Polish contribution

- Split Validator::of into two factory methods:
  - forInstanceOf using Class::isAssignableFrom
  - forType using Class::equals
- Moved anonymous implementation into TypedValidator with default access

Closes gh-30341

spring-context/src/main/java/org/springframework/validation/TypedValidator.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2002-2023 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.validation;
+
+import java.util.function.BiConsumer;
+import java.util.function.Predicate;
+
+import org.springframework.util.Assert;
+
+/**
+ * Validator instance returned by {@link Validator#forInstanceOf(Class, BiConsumer)}
+ * and {@link Validator#forType(Class, BiConsumer)}.
+ *
+ * @author Toshiaki Maki
+ * @author Arjen Poutsma
+ * @since 6.1
+ * @param <T> the target object type
+ */
+final class TypedValidator<T> implements Validator {
+
+	private final Class<T> targetClass;
+
+	private final Predicate<Class<?>> supports;
+
+	private final BiConsumer<T, Errors> validate;
+
+
+	public TypedValidator(Class<T> targetClass, Predicate<Class<?>> supports, BiConsumer<T, Errors> validate) {
+		Assert.notNull(targetClass, "TargetClass must not be null");
+		Assert.notNull(supports, "Supports function must not be null");
+		Assert.notNull(validate, "Validate function must not be null");
+
+		this.targetClass = targetClass;
+		this.supports = supports;
+		this.validate = validate;
+	}
+
+
+	@Override
+	public boolean supports(Class<?> clazz) {
+		return this.supports.test(clazz);
+	}
+
+	@Override
+	public void validate(Object target, Errors errors) {
+		this.validate.accept(this.targetClass.cast(target), errors);
+	}
+
+}

spring-context/src/main/java/org/springframework/validation/Validator.java

@@ -18,8 +18,6 @@ package org.springframework.validation;
 
 import java.util.function.BiConsumer;
 
-import org.springframework.util.Assert;
-
 /**
  * A validator for application-specific objects.
  *
@@ -30,33 +28,25 @@ import org.springframework.util.Assert;
  * of an application, and supports the encapsulation of validation
  * logic as a first-class citizen in its own right.
  *
- * <p>Find below a simple but complete {@code Validator}
- * implementation, which validates that the various {@link String}
- * properties of a {@code UserLogin} instance are not empty
- * (that is they are not {@code null} and do not consist
+ * <p>Implementations can be created via the static factory methods
+ * {@link #forInstanceOf(Class, BiConsumer)} or
+ * {@link #forType(Class, BiConsumer)}.
+ * Below is a simple but complete {@code Validator} that validates that the
+ * various {@link String} properties of a {@code UserLogin} instance are not
+ * empty (they are not {@code null} and do not consist
  * wholly of whitespace), and that any password that is present is
  * at least {@code 'MINIMUM_PASSWORD_LENGTH'} characters in length.
  *
- * <pre class="code">public class UserLoginValidator implements Validator {
- *
- *    private static final int MINIMUM_PASSWORD_LENGTH = 6;
- *
- *    public boolean supports(Class clazz) {
- *       return UserLogin.class.isAssignableFrom(clazz);
- *    }
- *
- *    public void validate(Object target, Errors errors) {
+ * <pre class="code">Validator userLoginValidator = Validator.forInstance(UserLogin.class, (login, errors) -> {
  *   ValidationUtils.rejectIfEmptyOrWhitespace(errors, "userName", "field.required");
  *   ValidationUtils.rejectIfEmptyOrWhitespace(errors, "password", "field.required");
- *       UserLogin login = (UserLogin) target;
  *   if (login.getPassword() != null
  *         &amp;&amp; login.getPassword().trim().length() &lt; MINIMUM_PASSWORD_LENGTH) {
  *      errors.rejectValue("password", "field.min.length",
  *            new Object[]{Integer.valueOf(MINIMUM_PASSWORD_LENGTH)},
  *            "The password must be at least [" + MINIMUM_PASSWORD_LENGTH + "] characters in length.");
  *   }
- *    }
- * }</pre>
+ * });</pre>
  *
  * <p>See also the Spring reference manual for a fuller discussion of
  * the {@code Validator} interface and its role in an enterprise
@@ -64,6 +54,7 @@ import org.springframework.util.Assert;
  *
  * @author Rod Johnson
  * @author Toshiaki Maki
+ * @author Arjen Poutsma
  * @see SmartValidator
  * @see Errors
  * @see ValidationUtils
@@ -97,38 +88,54 @@ public interface Validator {
 	 */
 	void validate(Object target, Errors errors);
 
+
 	/**
-	 * Takes the {@link BiConsumer} containing the validation logic for the specific type
-	 * <code>&lt;T&gt;</code> and returns the {@link Validator} instance.<br>
-	 * This validator implements the <i>typical</i> {@link #supports(Class)} method
-	 * for the given <code>&lt;T&gt;</code>.<br>
+	 * Return a {@code Validator} that checks whether the target object
+	 * {@linkplain Class#isAssignableFrom(Class) is an instance of}
+	 * {@code targetClass}, resorting to {@code delegate} to populate
+	 * {@link Errors} if it is.
 	 *
-	 * By using this method, a {@link Validator}  can be implemented as follows:
-	 *
-	 * <pre class="code">Validator passwordEqualsValidator = Validator.of(PasswordResetForm.class, (form, errors) -> {
+	 * <p>For instance:
+	 * <pre class="code">Validator passwordEqualsValidator = Validator.forInstanceOf(PasswordResetForm.class, (form, errors) -> {
 	 *   if (!Objects.equals(form.getPassword(), form.getConfirmPassword())) {
 	 * 	   errors.rejectValue("confirmPassword",
 	 * 	         "PasswordEqualsValidator.passwordResetForm.password",
 	 * 	         "password and confirm password must be same.");
 	 * 	   }
 	 * 	 });</pre>
-	 * @param targetClass  the class of the object that is to be validated
-	 * @param delegate the validation logic to delegate for the specific type <code>&lt;T&gt;</code>
-	 * @param <T> the type of the object that is to be validated
-	 * @return the {@link Validator} instance
+	 * @param targetClass the class supported by the returned validator
+	 * @param delegate function invoked with the target object, if it is an
+	 * instance of type T
+	 * @param <T> the target object type
+	 * @return the created {@code Validator}
+	 * @since 6.1
 	 */
-	static <T> Validator of(Class<T> targetClass, BiConsumer<T, Errors> delegate) {
-		Assert.notNull(targetClass, "'targetClass' must not be null.");
-		return new Validator() {
-			@Override
-			public boolean supports(Class<?> clazz) {
-				return targetClass.isAssignableFrom(clazz);
+	static <T> Validator forInstanceOf(Class<T> targetClass, BiConsumer<T, Errors> delegate) {
+		return new TypedValidator<>(targetClass, targetClass::isAssignableFrom, delegate);
 	}
 
-			@Override
-			public void validate(Object target, Errors errors) {
-				delegate.accept(targetClass.cast(target), errors);
-			}
-		};
+	/**
+	 * Return a {@code Validator} that checks whether the target object's class
+	 * is identical to {@code targetClass}, resorting to {@code delegate} to
+	 * populate {@link Errors} if it is.
+	 *
+	 * <p>For instance:
+	 * <pre class="code">Validator passwordEqualsValidator = Validator.forType(PasswordResetForm.class, (form, errors) -> {
+	 *   if (!Objects.equals(form.getPassword(), form.getConfirmPassword())) {
+	 * 	   errors.rejectValue("confirmPassword",
+	 * 	         "PasswordEqualsValidator.passwordResetForm.password",
+	 * 	         "password and confirm password must be same.");
+	 * 	   }
+	 * 	 });</pre>
+	 * @param targetClass the exact class supported by the returned validator (no subclasses)
+	 * @param delegate function invoked with the target object, if it is an
+	 * instance of type T
+	 * @param <T> the target object type
+	 * @return the created {@code Validator}
+	 * @since 6.1
+	 */
+	static <T> Validator forType(Class<T> targetClass, BiConsumer<T, Errors> delegate) {
+		return new TypedValidator<>(targetClass, targetClass::equals, delegate);
 	}
+
 }

spring-context/src/test/java/org/springframework/validation/DataBinderTests.java

@@ -82,7 +82,7 @@ import static org.assertj.core.api.Assertions.entry;
  */
 class DataBinderTests {
 
-	Validator spouseValidator = Validator.of(TestBean.class, (tb, errors) -> {
+	private final Validator spouseValidator = Validator.forInstanceOf(TestBean.class, (tb, errors) -> {
 				if (tb == null || "XXX".equals(tb.getName())) {
 					errors.rejectValue("", "SPOUSE_NOT_AVAILABLE");
 					return;

spring-context/src/test/java/org/springframework/validation/ValidationUtilsTests.java

@@ -33,9 +33,9 @@ import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException
  */
 public class ValidationUtilsTests {
 
-	Validator emptyValidator = Validator.of(TestBean.class, (testBean, errors) -> ValidationUtils.rejectIfEmpty(errors, "name", "EMPTY", "You must enter a name!"));
+	private final Validator emptyValidator = Validator.forInstanceOf(TestBean.class, (testBean, errors) -> ValidationUtils.rejectIfEmpty(errors, "name", "EMPTY", "You must enter a name!"));
 
-	Validator emptyOrWhitespaceValidator = Validator.of(TestBean.class, (testBean, errors) -> ValidationUtils.rejectIfEmptyOrWhitespace(errors, "name", "EMPTY_OR_WHITESPACE", "You must enter a name!"));
+	private final Validator emptyOrWhitespaceValidator = Validator.forInstanceOf(TestBean.class, (testBean, errors) -> ValidationUtils.rejectIfEmptyOrWhitespace(errors, "name", "EMPTY_OR_WHITESPACE", "You must enter a name!"));
 
 	@Test
 	public void testInvokeValidatorWithNullValidator() throws Exception {

spring-context/src/test/java/org/springframework/validation/ValidatorTests.java

@@ -0,0 +1,48 @@
+/*
+ * Copyright 2002-2023 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.validation;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.beans.testfixture.beans.TestBean;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Arjen Poutsma
+ */
+class ValidatorTests {
+
+	@Test
+	public void testSupportsForInstanceOf() {
+		Validator validator = Validator.forInstanceOf(TestBean.class, (testBean, errors) -> {});
+		assertThat(validator.supports(TestBean.class)).isTrue();
+		assertThat(validator.supports(TestBeanSubclass.class)).isTrue();
+	}
+
+	@Test
+	public void testSupportsForType() {
+		Validator validator = Validator.forType(TestBean.class, (testBean, errors) -> {});
+		assertThat(validator.supports(TestBean.class)).isTrue();
+		assertThat(validator.supports(TestBeanSubclass.class)).isFalse();
+	}
+
+
+	private static class TestBeanSubclass extends TestBean {
+	}
+
+}