Add Builder for ClassifierCompositeItemWriter

resolves BATCH-2599

Added null check for setClassifier in ClassifierCompositeItemWriter.

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/support/ClassifierCompositeItemWriter.java

@@ -24,6 +24,7 @@ import java.util.Map;
 import org.springframework.classify.Classifier;
 import org.springframework.classify.ClassifierSupport;
 import org.springframework.batch.item.ItemWriter;
+import org.springframework.util.Assert;
 
 /**
  * Calls one of a collection of ItemWriters for each item, based on a router
@@ -32,6 +33,7 @@ import org.springframework.batch.item.ItemWriter;
  * The implementation is thread-safe if all delegates are thread-safe.
  * 
  * @author Dave Syer
+ * @author Glenn Renfro
  * @since 2.0
  */
 public class ClassifierCompositeItemWriter<T> implements ItemWriter<T> {
@@ -42,6 +44,7 @@ public class ClassifierCompositeItemWriter<T> implements ItemWriter<T> {
 	 * @param classifier the classifier to set
 	 */
 	public void setClassifier(Classifier<T, ItemWriter<? super T>> classifier) {
+		Assert.notNull(classifier, "A classifier is required.");
 		this.classifier = classifier;
 	}
 

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/support/builder/ClassifierCompositeItemWriterBuilder.java

@@ -0,0 +1,63 @@
+/*
+ * Copyright 2017 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.batch.item.support.builder;
+
+import org.springframework.batch.item.ItemWriter;
+import org.springframework.batch.item.support.ClassifierCompositeItemWriter;
+import org.springframework.classify.Classifier;
+import org.springframework.classify.ClassifierSupport;
+import org.springframework.util.Assert;
+
+/**
+ * Creates a fully qualified ClassifierCompositeItemWriter.
+ *
+ * @author Glenn Renfro
+ *
+ * @since 4.0
+ */
+public class ClassifierCompositeItemWriterBuilder<T> {
+
+	private Classifier<T, ItemWriter<? super T>> classifier;
+
+	/**
+	 * Establish the classifier to be used for the selection of which {@link ItemWriter}
+	 * to use.
+	 *
+	 * @param classifier the classifier to set
+	 * @return this instance for method chaining
+	 * @see org.springframework.batch.item.support.ClassifierCompositeItemWriter#setClassifier(Classifier)
+	 */
+	public ClassifierCompositeItemWriterBuilder<T> classifier(Classifier<T, ItemWriter<? super T>> classifier) {
+		this.classifier = classifier;
+
+		return this;
+	}
+
+	/**
+	 * Returns a fully constructed {@link ClassifierCompositeItemWriter}.
+	 *
+	 * @return a new {@link ClassifierCompositeItemWriter}
+	 */
+	public ClassifierCompositeItemWriter<T> build() {
+		Assert.notNull(classifier, "A classifier is required.");
+
+		ClassifierCompositeItemWriter<T> writer = new ClassifierCompositeItemWriter<>();
+		writer.setClassifier(this.classifier);
+		return writer;
+	}
+
+}