DATAJPA-1449 - Removal of Specifications.

`Specifications` is deprecated since 2.0. This removes it completely.

 * Moves static helper code into a private helper class.
 * Removes references to `Specifications` from `Specification`.

 Original pull request: #300.

Author: koraktor

src/main/java/org/springframework/data/jpa/domain/Specification.java

@@ -15,7 +15,8 @@
  */
 package org.springframework.data.jpa.domain;
 
-import static org.springframework.data.jpa.domain.Specifications.CompositionType.*;
+import static org.springframework.data.jpa.domain.SpecificationComposition.*;
+import static org.springframework.data.jpa.domain.SpecificationComposition.CompositionType.*;
 
 import java.io.Serializable;
 
@@ -35,11 +36,12 @@
  * @author Sebastian Staudt
  * @author Mark Paluch
  */
-@SuppressWarnings("deprecation")
 public interface Specification<T> extends Serializable {
 
 	long serialVersionUID = 1L;
 
+	Specification EMPTY_SPEC = (root, query, criteriaBuilder) -> null;
+
 	/**
 	 * Negates the given {@link Specification}.
 	 *
@@ -49,7 +51,7 @@ public interface Specification<T> extends Serializable {
 	 * @since 2.0
 	 */
 	static <T> Specification<T> not(Specification<T> spec) {
-		return Specifications.negated(spec);
+		return negated(spec);
 	}
 
 	/**
@@ -61,7 +63,7 @@ static <T> Specification<T> not(Specification<T> spec) {
 	 * @since 2.0
 	 */
 	static <T> Specification<T> where(Specification<T> spec) {
-		return Specifications.where(spec);
+		return spec == null ? EMPTY_SPEC : spec;
 	}
 
 	/**
@@ -72,7 +74,7 @@ static <T> Specification<T> where(Specification<T> spec) {
 	 * @since 2.0
 	 */
 	default Specification<T> and(Specification<T> other) {
-		return Specifications.composed(this, other, AND);
+		return composed(this, other, AND);
 	}
 
 	/**
@@ -83,7 +85,7 @@ default Specification<T> and(Specification<T> other) {
 	 * @since 2.0
 	 */
 	default Specification<T> or(Specification<T> other) {
-		return Specifications.composed(this, other, OR);
+		return composed(this, other, OR);
 	}
 
 	/**

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

@@ -0,0 +1,56 @@
+package org.springframework.data.jpa.domain;
+
+import org.springframework.lang.Nullable;
+
+import javax.persistence.criteria.CriteriaBuilder;
+import javax.persistence.criteria.Predicate;
+
+/**
+ * Helper class to support specification compositions
+ *
+ * @author Sebastian Staudt
+ * @see Specification
+ */
+class SpecificationComposition {
+
+    /**
+     * Enum for the composition types for {@link Predicate}s. Can not be turned into lambdas as we need to be
+     * serializable.
+     *
+     * @author Thomas Darimont
+     */
+    enum CompositionType {
+
+        AND {
+            @Override
+            public Predicate combine(CriteriaBuilder builder, Predicate lhs, Predicate rhs) {
+                return builder.and(lhs, rhs);
+            }
+        },
+
+        OR {
+            @Override
+            public Predicate combine(CriteriaBuilder builder, Predicate lhs, Predicate rhs) {
+                return builder.or(lhs, rhs);
+            }
+        };
+
+        abstract Predicate combine(CriteriaBuilder builder, Predicate lhs, Predicate rhs);
+    }
+
+    static <T> Specification<T> negated(@Nullable Specification<T> spec) {
+        return (root, query, builder) -> spec == null ? null : builder.not(spec.toPredicate(root, query, builder));
+    }
+
+    static <T> Specification<T> composed(@Nullable Specification<T> lhs, @Nullable Specification<T> rhs, CompositionType compositionType) {
+
+        return (root, query, builder) -> {
+
+            Predicate otherPredicate = rhs == null ? null : rhs.toPredicate(root, query, builder);
+            Predicate thisPredicate = lhs == null ? null : lhs.toPredicate(root, query, builder);
+
+            return thisPredicate == null ? otherPredicate
+                    : otherPredicate == null ? thisPredicate : compositionType.combine(builder, thisPredicate, otherPredicate);
+        };
+    }
+}