Support nested @PropertyMapping annotations

Update `AnnotationsPropertySource` so that nested annotations are
supported. Prior to this commit, annotations annotated with
`@PropertyMapping` that contained nested annotation attributes would
result in instances of `TypeMappedAnnotation` being used as properties.
This usually led to errors due to not being able to convert those to
Strings. This commit makes it so that nested annotations are recursively
mapped to properties. This should allow for more complex configuration
to be mapped from annotations.

See gh-23146

Author: Stefan Zwanenburg

spring-boot-project/spring-boot-test-autoconfigure/src/main/java/org/springframework/boot/test/autoconfigure/properties/AnnotationsPropertySource.java

@@ -90,7 +90,7 @@ private void collectProperties(String prefix, SkipPropertyMapping defaultSkip, M
 			}
 		}
 		String name = getName(prefix, attributeMapping, attribute);
-		putProperties(name, value.get(), properties);
+		putProperties(name, skip, value.get(), properties);
 	}
 
 	private String getName(String prefix, MergedAnnotation<?> attributeMapping, Method attribute) {
@@ -118,11 +118,17 @@ private String dotAppend(String prefix, String postfix) {
 		return postfix;
 	}
 
-	private void putProperties(String name, Object value, Map<String, Object> properties) {
+	private void putProperties(String name, SkipPropertyMapping defaultSkip, Object value,
+			Map<String, Object> properties) {
 		if (ObjectUtils.isArray(value)) {
 			Object[] array = ObjectUtils.toObjectArray(value);
 			for (int i = 0; i < array.length; i++) {
-				properties.put(name + "[" + i + "]", array[i]);
+				putProperties(name + "[" + i + "]", defaultSkip, array[i], properties);
+			}
+		}
+		else if (value instanceof MergedAnnotation<?>) {
+			for (Method attribute : ((MergedAnnotation<?>) value).getType().getDeclaredMethods()) {
+				collectProperties(name, defaultSkip, (MergedAnnotation<?>) value, attribute, properties);
 			}
 		}
 		else {

spring-boot-project/spring-boot-test-autoconfigure/src/test/java/org/springframework/boot/test/autoconfigure/properties/AnnotationsPropertySourceTests.java

@@ -21,6 +21,9 @@
 
 import org.junit.jupiter.api.Test;
 
+import org.springframework.boot.test.autoconfigure.properties.AnnotationsPropertySourceTests.DeeplyNestedAnnotations.Level1;
+import org.springframework.boot.test.autoconfigure.properties.AnnotationsPropertySourceTests.DeeplyNestedAnnotations.Level2;
+import org.springframework.boot.test.autoconfigure.properties.AnnotationsPropertySourceTests.NestedAnnotations.Entry;
 import org.springframework.core.annotation.AliasFor;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -172,6 +175,26 @@ void enumValueNotMapped() {
 		assertThat(source.containsProperty("testenum.value")).isFalse();
 	}
 
+	@Test
+	void nestedAnnotationsMapped() {
+		AnnotationsPropertySource source = new AnnotationsPropertySource(PropertyMappedWithNestedAnnotations.class);
+		assertThat(source.getProperty("testnested")).isNull();
+		assertThat(source.getProperty("testnested.entries[0]")).isNull();
+		assertThat(source.getProperty("testnested.entries[0].value")).isEqualTo("one");
+		assertThat(source.getProperty("testnested.entries[1]")).isNull();
+		assertThat(source.getProperty("testnested.entries[1].value")).isEqualTo("two");
+	}
+
+	@Test
+	void deeplyNestedAnnotationsMapped() {
+		AnnotationsPropertySource source = new AnnotationsPropertySource(
+				PropertyMappedWithDeeplyNestedAnnotations.class);
+		assertThat(source.getProperty("testdeeplynested")).isNull();
+		assertThat(source.getProperty("testdeeplynested.level1")).isNull();
+		assertThat(source.getProperty("testdeeplynested.level1.level2")).isNull();
+		assertThat(source.getProperty("testdeeplynested.level1.level2.value")).isEqualTo("level2");
+	}
+
 	static class NoAnnotation {
 
 	}
@@ -396,4 +419,51 @@ enum EnumItem {
 
 	}
 
+	@Retention(RetentionPolicy.RUNTIME)
+	@PropertyMapping("testnested")
+	@interface NestedAnnotations {
+
+		Entry[] entries();
+
+		@Retention(RetentionPolicy.RUNTIME)
+		@interface Entry {
+
+			String value();
+
+		}
+
+	}
+
+	@NestedAnnotations(entries = { @Entry("one"), @Entry("two") })
+	static class PropertyMappedWithNestedAnnotations {
+
+	}
+
+	@Retention(RetentionPolicy.RUNTIME)
+	@PropertyMapping("testdeeplynested")
+	@interface DeeplyNestedAnnotations {
+
+		Level1 level1();
+
+		@Retention(RetentionPolicy.RUNTIME)
+		@interface Level1 {
+
+			Level2 level2();
+
+		}
+
+		@Retention(RetentionPolicy.RUNTIME)
+		@interface Level2 {
+
+			String value();
+
+		}
+
+	}
+
+	@DeeplyNestedAnnotations(level1 = @Level1(level2 = @Level2("level2")))
+	static class PropertyMappedWithDeeplyNestedAnnotations {
+
+	}
+
 }