Handle null host value in MailHealthIndicator

If both the host and port are omitted from the mail properties, the
`location` field will be omitted from the health indicator details.

Fixes gh-38007

Author: scottfrederick

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/mail/MailHealthIndicator.java

@@ -20,11 +20,13 @@
 import org.springframework.boot.actuate.health.Health.Builder;
 import org.springframework.boot.actuate.health.HealthIndicator;
 import org.springframework.mail.javamail.JavaMailSenderImpl;
+import org.springframework.util.StringUtils;
 
 /**
  * {@link HealthIndicator} for configured smtp server(s).
  *
  * @author Johannes Edmeier
+ * @author Scott Frederick
  * @since 2.0.0
  */
 public class MailHealthIndicator extends AbstractHealthIndicator {
@@ -38,9 +40,15 @@ public MailHealthIndicator(JavaMailSenderImpl mailSender) {
 
 	@Override
 	protected void doHealthCheck(Builder builder) throws Exception {
+		String host = this.mailSender.getHost();
 		int port = this.mailSender.getPort();
-		builder.withDetail("location", (port != JavaMailSenderImpl.DEFAULT_PORT)
-				? this.mailSender.getHost() + ":" + this.mailSender.getPort() : this.mailSender.getHost());
+		StringBuilder location = new StringBuilder((host != null) ? host : "");
+		if (port != JavaMailSenderImpl.DEFAULT_PORT) {
+			location.append(":").append(port);
+		}
+		if (StringUtils.hasLength(location)) {
+			builder.withDetail("location", location.toString());
+		}
 		this.mailSender.testConnection();
 		builder.up();
 	}

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/mail/MailHealthIndicatorTests.java

@@ -44,6 +44,7 @@
  *
  * @author Johannes Edmeier
  * @author Stephane Nicoll
+ * @author Scott Frederick
  */
 class MailHealthIndicatorTests {
 
@@ -61,6 +62,52 @@ void setup() {
 		this.indicator = new MailHealthIndicator(this.mailSender);
 	}
 
+	@Test
+	void smtpOnDefaultHostAndPortIsUp() {
+		given(this.mailSender.getHost()).willReturn(null);
+		given(this.mailSender.getPort()).willReturn(-1);
+		given(this.mailSender.getProtocol()).willReturn("success");
+		Health health = this.indicator.health();
+		assertThat(health.getStatus()).isEqualTo(Status.UP);
+		assertThat(health.getDetails()).doesNotContainKey("location");
+	}
+
+	@Test
+	void smtpOnDefaultHostAndPortIsDown() throws MessagingException {
+		given(this.mailSender.getHost()).willReturn(null);
+		given(this.mailSender.getPort()).willReturn(-1);
+		willThrow(new MessagingException("A test exception")).given(this.mailSender).testConnection();
+		Health health = this.indicator.health();
+		assertThat(health.getStatus()).isEqualTo(Status.DOWN);
+		assertThat(health.getDetails()).doesNotContainKey("location");
+		Object errorMessage = health.getDetails().get("error");
+		assertThat(errorMessage).isNotNull();
+		assertThat(errorMessage.toString().contains("A test exception")).isTrue();
+	}
+
+	@Test
+	void smtpOnDefaultHostAndCustomPortIsUp() {
+		given(this.mailSender.getHost()).willReturn(null);
+		given(this.mailSender.getPort()).willReturn(1234);
+		given(this.mailSender.getProtocol()).willReturn("success");
+		Health health = this.indicator.health();
+		assertThat(health.getStatus()).isEqualTo(Status.UP);
+		assertThat(health.getDetails().get("location")).isEqualTo(":1234");
+	}
+
+	@Test
+	void smtpOnDefaultHostAndCustomPortIsDown() throws MessagingException {
+		given(this.mailSender.getHost()).willReturn(null);
+		given(this.mailSender.getPort()).willReturn(1234);
+		willThrow(new MessagingException("A test exception")).given(this.mailSender).testConnection();
+		Health health = this.indicator.health();
+		assertThat(health.getStatus()).isEqualTo(Status.DOWN);
+		assertThat(health.getDetails().get("location")).isEqualTo(":1234");
+		Object errorMessage = health.getDetails().get("error");
+		assertThat(errorMessage).isNotNull();
+		assertThat(errorMessage.toString().contains("A test exception")).isTrue();
+	}
+
 	@Test
 	void smtpOnDefaultPortIsUp() {
 		given(this.mailSender.getPort()).willReturn(-1);