IN PROGRESS - BATCH-520: Add a Delegating File Reader For Multiple Files

added SortedMultiResourceItemReader which sorts the injected Resources using Comparator

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

@@ -16,8 +16,7 @@ import org.springframework.util.Assert;
  * Reset (rollback) capability is implemented by item buffering. To restart
  * correctly resource ordering needs to be preserved between runs.
  * 
- * TODO more robust restart e.g. sort resources alphabetically and store
- * filename instead of resource array index.
+ * @see SortedMultiResourceItemReader
  * 
  * @author Robert Kasanicky
  */

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

@@ -0,0 +1,49 @@
+package org.springframework.batch.item;
+
+import java.util.Arrays;
+import java.util.Comparator;
+
+import org.springframework.core.io.Resource;
+
+/**
+ * {@link MultiResourceItemReader} which orders the injected resources using
+ * {@link #setComparator(Comparator)} to avoid potential problems caused by
+ * resource re-ordering on restart.
+ * 
+ * @see MultiResourceItemReader
+ * 
+ * @author Robert Kasanicky
+ */
+public class SortedMultiResourceItemReader extends MultiResourceItemReader {
+
+	private Comparator comparator = new Comparator() {
+
+		/**
+		 * Compares resource filenames.
+		 */
+		public int compare(Object o1, Object o2) {
+			Resource r1 = (Resource) o1;
+			Resource r2 = (Resource) o2;
+			return r1.getFilename().compareTo(r2.getFilename());
+		}
+
+	};
+
+	/**
+	 * @param comparator used to order the injected resources, by default
+	 * compares {@link Resource#getFilename()} values.
+	 */
+	public void setComparator(Comparator comparator) {
+		this.comparator = comparator;
+	}
+
+	/**
+	 * Orders the resources using {@link #setComparator(Comparator)} before
+	 * setting them.
+	 */
+	public void setResources(Resource[] resources) {
+		Arrays.sort(resources, comparator);
+		super.setResources(resources);
+	}
+
+}

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/SortedMultiResourceItemReaderTests.java

@@ -0,0 +1,62 @@
+package org.springframework.batch.item;
+
+import java.util.Comparator;
+
+import junit.framework.TestCase;
+
+import org.springframework.core.io.FileSystemResource;
+import org.springframework.core.io.Resource;
+
+/**
+ * Tests for {@link SortedMultiResourceItemReader}.
+ */
+public class SortedMultiResourceItemReaderTests extends TestCase {
+
+	private SortedMultiResourceItemReader tested = new SortedMultiResourceItemReader();
+
+	private Resource r1 = new FileSystemResource("b");
+
+	private Resource r2 = new FileSystemResource("a");
+
+	private Resource r3 = new FileSystemResource("c");
+
+	private Resource[] resources = { r1, r2, r3 };
+
+	/**
+	 * Resources are ordered according to filename by default.
+	 */
+	public void testResourceOrdering() {
+
+		tested.setResources(resources);
+
+		assertSame(r2, resources[0]);
+		assertSame(r1, resources[1]);
+		assertSame(r3, resources[2]);
+
+	}
+
+	/**
+	 * Resources are ordered according to injected comparator.
+	 */
+	public void testResourceOrderingWithCustomComparator() {
+		Comparator comp = new Comparator() {
+
+			/**
+			 * Reversed ordering by filename.
+			 */
+			public int compare(Object o1, Object o2) {
+				Resource r1 = (Resource) o1;
+				Resource r2 = (Resource) o2;
+				return -r1.getFilename().compareTo(r2.getFilename());
+			}
+
+		};
+
+		tested.setComparator(comp);
+		tested.setResources(resources);
+
+		assertSame(r3, resources[0]);
+		assertSame(r1, resources[1]);
+		assertSame(r2, resources[2]);
+	}
+}