DATACMNS-1235 - Polishing.

Introduced BeanLookup to easily create a Lazy<T>-based lookup of a unique bean by type on a ListableBeanFactory. This is in support of making it easy for downstream Spring Data modules to consume the EntityPathResolver declared in an ApplicationContext using XML configuration.

QuerydslWebConfiguration now uses a plain ObjectProvider to access the bean defined falling back to our default.

Original pull request: #265.

src/main/java/org/springframework/data/util/BeanLookup.java

@@ -0,0 +1,80 @@
+/*
+ * Copyright 2018 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.util;
+
+import lombok.experimental.UtilityClass;
+
+import java.util.Map;
+
+import javax.annotation.Nullable;
+
+import org.springframework.beans.factory.BeanFactory;
+import org.springframework.beans.factory.ListableBeanFactory;
+import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
+import org.springframework.util.Assert;
+
+/**
+ * Simple helper to allow lenient lookup of beans of a given type from a {@link ListableBeanFactory}. This is not user
+ * facing API but a mere helper for Spring Data configuration code.
+ * 
+ * @author Oliver Gierke
+ * @since 2.1
+ * @soundtrack Dave Matthews Band - Bartender (DMB Live 25)
+ */
+@UtilityClass
+public class BeanLookup {
+
+	/**
+	 * Returns a {@link Lazy} for the unique bean of the given type from the given {@link BeanFactory} (which needs to be
+	 * a {@link ListableBeanFactory}). The lookup will produce a {@link NoUniqueBeanDefinitionException} in case multiple
+	 * beans of the given type are available in the given {@link BeanFactory}.
+	 * 
+	 * @param type must not be {@literal null}.
+	 * @param beanFactory the {@link BeanFactory} to lookup the bean from.
+	 * @return a {@link Lazy} for the unique bean of the given type or the instance provided by the fallback in case no
+	 *         bean of the given type can be found.
+	 */
+	public static <T> Lazy<T> lazyIfAvailable(Class<T> type, BeanFactory beanFactory) {
+
+		Assert.notNull(type, "Type must not be null!");
+		Assert.isInstanceOf(ListableBeanFactory.class, beanFactory);
+
+		return Lazy.of(() -> lookupBean(type, (ListableBeanFactory) beanFactory));
+	}
+
+	/**
+	 * Looks up the unique bean of the given type from the given {@link ListableBeanFactory}.
+	 * 
+	 * @param type must not be {@literal null}.
+	 * @param beanFactory must not be {@literal null}.
+	 * @return
+	 */
+	@Nullable
+	private static <T> T lookupBean(Class<T> type, ListableBeanFactory beanFactory) {
+
+		Map<String, T> names = beanFactory.getBeansOfType(type, false, false);
+
+		switch (names.size()) {
+
+			case 0:
+				return null;
+			case 1:
+				return names.values().iterator().next();
+			default:
+				throw new NoUniqueBeanDefinitionException(type, names.keySet());
+		}
+	}
+}