Fix StatusAggregator static initialization

Prior to this commit, there was a cycle between `StatusAggregator` and
`SimpleStatusAggregator`, which caused a static initialization bug -
depending on which class (the implementation or its interface) was
loaded first.

This commit turns the static field of the `StatusAggregator` interface
into a static method to avoid this problem.

Fixes gh-21211

Author: bclozel

spring-boot-project/spring-boot-actuator-autoconfigure/src/main/java/org/springframework/boot/actuate/autoconfigure/availability/AvailabilityProbesHealthEndpointGroup.java

@@ -56,7 +56,7 @@ public boolean showDetails(SecurityContext securityContext) {
 
 	@Override
 	public StatusAggregator getStatusAggregator() {
-		return StatusAggregator.DEFAULT;
+		return StatusAggregator.getDefault();
 	}
 
 	@Override

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/availability/AvailabilityProbesHealthEndpointGroupTests.java

@@ -57,7 +57,7 @@ void showDetailsReturnsFalse() {
 
 	@Test
 	void getStatusAggregatorReturnsDefaultStatusAggregator() {
-		assertThat(this.group.getStatusAggregator()).isEqualTo(StatusAggregator.DEFAULT);
+		assertThat(this.group.getStatusAggregator()).isEqualTo(StatusAggregator.getDefault());
 	}
 
 	@Test

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -37,13 +37,17 @@
 public class SimpleStatusAggregator implements StatusAggregator {
 
 	private static final List<String> DEFAULT_ORDER;
+
+	static final StatusAggregator INSTANCE;
+
 	static {
 		List<String> defaultOrder = new ArrayList<>();
 		defaultOrder.add(Status.DOWN.getCode());
 		defaultOrder.add(Status.OUT_OF_SERVICE.getCode());
 		defaultOrder.add(Status.UP.getCode());
 		defaultOrder.add(Status.UNKNOWN.getCode());
 		DEFAULT_ORDER = Collections.unmodifiableList(getUniformCodes(defaultOrder.stream()));
+		INSTANCE = new SimpleStatusAggregator();
 	}
 
 	private final List<String> order;

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

@@ -33,10 +33,13 @@
 public interface StatusAggregator {
 
 	/**
-	 * A {@link StatusAggregator} instance using default ordering rules.
+	 * Return {@link StatusAggregator} instance using default ordering rules.
+	 * @return a {@code StatusAggregator} with default ordering rules.
 	 * @since 2.3.0
 	 */
-	StatusAggregator DEFAULT = new SimpleStatusAggregator();
+	static StatusAggregator getDefault() {
+		return SimpleStatusAggregator.INSTANCE;
+	}
 
 	/**
 	 * Return the aggregate status for the given set of statuses.

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -29,7 +29,14 @@
 class SimpleStatusAggregatorTests {
 
 	@Test
-	void getAggregateStatusWhenUsingDefaultOrder() {
+	void getAggregateStatusWhenUsingDefaultInstance() {
+		StatusAggregator aggregator = StatusAggregator.getDefault();
+		Status status = aggregator.getAggregateStatus(Status.DOWN, Status.UP, Status.UNKNOWN, Status.OUT_OF_SERVICE);
+		assertThat(status).isEqualTo(Status.DOWN);
+	}
+
+	@Test
+	void getAggregateStatusWhenUsingNewDefaultOrder() {
 		SimpleStatusAggregator aggregator = new SimpleStatusAggregator();
 		Status status = aggregator.getAggregateStatus(Status.DOWN, Status.UP, Status.UNKNOWN, Status.OUT_OF_SERVICE);
 		assertThat(status).isEqualTo(Status.DOWN);