Add an ItemProcessor that delegates to a java.util.function.Function

This commit adds a new `ItemProcessor` that delegates to a
`java.util.function.Function`.  It also enables users to configure a
`Function` as an `ItemProcessor` via the builders (so they don't need to
worry about the `ItemProcessor` wrapper in the first place).

Resolves BATCH-2641

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/function/FunctionItemProcessor.java

@@ -0,0 +1,45 @@
+/*
+ * 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.function;
+
+import java.util.function.Function;
+
+import org.springframework.batch.item.ItemProcessor;
+import org.springframework.util.Assert;
+
+/**
+ * An {@link ItemProcessor} implementation that delegates to a {@link Function}
+ *
+ * @author Michael Minella
+ * @since 4.0
+ */
+public class FunctionItemProcessor<I, O> implements ItemProcessor<I, O>{
+
+	private final Function<I, O> function;
+
+	/**
+	 * @param function the delegate.  Must not be null
+	 */
+	public FunctionItemProcessor(Function<I, O> function) {
+		Assert.notNull(function, "A function is required");
+		this.function = function;
+	}
+
+	@Override
+	public O process(I item) throws Exception {
+		return this.function.apply(item);
+	}
+}