Fallback to BeanPropertyBindingResult for unbound property validation

gh-17424 updated `ValidationBindHandler` so that it would
only look at bound values when validating. This commit updates
`ValidationBindHandler` to use Spring Framework's `BeanPropertyBindingResult`.
This means that for fields that are not bound, JavaBean accessor methods can
be used to get the value to validate.

Fixes gh-25356

Author: mbhave

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/properties/bind/validation/ValidationBindHandler.java

@@ -22,6 +22,7 @@
 import java.util.Set;
 import java.util.stream.Collectors;
 
+import org.springframework.beans.NotReadablePropertyException;
 import org.springframework.boot.context.properties.bind.AbstractBindHandler;
 import org.springframework.boot.context.properties.bind.BindContext;
 import org.springframework.boot.context.properties.bind.BindHandler;
@@ -33,6 +34,7 @@
 import org.springframework.core.ResolvableType;
 import org.springframework.util.ObjectUtils;
 import org.springframework.validation.AbstractBindingResult;
+import org.springframework.validation.BeanPropertyBindingResult;
 import org.springframework.validation.Validator;
 
 /**
@@ -143,14 +145,14 @@ private void validateAndPush(ConfigurationPropertyName name, Object target, Clas
 	/**
 	 * {@link AbstractBindingResult} implementation backed by the bound properties.
 	 */
-	private class ValidationResult extends AbstractBindingResult {
+	private class ValidationResult extends BeanPropertyBindingResult {
 
 		private final ConfigurationPropertyName name;
 
-		private Object target;
+		private final Object target;
 
 		protected ValidationResult(ConfigurationPropertyName name, Object target) {
-			super(null);
+			super(target, null);
 			this.name = name;
 			this.target = target;
 		}
@@ -160,11 +162,6 @@ public String getObjectName() {
 			return this.name.toString();
 		}
 
-		@Override
-		public Object getTarget() {
-			return this.target;
-		}
-
 		@Override
 		public Class<?> getFieldType(String field) {
 			ResolvableType type = getBoundField(ValidationBindHandler.this.boundTypes, field);
@@ -177,7 +174,29 @@ public Class<?> getFieldType(String field) {
 
 		@Override
 		protected Object getActualFieldValue(String field) {
-			return getBoundField(ValidationBindHandler.this.boundResults, field);
+			Object boundField = getBoundField(ValidationBindHandler.this.boundResults, field);
+			if (boundField != null) {
+				return boundField;
+			}
+			try {
+				return super.getActualFieldValue(field);
+			}
+			catch (Exception ex) {
+				if (isPropertyNotReadable(ex)) {
+					return null;
+				}
+				throw ex;
+			}
+		}
+
+		private boolean isPropertyNotReadable(Throwable ex) {
+			while (ex != null) {
+				if (ex instanceof NotReadablePropertyException) {
+					return true;
+				}
+				ex = ex.getCause();
+			}
+			return false;
 		}
 
 		private <T> T getBoundField(Map<ConfigurationPropertyName, T> boundFields, String field) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/ConfigurationPropertiesTests.java

@@ -687,6 +687,13 @@ void loadWhenConfigurationPropertiesIsAlsoValidatorShouldApplyValidator() {
 		});
 	}
 
+	@Test
+	void loadWhenConfigurationPropertiesWithValidDefaultValuesShouldNotFail() {
+		AnnotationConfigApplicationContext context = load(ValidatorPropertiesWithDefaultValues.class);
+		ValidatorPropertiesWithDefaultValues bean = context.getBean(ValidatorPropertiesWithDefaultValues.class);
+		assertThat(bean.getBar()).isEqualTo("a");
+	}
+
 	@Test
 	void loadWhenSetterThrowsValidationExceptionShouldFail() {
 		assertThatExceptionOfType(BeanCreationException.class)

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/properties/ValidatorPropertiesWithDefaultValues.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright 2012-2021 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.context.properties;
+
+import org.springframework.validation.Errors;
+import org.springframework.validation.ValidationUtils;
+import org.springframework.validation.Validator;
+
+/**
+ * Used for testing validation of properties that have default field values.
+ *
+ * @author Madhura Bhave
+ */
+@EnableConfigurationProperties
+@ConfigurationProperties
+class ValidatorPropertiesWithDefaultValues implements Validator {
+
+	private String bar = "a";
+
+	@Override
+	public boolean supports(Class<?> type) {
+		return type == ValidatorPropertiesWithDefaultValues.class;
+	}
+
+	@Override
+	public void validate(Object target, Errors errors) {
+		ValidationUtils.rejectIfEmpty(errors, "bar", "foo.empty");
+	}
+
+	public String getBar() {
+		return this.bar;
+	}
+
+	public void setBar(String bar) {
+		this.bar = bar;
+	}
+
+}

src/checkstyle/checkstyle-suppressions.xml

@@ -49,4 +49,5 @@
 	<suppress files="LinuxDomainSocket" checks="FinalClass" message="SockaddrUn" />
 	<suppress files="BsdDomainSocket" checks="FinalClass" message="SockaddrUn" />
 	<suppress files="StringSequence" checks="SpringMethodVisibility" message="isEmpty"/>
+	<suppress files="ValidatorPropertiesWithDefaultValues\.java" checks="SpringMethodVisibility" />
 </suppressions>