DATAJPA-403 - Improve predicate building by reusing existing joins if possible.

QueryUtils now checks already existing joins and reuses them when building expressions. Inspired by the pull request #41 by Alexandre Payment but polished and added integration test to make sure the joins really get reused.

src/test/java/org/springframework/data/jpa/repository/query/QueryUtilsIntegrationTests.java

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+/*
+ * Copyright 2013 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.jpa.repository.query;
+
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+
+import javax.persistence.EntityManager;
+import javax.persistence.PersistenceContext;
+import javax.persistence.criteria.CriteriaBuilder;
+import javax.persistence.criteria.CriteriaQuery;
+import javax.persistence.criteria.Root;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.springframework.data.jpa.domain.sample.User;
+import org.springframework.data.mapping.PropertyPath;
+import org.springframework.test.context.ContextConfiguration;
+import org.springframework.test.context.junit4.SpringJUnit4ClassRunner;
+
+/**
+ * Integration tests for {@link QueryUtils}.
+ * 
+ * @author Oliver Gierke
+ */
+@RunWith(SpringJUnit4ClassRunner.class)
+@ContextConfiguration("classpath:infrastructure.xml")
+public class QueryUtilsIntegrationTests {
+
+	@PersistenceContext EntityManager em;
+
+	/**
+	 * @see DATAJPA-403
+	 */
+	@Test
+	public void reusesExistingJoinForExpression() {
+
+		CriteriaBuilder builder = em.getCriteriaBuilder();
+		CriteriaQuery<User> query = builder.createQuery(User.class);
+		Root<User> from = query.from(User.class);
+
+		PropertyPath managerFirstname = PropertyPath.from("manager.firstname", User.class);
+		PropertyPath managerLastname = PropertyPath.from("manager.lastname", User.class);
+
+		QueryUtils.toExpressionRecursively(from, managerLastname);
+		QueryUtils.toExpressionRecursively(from, managerFirstname);
+
+		assertThat(from.getJoins(), hasSize(1));
+	}
+}