Notify the use of logback specific system property

Logback documentation explains how to initialize the logging system and
namely how the `logback.configurationFile` system property can be used to
specify the configuration file to use.

Spring Boot has an abstraction on top of that. A user can define the
`logging.path` property regardless of the logging infrastructure it is
using.

Users following the logback documentation can be confused at first so
we're not logging a warning when we found out that the logback specific
property has been specified.

Closes gh-2382

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -1019,6 +1019,9 @@ NOTE: The logging system is initialized early in the application lifecycle and a
 logging properties will not be found in property files loaded via `@PropertySource`
 annotations.
 
+TIP: Logging properties are independent of the actual logging infrastructure. As a
+result, specific configuration keys (such as `logback.configurationFile` for Logback)
+are not managed by spring Boot.
 
 
 [[boot-features-custom-log-levels]]

spring-boot/src/main/java/org/springframework/boot/logging/logback/LogbackLoggingSystem.java

@@ -56,6 +56,8 @@ import ch.qos.logback.core.status.Status;
  */
 public class LogbackLoggingSystem extends Slf4JLoggingSystem {
 
+	private static final String CONFIGURATION_FILE_PROPERTY = "logback.configurationFile";
+
 	private static final Map<LogLevel, Level> LEVELS;
 
 	static {
@@ -102,6 +104,11 @@ public class LogbackLoggingSystem extends Slf4JLoggingSystem {
 			String configLocation, LogFile logFile) {
 		getLogger(null).getLoggerContext().getTurboFilterList().remove(FILTER);
 		super.initialize(initializationContext, configLocation, logFile);
+		if (StringUtils.hasText(System.getProperty(CONFIGURATION_FILE_PROPERTY))) {
+			getLogger(LogbackLoggingSystem.class.getName()).warn(
+					"Ignoring '"+CONFIGURATION_FILE_PROPERTY+"' system property. " +
+							"Please use 'logging.path' instead.");
+		}
 	}
 
 	@Override

spring-boot/src/test/java/org/springframework/boot/logging/logback/LogbackLoggingSystemTests.java

@@ -141,6 +141,21 @@ public class LogbackLoggingSystemTests extends AbstractLoggingSystemTests {
 		assertFalse(new File(tmpDir() + "/tmp.log").exists());
 	}
 
+	@Test
+	public void testLogbackSpecificSystemProperty() throws Exception {
+		System.setProperty("logback.configurationFile", "/foo/my-file.xml");
+		try {
+			this.loggingSystem.beforeInitialize();
+			this.loggingSystem.initialize(this.initializationContext, null, null);
+			String output = this.output.toString().trim();
+			assertTrue("Wrong output:\n" + output, output.contains("Ignoring " +
+					"'logback.configurationFile' system property. Please use 'logging.path' instead."));
+		}
+		finally {
+			System.clearProperty("logback.configurationFile");
+		}
+	}
+
 	@Test(expected = IllegalStateException.class)
 	public void testNonexistentConfigLocation() throws Exception {
 		this.loggingSystem.beforeInitialize();