Merge branch '2.3.x'

Closes gh-22897

spring-boot-project/spring-boot-docs/src/docs/asciidoc/spring-boot-features.adoc

@@ -538,6 +538,9 @@ Or you could supply the JSON by using a command line argument:
 
 If you are deploying to a classic Application Server, you could also use a JNDI variable named `java:comp/env/spring.application.json`.
 
+NOTE: Although `null` values from the JSON will be added to the resulting property source, the `PropertySourcesPropertyResolver` treats `null` properties as missing values.
+This means that the JSON cannot override properties from lower order property sources with a `null` value.
+
 
 
 [[boot-features-external-config-application-property-files]]

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/env/SpringApplicationJsonEnvironmentPostProcessor.java

 /*
- * 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.
@@ -38,6 +38,7 @@ import org.springframework.core.env.MutablePropertySources;
 import org.springframework.core.env.PropertySource;
 import org.springframework.core.env.StandardEnvironment;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.CollectionUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.web.context.support.StandardServletEnvironment;
 
@@ -123,9 +124,17 @@ public class SpringApplicationJsonEnvironmentPostProcessor implements Environmen
 	@SuppressWarnings("unchecked")
 	private void extract(String name, Map<String, Object> result, Object value) {
 		if (value instanceof Map) {
+			if (CollectionUtils.isEmpty((Map<?, ?>) value)) {
+				result.put(name, value);
+				return;
+			}
 			flatten(name, result, (Map<String, Object>) value);
 		}
 		else if (value instanceof Collection) {
+			if (CollectionUtils.isEmpty((Collection<?>) value)) {
+				result.put(name, value);
+				return;
+			}
 			int index = 0;
 			for (Object object : (Collection<Object>) value) {
 				extract(name + "[" + index + "]", result, object);

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/env/SpringApplicationJsonEnvironmentPostProcessorTests.java

 /*
- * 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.
@@ -17,6 +17,7 @@
 package org.springframework.boot.env;
 
 import java.util.Collections;
+import java.util.Map;
 
 import org.junit.jupiter.api.Test;
 
@@ -26,6 +27,7 @@ import org.springframework.core.env.ConfigurableEnvironment;
 import org.springframework.core.env.MapPropertySource;
 import org.springframework.core.env.PropertySource;
 import org.springframework.core.env.StandardEnvironment;
+import org.springframework.mock.env.MockPropertySource;
 import org.springframework.test.context.support.TestPropertySourceUtils;
 import org.springframework.web.context.support.StandardServletEnvironment;
 
@@ -170,6 +172,39 @@ class SpringApplicationJsonEnvironmentPostProcessorTests {
 		assertThat(this.environment.getPropertySources()).containsSequence(custom, json, servlet, jndi);
 	}
 
+	@Test
+	void nullValuesShouldBeAddedToPropertySource() {
+		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
+				"SPRING_APPLICATION_JSON={\"foo\":null}");
+		this.processor.postProcessEnvironment(this.environment, null);
+		assertThat(this.environment.containsProperty("foo")).isTrue();
+	}
+
+	@Test
+	void emptyValuesForCollectionShouldNotBeIgnored() {
+		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
+				"SPRING_APPLICATION_JSON={\"foo\":[]}");
+		MockPropertySource source = new MockPropertySource();
+		source.setProperty("foo", "bar");
+		this.environment.getPropertySources().addLast(source);
+		assertThat(this.environment.resolvePlaceholders("${foo}")).isEqualTo("bar");
+		this.environment.getPropertySources().addLast(source);
+		this.processor.postProcessEnvironment(this.environment, null);
+		assertThat(this.environment.resolvePlaceholders("${foo}")).isEmpty();
+	}
+
+	@Test
+	@SuppressWarnings("unchecked")
+	void emptyMapValuesShouldNotBeIgnored() {
+		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
+				"SPRING_APPLICATION_JSON={\"foo\":{}}");
+		MockPropertySource source = new MockPropertySource();
+		source.setProperty("foo.baz", "bar");
+		this.environment.getPropertySources().addLast(source);
+		this.processor.postProcessEnvironment(this.environment, null);
+		assertThat(this.environment.getProperty("foo", Map.class)).isEmpty();
+	}
+
 	private void testServletPropertySource(String servletPropertySourceName) {
 		this.environment.getPropertySources().addFirst(getPropertySource(servletPropertySourceName, "servlet"));
 		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,