Polish default value for arrays

See gh-1996

spring-boot-docs/src/main/asciidoc/appendix-configuration-metadata.adoc

@@ -144,8 +144,9 @@ The JSON object contained in the `properties` array can contain the following at
 
 |`defaultValue`
 | Object
-| The default value which will be used if the property is not specified. May be omitted
-  if the default value is not known.
+| The default value which will be used if the property is not specified. Can also be an
+  array of value(s) if the type of the property is an array. May be omitted if the default
+  value is not known.
 
 |`deprecated`
 | boolean

spring-boot-tools/spring-boot-configuration-processor/src/main/java/org/springframework/boot/configurationprocessor/fieldvalues/javac/JavaCompilerFieldValuesParser.java

@@ -124,7 +124,11 @@ public class JavaCompilerFieldValuesParser implements FieldValuesParser {
 			if (arrayValues != null) {
 				Object[] result = new Object[arrayValues.size()];
 				for (int i = 0; i < arrayValues.size(); i++) {
-					result[i] = getValue(arrayValues.get(i), null);
+					Object value = getValue(arrayValues.get(i), null);
+					if (value == null) { // One of the elements could not be resolved
+						return defaultValue;
+					}
+					result[i] = value;
 				}
 				return result;
 			}

spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/fieldvalues/AbstractFieldValuesProcessorTests.java

@@ -84,6 +84,7 @@ public abstract class AbstractFieldValuesProcessorTests {
 		assertThat(values.get("stringArrayConst"), equalToObject(new Object[]{"OK", "KO"}));
 		assertThat(values.get("stringArrayConstElements"), equalToObject(new Object[] {"c"}));
 		assertThat(values.get("integerArray"), equalToObject(new Object[] {42, 24}));
+		assertThat(values.get("unknownArray"), nullValue());
 	}
 
 	private Matcher<Object> equalToObject(Object object) {

spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationsample/fieldvalues/FieldValues.java

@@ -90,4 +90,6 @@ public class FieldValues {
 
 	private Integer[] integerArray = new Integer[] {42, 24};
 
+	private FieldValues[] unknownArray = new FieldValues[] {new FieldValues()};
+
 }