Adds a CAST expression to the SQL DSL.

Example: Expressions.cast(table_user.column("name"),"VARCHAR2")

Also adds a toString to AbstractSegment to avoid stack overflows.

Closes #1066
Original pull request: #1071.

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

@@ -15,7 +15,10 @@
  */
 package org.springframework.data.relational.core.sql;
 
+import java.util.Arrays;
+
 import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 
 /**
  * Abstract implementation to support {@link Segment} implementations.
@@ -64,4 +67,10 @@ abstract class AbstractSegment implements Segment {
 	public boolean equals(Object obj) {
 		return obj instanceof Segment && toString().equals(obj.toString());
 	}
+
+	@Override
+	public String toString() {
+		return StringUtils.collectionToDelimitedString(Arrays.asList(children), ", ", getClass().getSimpleName() + "(",
+				")");
+	}
 }

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

@@ -0,0 +1,60 @@
+/*
+ * Copyright 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.data.relational.core.sql;
+
+import org.springframework.util.Assert;
+
+/**
+ * Represents a CAST expression like {@code CAST(something AS JSON}.
+ *
+ * @author Jens Schauder
+ * @since 2.3
+ */
+public class Cast extends AbstractSegment implements Expression {
+
+	private final String targetType;
+	private final Expression expression;
+
+	private Cast(Expression expression, String targetType) {
+
+		super(expression);
+
+		Assert.notNull(targetType, "Cast target must not be null!");
+
+		this.expression = expression;
+		this.targetType = targetType;
+	}
+
+	/**
+	 * Creates a new CAST expression.
+	 *
+	 * @param expression the expression to cast. Must not be {@literal null}.
+	 * @param targetType the type to cast to. Must not be {@literal null}.
+	 * @return guaranteed to be not {@literal null}.
+	 */
+	static Expression create(Expression expression, String targetType) {
+		return new Cast(expression, targetType);
+	}
+
+	public String getTargetType() {
+		return targetType;
+	}
+
+	@Override
+	public String toString() {
+		return "CAST(" + expression + " AS " + targetType + ")";
+	}
+}

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

@@ -53,6 +53,13 @@ public abstract class Expressions {
 		return table.asterisk();
 	}
 
+	/**
+	 * @return a new {@link Cast} expression.
+	 */
+	public static Expression cast(Expression expression, String targetType) {
+		return Cast.create(expression, targetType);
+	}
+
 	// Utility constructor.
 	private Expressions() {}
 

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

@@ -0,0 +1,77 @@
+/*
+ * Copyright 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.data.relational.core.sql.render;
+
+import java.util.StringJoiner;
+
+import org.springframework.data.relational.core.sql.Cast;
+import org.springframework.data.relational.core.sql.Visitable;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
+/**
+ * Renders a CAST expression, by delegating to an {@link ExpressionVisitor} and building the expression out of the
+ * rendered parts.
+ * 
+ * @author Jens Schauder
+ * @since 2.3
+ */
+class CastVisitor extends TypedSubtreeVisitor<Cast> implements PartRenderer {
+
+	private final RenderContext context;
+	@Nullable private StringJoiner joiner;
+	@Nullable private ExpressionVisitor expressionVisitor;
+
+	CastVisitor(RenderContext context) {
+
+		this.context = context;
+	}
+
+	@Override
+	Delegation enterMatched(Cast cast) {
+
+		joiner = new StringJoiner(", ", "CAST(", " AS " + cast.getTargetType() + ")");
+
+		return super.enterMatched(cast);
+	}
+
+	@Override
+	Delegation enterNested(Visitable segment) {
+
+		expressionVisitor = new ExpressionVisitor(context, ExpressionVisitor.AliasHandling.IGNORE);
+		return Delegation.delegateTo(expressionVisitor);
+	}
+
+	@Override
+	Delegation leaveNested(Visitable segment) {
+
+		Assert.state(joiner != null, "Joiner must not be null.");
+		Assert.state(expressionVisitor != null, "ExpressionVisitor must not be null.");
+
+		joiner.add(expressionVisitor.getRenderedPart());
+		return super.leaveNested(segment);
+	}
+
+	@Override
+	public CharSequence getRenderedPart() {
+
+		if (joiner == null) {
+			throw new IllegalStateException("Joiner must not be null.");
+		}
+
+		return joiner.toString();
+	}
+}

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

@@ -15,15 +15,7 @@
  */
 package org.springframework.data.relational.core.sql.render;
 
-import org.springframework.data.relational.core.sql.AsteriskFromTable;
-import org.springframework.data.relational.core.sql.BindMarker;
-import org.springframework.data.relational.core.sql.Column;
-import org.springframework.data.relational.core.sql.Condition;
-import org.springframework.data.relational.core.sql.Expression;
-import org.springframework.data.relational.core.sql.Named;
-import org.springframework.data.relational.core.sql.SimpleFunction;
-import org.springframework.data.relational.core.sql.SubselectExpression;
-import org.springframework.data.relational.core.sql.Visitable;
+import org.springframework.data.relational.core.sql.*;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 
@@ -105,6 +97,11 @@ class ExpressionVisitor extends TypedSubtreeVisitor<Expression> implements PartR
 			}
 		} else if (segment instanceof AsteriskFromTable) {
 			value = NameRenderer.render(context, ((AsteriskFromTable) segment).getTable()) + ".*";
+		} else if (segment instanceof Cast) {
+
+			CastVisitor visitor = new CastVisitor(context);
+			partRenderer = visitor;
+			return Delegation.delegateTo(visitor);
 		} else {
 			// works for literals and just and possibly more
 			value = segment.toString();
@@ -138,6 +135,7 @@ class ExpressionVisitor extends TypedSubtreeVisitor<Expression> implements PartR
 	Delegation leaveMatched(Expression segment) {
 
 		if (partRenderer != null) {
+
 			value = partRenderer.getRenderedPart();
 			partRenderer = null;
 		}

spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/ExpressionVisitorUnitTests.java

@@ -68,7 +68,10 @@ public class ExpressionVisitorUnitTests {
 				fixture("Count *", Functions.count(Expressions.asterisk()), "COUNT(*)"), //
 				fixture("Function", SimpleFunction.create("Function", asList(SQL.literalOf("one"), SQL.literalOf("two"))), //
 						"Function('one', 'two')"), //
-				fixture("Null", SQL.nullLiteral(), "NULL")); //
+				fixture("Null", SQL.nullLiteral(), "NULL"), //
+				fixture("Cast", Expressions.cast(Column.create("col", Table.create("tab")), "JSON"), "CAST(tab.col AS JSON)"), //
+				fixture("Cast with alias", Expressions.cast(Column.create("col", Table.create("tab")).as("alias"), "JSON"),
+						"CAST(tab.col AS JSON)")); //
 	}
 
 	@Test // GH-1003

spring-data-relational/src/test/java/org/springframework/data/relational/core/sql/render/SelectRendererUnitTests.java

@@ -168,8 +168,7 @@ class SelectRendererUnitTests {
 				.join(department) //
 				.on(Conditions.isEqual(employee.column("department_id"), department.column("id")) //
 						.or(Conditions.isNotEqual(employee.column("tenant"), department.column("tenant")) //
-						))
-				.build();
+						)).build();
 
 		assertThat(SqlRenderer.toString(select)).isEqualTo("SELECT employee.id, department.name FROM employee " //
 				+ "JOIN department ON employee.department_id = department.id " //
@@ -183,12 +182,11 @@ class SelectRendererUnitTests {
 		Table department = SQL.table("department");
 
 		Select select = Select.builder().select(employee.column("id"), department.column("name")).from(employee) //
-				.join(department)
-				.on(Expressions.just("alpha")).equals(Expressions.just("beta")) //
+				.join(department).on(Expressions.just("alpha")).equals(Expressions.just("beta")) //
 				.build();
 
-		assertThat(SqlRenderer.toString(select)).isEqualTo("SELECT employee.id, department.name FROM employee "
-				+ "JOIN department ON alpha = beta");
+		assertThat(SqlRenderer.toString(select))
+				.isEqualTo("SELECT employee.id, department.name FROM employee " + "JOIN department ON alpha = beta");
 	}
 
 	@Test // DATAJDBC-309
@@ -458,4 +456,15 @@ class SelectRendererUnitTests {
 		final String rendered = SqlRenderer.toString(select);
 		assertThat(rendered).isEqualTo("SELECT User.name, User.age FROM User WHERE User.age > 20");
 	}
+
+	@Test // GH-1066
+	void shouldRenderCast() {
+
+		Table table_user = SQL.table("User");
+		Select select = StatementBuilder.select(Expressions.cast(table_user.column("name"), "VARCHAR2")).from(table_user)
+				.build();
+
+		final String rendered = SqlRenderer.toString(select);
+		assertThat(rendered).isEqualTo("SELECT CAST(User.name AS VARCHAR2) FROM User");
+	}
 }