Allow to reset a log level

This commit ensures that `setLogLevel` on the `LoggingSystem` accepts
a `null` level. A `null` level means any customization sets on that
level should be removed and the default configuration should be used
instead.

Effectively, the level of the parent logger is going to be used when
`setLevel` is called with `null` for a given logger.

Most JMX clients do not accept to pass `null` for an argument so an
empty String is translated to null in that specific case.

Closes gh-8776

spring-boot-actuator/src/main/java/org/springframework/boot/actuate/endpoint/jmx/LoggersEndpointMBean.java

@@ -24,12 +24,15 @@ import org.springframework.boot.actuate.endpoint.mvc.MvcEndpoint;
 import org.springframework.boot.logging.LogLevel;
 import org.springframework.jmx.export.annotation.ManagedAttribute;
 import org.springframework.jmx.export.annotation.ManagedOperation;
-import org.springframework.util.Assert;
+import org.springframework.jmx.export.annotation.ManagedOperationParameter;
+import org.springframework.jmx.export.annotation.ManagedOperationParameters;
+import org.springframework.util.StringUtils;
 
 /**
  * Adapter to expose {@link LoggersEndpoint} as an {@link MvcEndpoint}.
  *
  * @author Vedran Pavic
+ * @author Stephane Nicoll
  * @since 1.5.0
  */
 public class LoggersEndpointMBean extends EndpointMBean {
@@ -45,15 +48,25 @@ public class LoggersEndpointMBean extends EndpointMBean {
 	}
 
 	@ManagedOperation(description = "Get log level for a given logger")
+	@ManagedOperationParameters({
+			@ManagedOperationParameter(name = "loggerName", description = "Name of the logger") })
 	public Object getLogger(String loggerName) {
 		return convert(getEndpoint().invoke(loggerName));
 	}
 
 	@ManagedOperation(description = "Set log level for a given logger")
+	@ManagedOperationParameters({
+			@ManagedOperationParameter(name = "loggerName", description = "Name of the logger"),
+			@ManagedOperationParameter(name = "logLevel", description = "New log level (can be null or empty String to remove the custom level)") })
 	public void setLogLevel(String loggerName, String logLevel) {
-		Assert.notNull(logLevel, "LogLevel must not be null");
-		LogLevel level = LogLevel.valueOf(logLevel.toUpperCase());
-		getEndpoint().setLogLevel(loggerName, level);
+		getEndpoint().setLogLevel(loggerName, determineLogLevel(logLevel));
+	}
+
+	private LogLevel determineLogLevel(String logLevel) {
+		if (StringUtils.hasText(logLevel)) {
+			return LogLevel.valueOf(logLevel.toUpperCase());
+		}
+		return null;
 	}
 
 	@Override

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/LoggersEndpointTests.java

@@ -80,6 +80,12 @@ public class LoggersEndpointTests extends AbstractEndpointTests<LoggersEndpoint>
 		verify(getLoggingSystem()).setLogLevel("ROOT", LogLevel.DEBUG);
 	}
 
+	@Test
+	public void setLogLevelToNull() {
+		getEndpointBean().setLogLevel("ROOT", null);
+		verify(getLoggingSystem()).setLogLevel("ROOT", null);
+	}
+
 	private LoggingSystem getLoggingSystem() {
 		return this.context.getBean(LoggingSystem.class);
 	}

spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/mvc/LoggersMvcEndpointTests.java

@@ -64,6 +64,7 @@ import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.
  * @author Ben Hale
  * @author Phillip Webb
  * @author Eddú Meléndez
+ * @author Stephane Nicoll
  */
 @RunWith(SpringRunner.class)
 @SpringBootTest
@@ -181,6 +182,22 @@ public class LoggersMvcEndpointTests {
 		verifyZeroInteractions(this.loggingSystem);
 	}
 
+	@Test
+	public void setLoggerWithNullLogLevel() throws Exception {
+		this.mvc.perform(post(PATH + "/ROOT").contentType(MediaType.APPLICATION_JSON)
+				.content("{\"configuredLevel\": null}"))
+				.andExpect(status().isNoContent());
+		verify(this.loggingSystem).setLogLevel("ROOT", null);
+	}
+
+	@Test
+	public void setLoggerWithNoLogLevel() throws Exception {
+		this.mvc.perform(post(PATH + "/ROOT").contentType(MediaType.APPLICATION_JSON)
+				.content("{}"))
+				.andExpect(status().isNoContent());
+		verify(this.loggingSystem).setLogLevel("ROOT", null);
+	}
+
 	@Test
 	public void logLevelForLoggerWithNameThatCouldBeMistakenForAPathExtension()
 			throws Exception {