Restore empty YAML arrays in JSON and YAML config responses

spring-cloud-config-server/src/main/java/org/springframework/cloud/config/server/environment/EnvironmentController.java

@@ -438,6 +438,28 @@ private Map<String, Object> convertToProperties(Environment profiles) {
 
 	private void postProcessProperties(Map<String, Object> propertiesMap) {
 		propertiesMap.keySet().removeIf(key -> key.equals("spring.profiles"));
+		restoreEmptyCollections(propertiesMap);
+	}
+
+	/**
+	 * Flattened YAML properties represent empty collections as an empty string (see
+	 * {@code YamlProcessor#buildFlattenedMap}). When rebuilding nested YAML/JSON, restore
+	 * those markers to empty lists unless indexed entries exist for the same prefix.
+	 */
+	private void restoreEmptyCollections(Map<String, Object> propertiesMap) {
+		Set<String> arrayPrefixes = new HashSet<>();
+		for (String key : propertiesMap.keySet()) {
+			int indexOfBracket = key.indexOf('[');
+			if (indexOfBracket > 0) {
+				arrayPrefixes.add(key.substring(0, indexOfBracket));
+			}
+		}
+		for (Entry<String, Object> entry : propertiesMap.entrySet()) {
+			String key = entry.getKey();
+			if (!key.contains("[") && "".equals(entry.getValue()) && !arrayPrefixes.contains(key)) {
+				entry.setValue(Collections.emptyList());
+			}
+		}
 	}
 
 	/**

spring-cloud-config-server/src/test/java/org/springframework/cloud/config/server/environment/EnvironmentControllerTests.java

@@ -197,6 +197,26 @@ public void arrayInYaml() throws Exception {
 		assertThat(yaml).isEqualTo("a:\n  b:\n  - c\n  - d\n");
 	}
 
+	@Test
+	public void emptyArrayInJson() throws Exception {
+		Map<String, Object> map = new LinkedHashMap<>();
+		map.put("network.urls", "");
+		this.environment.add(new PropertySource("one", map));
+		when(this.repository.findOne("foo", "bar", null, false)).thenReturn(this.environment);
+		String json = this.controller.jsonProperties("foo", "bar", false).getBody();
+		JSONAssert.assertEquals("{\"network\":{\"urls\":[]}}", json, JSONCompareMode.STRICT);
+	}
+
+	@Test
+	public void emptyArrayInYaml() throws Exception {
+		Map<String, Object> map = new LinkedHashMap<>();
+		map.put("network.urls", "");
+		this.environment.add(new PropertySource("one", map));
+		when(this.repository.findOne("foo", "bar", null, false)).thenReturn(this.environment);
+		String yaml = this.controller.yaml("foo", "bar", false).getBody();
+		assertThat(yaml).isEqualTo("network:\n  urls: []\n");
+	}
+
 	@Test
 	public void yamlWithBrackets() throws Exception {
 		Map<String, Object> map = new LinkedHashMap<String, Object>();

spring-cloud-config-server/src/test/java/org/springframework/cloud/config/server/environment/NativeEnvironmentRepositoryTests.java

@@ -23,13 +23,16 @@
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Disabled;
 import org.junit.jupiter.api.Test;
+import org.skyscreamer.jsonassert.JSONAssert;
+import org.skyscreamer.jsonassert.JSONCompareMode;
 
 import org.springframework.boot.WebApplicationType;
 import org.springframework.boot.builder.SpringApplicationBuilder;
 import org.springframework.cloud.config.environment.Environment;
 import org.springframework.cloud.config.environment.PropertySource;
 import org.springframework.cloud.config.server.environment.SearchPathLocator.Locations;
 import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.http.ResponseEntity;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
@@ -127,6 +130,26 @@ public void prefixedMultiDocProperties() {
 			.isNotEqualTo(environment.getPropertySources().get(1).getName());
 	}
 
+	@Test
+	public void emptyYamlArrayInJsonOutput() throws Exception {
+		this.repository.setSearchLocations("classpath:/test/empty-array");
+		Environment environment = this.repository.findOne("application", "default", "master");
+		EnvironmentRepository repository = new EnvironmentRepository() {
+			@Override
+			public Environment findOne(String application, String profile, String label) {
+				return environment;
+			}
+
+			@Override
+			public Environment findOne(String application, String profile, String label, boolean includeOrigin) {
+				return environment;
+			}
+		};
+		EnvironmentController controller = new EnvironmentController(repository);
+		ResponseEntity<String> json = controller.labelledJsonProperties("application", "default", "master", false);
+		JSONAssert.assertEquals("{\"network\":{\"urls\":[]}}", json.getBody(), JSONCompareMode.STRICT);
+	}
+
 	@Test
 	public void prefixedWithFile() {
 		this.repository.setSearchLocations("file:./src/test/resources/test");

spring-cloud-config-server/src/test/resources/test/empty-array/application.yml

@@ -0,0 +1,2 @@
+network:
+  urls: []