DATAJDBC-309 - Refactoring.

Refactor SQL rendering from a shared stack-based implementation to independent delegating visitors. Introduce DelegatingVisitor and TypedSubtreeVisitor base classes.

Introduce SelectList container.

Extract nested renderes to top-level types.

TODO: Replace null as delegation signal with Delegation object.

Author: mp911de

spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/DefaultSelect.java

@@ -30,7 +30,7 @@
 class DefaultSelect implements Select {
 
 	private final boolean distinct;
-	private final List<Expression> selectList;
+	private final SelectList selectList;
 	private final From from;
 	private final long limit;
 	private final long offset;
@@ -42,7 +42,7 @@ class DefaultSelect implements Select {
 				  List<Join> joins, @Nullable Condition where, List<OrderByField> orderBy) {
 
 		this.distinct = distinct;
-		this.selectList = new ArrayList<>(selectList);
+		this.selectList = new SelectList(new ArrayList<>(selectList));
 		this.from = new From(from);
 		this.limit = limit;
 		this.offset = offset;
@@ -85,7 +85,7 @@ public void visit(Visitor visitor) {
 
 		visitor.enter(this);
 
-		selectList.forEach(it -> it.visit(visitor));
+		selectList.visit(visitor);
 		from.visit(visitor);
 		joins.forEach(it -> it.visit(visitor));
 

spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/Select.java

@@ -35,7 +35,6 @@
  */
 public interface Select extends Segment, Visitable {
 
-
 	/**
 	 * Creates a new {@link SelectBuilder}.
 	 *

spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/SelectList.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2019 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
+ *
+ *      http://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.data.relational.core.sql;
+
+import java.util.List;
+
+import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
+
+/**
+ * @author Mark Paluch
+ */
+public class SelectList extends AbstractSegment {
+
+	private final List<Expression> selectList;
+
+	SelectList(List<Expression> selectList) {
+		super(selectList.toArray(new Expression[0]));
+		this.selectList = selectList;
+	}
+
+	@Override
+	public void visit(Visitor visitor) {
+
+		Assert.notNull(visitor, "Visitor must not be null!");
+
+		visitor.enter(this);
+		for (Segment child : selectList) {
+			child.visit(visitor);
+		}
+		visitor.leave(this);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see java.lang.Object#toString()
+	 */
+	@Override
+	public String toString() {
+		return StringUtils.collectionToDelimitedString(selectList, ", ");
+	}
+}

spring-data-relational/src/main/java/org/springframework/data/relational/core/sql/SubselectExpression.java

@@ -20,7 +20,7 @@
  *
  * @author Jens Schauder
  */
-class SubselectExpression extends AbstractSegment implements Expression {
+public class SubselectExpression extends AbstractSegment implements Expression {
 
 	private final Select subselect;