Make sure health endpoint is available with no contributor

This commit makes sure that the health endpoint returns a default health
status when no contributors are available. Previously, it was returning
`null` which leads to a 404 when exposed via HTTP.

Closes gh-18676

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthEndpoint.java

@@ -62,7 +62,8 @@ public class HealthEndpoint extends HealthEndpointSupport<HealthContributor, Hea
 
 	@ReadOperation
 	public HealthComponent health() {
-		return health(ApiVersion.V3, EMPTY_PATH);
+		HealthComponent health = health(ApiVersion.V3, EMPTY_PATH);
+		return (health != null) ? health : DEFAULT_HEALTH;
 	}
 
 	@ReadOperation

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthEndpointSupport.java

@@ -34,6 +34,8 @@ import org.springframework.util.Assert;
  */
 abstract class HealthEndpointSupport<C, T> {
 
+	static final Health DEFAULT_HEALTH = Health.up().build();
+
 	private final ContributorRegistry<C> registry;
 
 	private final HealthEndpointGroups groups;

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/HealthEndpointWebExtension.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.actuate.health;
 
+import java.util.Arrays;
 import java.util.Map;
 import java.util.Set;
 
@@ -79,7 +80,9 @@ public class HealthEndpointWebExtension extends HealthEndpointSupport<HealthCont
 			boolean showAll, String... path) {
 		HealthResult<HealthComponent> result = getHealth(apiVersion, securityContext, showAll, path);
 		if (result == null) {
-			return new WebEndpointResponse<>(WebEndpointResponse.STATUS_NOT_FOUND);
+			return (Arrays.equals(path, NO_PATH))
+					? new WebEndpointResponse<>(DEFAULT_HEALTH, WebEndpointResponse.STATUS_OK)
+					: new WebEndpointResponse<>(WebEndpointResponse.STATUS_NOT_FOUND);
 		}
 		HealthComponent health = result.getHealth();
 		HealthEndpointGroup group = result.getGroup();

spring-boot-project/spring-boot-actuator/src/main/java/org/springframework/boot/actuate/health/ReactiveHealthEndpointWebExtension.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.actuate.health;
 
+import java.util.Arrays;
 import java.util.Map;
 import java.util.Set;
 
@@ -81,7 +82,9 @@ public class ReactiveHealthEndpointWebExtension
 			SecurityContext securityContext, boolean showAll, String... path) {
 		HealthResult<Mono<? extends HealthComponent>> result = getHealth(apiVersion, securityContext, showAll, path);
 		if (result == null) {
-			return Mono.just(new WebEndpointResponse<>(WebEndpointResponse.STATUS_NOT_FOUND));
+			return (Arrays.equals(path, NO_PATH))
+					? Mono.just(new WebEndpointResponse<>(DEFAULT_HEALTH, WebEndpointResponse.STATUS_OK))
+					: Mono.just(new WebEndpointResponse<>(WebEndpointResponse.STATUS_NOT_FOUND));
 		}
 		HealthEndpointGroup group = result.getGroup();
 		return result.getHealth().map((health) -> {

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/health/HealthEndpointTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.actuate.health;
 
+import java.util.Collections;
 import java.util.Map;
 
 import org.junit.jupiter.api.Test;
@@ -51,6 +52,15 @@ class HealthEndpointTests
 		assertThat(health).isInstanceOf(SystemHealth.class);
 	}
 
+	@Test
+	void healthWithNoContributorReturnsUp() {
+		assertThat(this.registry).isEmpty();
+		HealthComponent health = create(this.registry,
+				HealthEndpointGroups.of(mock(HealthEndpointGroup.class), Collections.emptyMap())).health();
+		assertThat(health.getStatus()).isEqualTo(Status.UP);
+		assertThat(health).isInstanceOf(Health.class);
+	}
+
 	@Test
 	void healthWhenPathDoesNotExistReturnsNull() {
 		this.registry.registerContributor("test", createContributor(this.up));

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/health/HealthEndpointWebExtensionTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.actuate.health;
 
+import java.util.Collections;
 import java.util.Map;
 
 import org.junit.jupiter.api.Test;
@@ -58,6 +59,18 @@ class HealthEndpointWebExtensionTests
 		assertThat(response.getStatus()).isEqualTo(200);
 	}
 
+	@Test
+	void healthWithNoContributorReturnsUp() {
+		assertThat(this.registry).isEmpty();
+		WebEndpointResponse<HealthComponent> response = create(this.registry,
+				HealthEndpointGroups.of(mock(HealthEndpointGroup.class), Collections.emptyMap()))
+						.health(ApiVersion.LATEST, SecurityContext.NONE);
+		assertThat(response.getStatus()).isEqualTo(200);
+		HealthComponent health = response.getBody();
+		assertThat(health.getStatus()).isEqualTo(Status.UP);
+		assertThat(health).isInstanceOf(Health.class);
+	}
+
 	@Test
 	void healthWhenPathDoesNotExistReturnsHttp404() {
 		this.registry.registerContributor("test", createContributor(this.up));

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/health/ReactiveHealthEndpointWebExtensionTests.java

@@ -16,6 +16,7 @@
 
 package org.springframework.boot.actuate.health;
 
+import java.util.Collections;
 import java.util.Map;
 
 import org.junit.jupiter.api.Test;
@@ -60,6 +61,18 @@ class ReactiveHealthEndpointWebExtensionTests extends
 		assertThat(response.getStatus()).isEqualTo(200);
 	}
 
+	@Test
+	void healthWithNoContributorReturnsUp() {
+		assertThat(this.registry).isEmpty();
+		WebEndpointResponse<? extends HealthComponent> response = create(this.registry,
+				HealthEndpointGroups.of(mock(HealthEndpointGroup.class), Collections.emptyMap()))
+						.health(ApiVersion.LATEST, SecurityContext.NONE).block();
+		assertThat(response.getStatus()).isEqualTo(200);
+		HealthComponent health = response.getBody();
+		assertThat(health.getStatus()).isEqualTo(Status.UP);
+		assertThat(health).isInstanceOf(Health.class);
+	}
+
 	@Test
 	void healthWhenPathDoesNotExistReturnsHttp404() {
 		this.registry.registerContributor("test", createContributor(this.up));