Revise PredicateSpecification for improved reuse.

Accept From instead of Root and apply the PredicateSpecification type parameter to From's target type. Dropping the type reduces our assumptions, it allows also for improved PredicateSpecification reuse without requiring a specific design choice of whether the PredicateSpecification applies to Root directly or a different From (Join, Embeddable, …).

Closes #4035

Author: mp911de

spring-data-jpa/src/main/java/org/springframework/data/jpa/domain/PredicateSpecification.java

@@ -16,8 +16,8 @@
 package org.springframework.data.jpa.domain;
 
 import jakarta.persistence.criteria.CriteriaBuilder;
+import jakarta.persistence.criteria.From;
 import jakarta.persistence.criteria.Predicate;
-import jakarta.persistence.criteria.Root;
 
 import java.io.Serializable;
 import java.util.Arrays;
@@ -34,12 +34,16 @@
  * <p>
  * Specifications can be composed into higher order functions from other specifications using
  * {@link #and(PredicateSpecification)}, {@link #or(PredicateSpecification)} or factory methods such as
- * {@link #allOf(Iterable)}.
+ * {@link #allOf(Iterable)} with reduced type interference of the query source type.
+ * <p>
+ * PredicateSpecifications are building blocks for composition and do not express their type opinion towards a specific
+ * entity source or join source type for improved reuse.
  * <p>
  * Composition considers whether one or more specifications contribute to the overall predicate by returning a
  * {@link Predicate} or {@literal null}. Specifications returning {@literal null}, such as {@link #unrestricted()}, are
  * considered to not contribute to the overall predicate, and their result is not considered in the final predicate.
  *
+ * @param <T> the type of the {@link From From target} to which the specification is applied.
  * @author Mark Paluch
  * @author Peter Aisher
  * @since 4.0
@@ -57,17 +61,17 @@ public interface PredicateSpecification<T> extends Serializable {
 	 * not(unrestricted()) // equivalent to `unrestricted()`
 	 * </pre>
 	 *
-	 * @param <T> the type of the {@link Root} the resulting {@literal PredicateSpecification} operates on.
+	 * @param <T> the type of the {@link From} the resulting {@literal PredicateSpecification} operates on.
 	 * @return guaranteed to be not {@literal null}.
 	 */
 	static <T> PredicateSpecification<T> unrestricted() {
-		return (root, builder) -> null;
+		return (from, builder) -> null;
 	}
 
 	/**
 	 * Simple static factory method to add some syntactic sugar around a {@literal PredicateSpecification}.
 	 *
-	 * @param <T> the type of the {@link Root} the resulting {@literal PredicateSpecification} operates on.
+	 * @param <T> the type of the {@link From} the resulting {@literal PredicateSpecification} operates on.
 	 * @param spec must not be {@literal null}.
 	 * @return guaranteed to be not {@literal null}.
 	 * @since 2.0
@@ -112,17 +116,17 @@ default PredicateSpecification<T> or(PredicateSpecification<T> other) {
 	/**
 	 * Negates the given {@link PredicateSpecification}.
 	 *
-	 * @param <T> the type of the {@link Root} the resulting {@literal PredicateSpecification} operates on.
+	 * @param <T> the type of the {@link From} the resulting {@literal PredicateSpecification} operates on.
 	 * @param spec can be {@literal null}.
 	 * @return guaranteed to be not {@literal null}.
 	 */
 	static <T> PredicateSpecification<T> not(PredicateSpecification<T> spec) {
 
 		Assert.notNull(spec, "Specification must not be null");
 
-		return (root, builder) -> {
+		return (from, builder) -> {
 
-			Predicate predicate = spec.toPredicate(root, builder);
+			Predicate predicate = spec.toPredicate(from, builder);
 			return predicate != null ? builder.not(predicate) : null;
 		};
 	}
@@ -187,13 +191,13 @@ static <T> PredicateSpecification<T> anyOf(Iterable<PredicateSpecification<T>> s
 
 	/**
 	 * Creates a WHERE clause for a query of the referenced entity in form of a {@link Predicate} for the given
-	 * {@link Root} and {@link CriteriaBuilder}.
+	 * {@link From} and {@link CriteriaBuilder}.
 	 *
-	 * @param root must not be {@literal null}.
+	 * @param from must not be {@literal null}.
 	 * @param criteriaBuilder must not be {@literal null}.
 	 * @return a {@link Predicate}, may be {@literal null}.
 	 */
 	@Nullable
-	Predicate toPredicate(Root<T> root, CriteriaBuilder criteriaBuilder);
+	Predicate toPredicate(From<?, T> from, CriteriaBuilder criteriaBuilder);
 
 }

spring-data-jpa/src/main/java/org/springframework/data/jpa/domain/SpecificationComposition.java

@@ -19,6 +19,7 @@
 import jakarta.persistence.criteria.CriteriaDelete;
 import jakarta.persistence.criteria.CriteriaQuery;
 import jakarta.persistence.criteria.CriteriaUpdate;
+import jakarta.persistence.criteria.From;
 import jakarta.persistence.criteria.Predicate;
 import jakarta.persistence.criteria.Root;
 
@@ -127,9 +128,9 @@ static <T> PredicateSpecification<T> composed(PredicateSpecification<T> lhs, Pre
 		};
 	}
 
-	private static <T> @Nullable Predicate toPredicate(@Nullable PredicateSpecification<T> specification, Root<T> root,
+	private static <T> @Nullable Predicate toPredicate(@Nullable PredicateSpecification<T> specification, From<?, T> from,
 			CriteriaBuilder builder) {
-		return specification == null ? null : specification.toPredicate(root, builder);
+		return specification == null ? null : specification.toPredicate(from, builder);
 	}
 
 }

spring-data-jpa/src/test/java/org/springframework/data/jpa/domain/PredicateSpecificationUnitTests.java

@@ -20,6 +20,7 @@
 import static org.springframework.util.SerializationUtils.*;
 
 import jakarta.persistence.criteria.CriteriaBuilder;
+import jakarta.persistence.criteria.From;
 import jakarta.persistence.criteria.Predicate;
 import jakarta.persistence.criteria.Root;
 
@@ -169,7 +170,7 @@ void notWithNullPredicate() {
 	static class SerializableSpecification implements Serializable, PredicateSpecification<Object> {
 
 		@Override
-		public Predicate toPredicate(Root<Object> root, CriteriaBuilder cb) {
+		public Predicate toPredicate(From<?, Object> root, CriteriaBuilder cb) {
 			return null;
 		}
 	}