Configure ping (formerly application) health indicator by default

This commit renames ApplicationHealthIndicator to PingHealthIndicator
and changes the auto-configuration so that it is now always configured
by default.

Closes gh-17926

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

@@ -23,7 +23,9 @@ package org.springframework.boot.actuate.health;
  * @author Christian Dupuis
  * @since 1.2.0
  * @see Status#UP
+ * @deprecated since 2.2 in favor of {@link PingHealthIndicator}.
  */
+@Deprecated
 public class ApplicationHealthIndicator extends AbstractHealthIndicator {
 
 	public ApplicationHealthIndicator() {

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

@@ -23,7 +23,6 @@ package org.springframework.boot.actuate.health;
  * @author Dave Syer
  * @author Phillip Webb
  * @since 1.0.0
- * @see ApplicationHealthIndicator
  */
 @FunctionalInterface
 public interface HealthIndicator extends HealthContributor {

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

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.health;
+
+/**
+ * Default implementation of {@link HealthIndicator} that returns {@link Status#UP}.
+ *
+ * @author Dave Syer
+ * @author Christian Dupuis
+ * @since 2.2.0
+ * @see Status#UP
+ */
+public class PingHealthIndicator extends AbstractHealthIndicator {
+
+	@Override
+	protected void doHealthCheck(Health.Builder builder) throws Exception {
+		builder.up();
+	}
+
+}

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

@@ -25,6 +25,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  *
  * @author Phillip Webb
  */
+@Deprecated
 class ApplicationHealthIndicatorTests {
 
 	@Test

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

@@ -0,0 +1,36 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.actuate.health;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link PingHealthIndicator}.
+ *
+ * @author Phillip Webb
+ */
+class PingHealthIndicatorTests {
+
+	@Test
+	void indicatesUp() {
+		PingHealthIndicator healthIndicator = new PingHealthIndicator();
+		assertThat(healthIndicator.health().getStatus()).isEqualTo(Status.UP);
+	}
+
+}