Align JSON null and empty handling with that of YAML

Closes gh-51583

core/spring-boot/src/main/java/org/springframework/boot/support/SpringApplicationJsonEnvironmentPostProcessor.java

@@ -131,14 +131,14 @@ public class SpringApplicationJsonEnvironmentPostProcessor implements Environmen
 	private void extract(String name, Map<String, Object> result, Object value) {
 		if (value instanceof Map<?, ?> map) {
 			if (CollectionUtils.isEmpty(map)) {
-				result.put(name, value);
+				result.put(name, "");
 				return;
 			}
 			flatten(name, result, (Map<String, Object>) value);
 		}
 		else if (value instanceof Collection<?> collection) {
 			if (CollectionUtils.isEmpty(collection)) {
-				result.put(name, value);
+				result.put(name, "");
 				return;
 			}
 			int index = 0;
@@ -148,7 +148,7 @@ public class SpringApplicationJsonEnvironmentPostProcessor implements Environmen
 			}
 		}
 		else {
-			result.put(name, value);
+			result.put(name, (value != null) ? value : "");
 		}
 	}
 

core/spring-boot/src/test/java/org/springframework/boot/support/SpringApplicationJsonEnvironmentPostProcessorTests.java

@@ -17,7 +17,6 @@
 package org.springframework.boot.support;
 
 import java.util.Collections;
-import java.util.Map;
 
 import org.junit.jupiter.api.Test;
 
@@ -28,7 +27,6 @@ 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;
 
@@ -175,36 +173,36 @@ class SpringApplicationJsonEnvironmentPostProcessorTests {
 	}
 
 	@Test
-	void nullValuesShouldBeAddedToPropertySource() {
+	void nullValueIsMappedToAnEmptyString() {
 		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
 				"SPRING_APPLICATION_JSON={\"foo\":null}");
 		this.processor.postProcessEnvironment(this.environment, getApplication());
-		assertThat(this.environment.containsProperty("foo")).isTrue();
+		assertThat(this.environment.getProperty("foo")).isEmpty();
 	}
 
 	@Test
-	void emptyValuesForCollectionShouldNotBeIgnored() {
+	void nullValueInArrayIsMappedToAnEmptyString() {
+		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
+				"SPRING_APPLICATION_JSON={\"foo\":[\"bar\", null]}");
+		this.processor.postProcessEnvironment(this.environment, getApplication());
+		assertThat(this.environment.getProperty("foo[0]")).isEqualTo("bar");
+		assertThat(this.environment.getProperty("foo[1]")).isEmpty();
+	}
+
+	@Test
+	void emptyArrayIsMappedToAnEmptyString() {
 		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("${foo}")).isEqualTo("bar");
-		this.environment.getPropertySources().addLast(source);
 		this.processor.postProcessEnvironment(this.environment, getApplication());
-		assertThat(this.environment.resolvePlaceholders("${foo}")).isEmpty();
+		assertThat(this.environment.getProperty("foo")).isEmpty();
 	}
 
 	@Test
-	@SuppressWarnings("unchecked")
-	void emptyMapValuesShouldNotBeIgnored() {
+	void emptyMapIsMappedToAnEmptyString() {
 		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, getApplication());
-		assertThat(this.environment.getProperty("foo", Map.class)).isEmpty();
+		assertThat(this.environment.getProperty("foo")).isEmpty();
 	}
 
 	private SpringApplication getApplication() {