Allow to disable debug property

Previously, adding `debug=false` in the environment had no effect as the
mere presence of the property was used to enable the debug mode. This
commit makes sure to also check the value and ignore the property if it
is set to `false`.

The documentation has also been updated to refer to the `trace` property.

Closes gh-5374

Author: snicoll

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

@@ -1072,9 +1072,14 @@ default `ERROR`, `WARN` and `INFO` level messages are logged. You can also enabl
 NOTE: you can also specify `debug=true` in your `application.properties`.
 
 When the debug mode is enabled, a selection of core loggers (embedded container, Hibernate
-and Spring) are configured to output more information. Enabling the debug mode does _not_
+and Spring Boot) are configured to output more information. Enabling the debug mode does _not_
 configure your application to log all messages with `DEBUG` level.
 
+Alternatively, you can enable a "`trace`" mode by starting your application with a `--trace`
+flag (or `trace=true` in your `application.properties`). This will enable trace logging for a
+selection of core loggers (embedded container, Hibernate schema generation and the whole Spring
+portfolio).
+
 [[boot-features-logging-color-coded-output]]
 ==== Color-coded output
 If your terminal supports ANSI, color output will be used to aid readability. You can set

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

@@ -284,15 +284,20 @@ private void setSystemProperty(String name, String value) {
 
 	private void initializeEarlyLoggingLevel(ConfigurableEnvironment environment) {
 		if (this.parseArgs && this.springBootLogging == null) {
-			if (environment.containsProperty("debug")) {
+			if (isSet(environment, "debug")) {
 				this.springBootLogging = LogLevel.DEBUG;
 			}
-			if (environment.containsProperty("trace")) {
+			if (isSet(environment, "trace")) {
 				this.springBootLogging = LogLevel.TRACE;
 			}
 		}
 	}
 
+	private boolean isSet(ConfigurableEnvironment environment, String property) {
+		String value = environment.getProperty(property);
+		return !(value == null || value.equals("false"));
+	}
+
 	private void initializeSystem(ConfigurableEnvironment environment,
 			LoggingSystem system, LogFile logFile) {
 		LoggingInitializationContext initializationContext = new LoggingInitializationContext(

spring-boot/src/main/resources/META-INF/additional-spring-configuration-metadata.json

@@ -160,6 +160,13 @@
     "type": "java.lang.String",
     "sourceType": "org.springframework.boot.context.config.ConfigFileApplicationListener",
     "description": "Unconditionally activate the specified comma separated profiles."
+  },
+  {
+    "name": "trace",
+    "type": "java.lang.Boolean",
+    "description": "Enable trace logs.",
+    "sourceType": "org.springframework.boot.logging.LoggingApplicationListener",
+    "defaultValue": false
   }
 ],"hints": [
   {

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

@@ -55,6 +55,7 @@
  * @author Dave Syer
  * @author Phillip Webb
  * @author Andy Wilkinson
+ * @author Stephane Nicoll
  */
 public class LoggingApplicationListenerTests {
 
@@ -231,6 +232,26 @@ public void parseTraceArg() throws Exception {
 		assertThat(this.outputCapture.toString()).contains("testattrace");
 	}
 
+	@Test
+	public void disableDebugArg() {
+		disableDebugTraceArg("debug=false");
+	}
+
+	@Test
+	public void disableTraceArg() {
+		disableDebugTraceArg("trace=false");
+	}
+
+	private void disableDebugTraceArg(String... environment) {
+		EnvironmentTestUtils.addEnvironment(this.context, environment);
+		this.initializer.initialize(this.context.getEnvironment(),
+				this.context.getClassLoader());
+		this.logger.debug("testatdebug");
+		this.logger.trace("testattrace");
+		assertThat(this.outputCapture.toString()).doesNotContain("testatdebug");
+		assertThat(this.outputCapture.toString()).doesNotContain("testattrace");
+	}
+
 	@Test
 	public void parseLevels() throws Exception {
 		EnvironmentTestUtils.addEnvironment(this.context,