDefaultProfiles());
+		}
+
+		return this.environment.getProperty(key);
+	}
+
+	private String doMatch(Pattern pattern, String key, String[] profiles) {
+		if (profiles.length == 0) {
+			return null;
+		}
+		if (profiles.length == 1) {
+			return profiles[0];
+		}
+		Matcher matcher = pattern.matcher(key);
+		if (matcher.matches()) {
+			try {
+				int index = Integer.parseInt(matcher.group(1));
+				if (index < profiles.length) {
+					return profiles[index];
+				}
+				LOGGER.warn("Index out of bounds for Spring default profiles: {}", index);
+				return null;
+			}
+			catch (Exception ex) {
+				LOGGER.warn("Unable to parse {} as integer value", matcher.group(1));
+				return null;
+			}
+
+		}
+		return String.join(",", profiles);
+	}
+
+	@Override
+	public String lookup(LogEvent event, String key) {
+		return lookup((key));
+	}
+
+	@Override
+	public void setLoggerContext(final LoggerContext loggerContext) {
+		if (loggerContext != null) {
+			this.environment = (Environment) loggerContext.getObject(Log4J2LoggingSystem.ENVIRONMENT_KEY);
+		}
+		else {
+			LOGGER.warn("Attempt to set LoggerContext reference to null in SpringLookup");
+		}
+	}
+
+}