Allow logging.file and logging.path to refer to logging system properties

Closes gh-5274

spring-boot/src/main/java/org/springframework/boot/logging/LoggingApplicationListener.java

@@ -247,16 +247,18 @@ public class LoggingApplicationListener implements GenericApplicationListener {
 	 */
 	protected void initialize(ConfigurableEnvironment environment,
 			ClassLoader classLoader) {
+		setSystemProperties(environment);
 		LogFile logFile = LogFile.get(environment);
-		setSystemProperties(environment, logFile);
+		if (logFile != null) {
+			logFile.applyToSystemProperties();
+		}
 		initializeEarlyLoggingLevel(environment);
 		initializeSystem(environment, this.loggingSystem, logFile);
 		initializeFinalLoggingLevels(environment, this.loggingSystem);
 		registerShutdownHookIfNecessary(environment, this.loggingSystem);
 	}
 
-	private void setSystemProperties(ConfigurableEnvironment environment,
-			LogFile logFile) {
+	private void setSystemProperties(ConfigurableEnvironment environment) {
 		RelaxedPropertyResolver propertyResolver = new RelaxedPropertyResolver(
 				environment, "logging.");
 		setSystemProperty(propertyResolver, EXCEPTION_CONVERSION_WORD,
@@ -265,9 +267,6 @@ public class LoggingApplicationListener implements GenericApplicationListener {
 		setSystemProperty(propertyResolver, FILE_LOG_PATTERN, "pattern.file");
 		setSystemProperty(propertyResolver, LOG_LEVEL_PATTERN, "pattern.level");
 		setSystemProperty(PID_KEY, new ApplicationPid().toString());
-		if (logFile != null) {
-			logFile.applyToSystemProperties();
-		}
 	}
 
 	private void setSystemProperty(RelaxedPropertyResolver propertyResolver,

spring-boot/src/test/java/org/springframework/boot/logging/LoggingApplicationListenerTests.java

@@ -35,6 +35,7 @@ import org.junit.rules.ExpectedException;
 import org.junit.rules.TemporaryFolder;
 import org.slf4j.bridge.SLF4JBridgeHandler;
 
+import org.springframework.boot.ApplicationPid;
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.context.event.ApplicationStartedEvent;
 import org.springframework.boot.logging.java.JavaLoggingSystem;
@@ -441,6 +442,16 @@ public class LoggingApplicationListenerTests {
 		assertThat(System.getProperty("PID"), is(not(nullValue())));
 	}
 
+	@Test
+	public void logFilePropertiesCanReferenceSystemProperties() {
+		EnvironmentTestUtils.addEnvironment(this.context,
+				"logging.file=target/${PID}.log");
+		this.initializer.initialize(this.context.getEnvironment(),
+				this.context.getClassLoader());
+		assertThat(System.getProperty("LOG_FILE"),
+				is(equalTo("target/" + new ApplicationPid().toString() + ".log")));
+	}
+
 	private boolean bridgeHandlerInstalled() {
 		Logger rootLogger = LogManager.getLogManager().getLogger("");
 		Handler[] handlers = rootLogger.getHandlers();