Add support to build StaxEventItemReader when no Resource is provided

Resolves #3736

Author: parikshitdutta

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/xml/builder/StaxEventItemReaderBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2019 the original author or authors.
+ * Copyright 2017-2020 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.
@@ -21,6 +21,9 @@
 
 import javax.xml.stream.XMLInputFactory;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
+
 import org.springframework.batch.item.xml.StaxEventItemReader;
 import org.springframework.batch.item.xml.StaxUtils;
 import org.springframework.core.io.Resource;
@@ -33,10 +36,13 @@
  *
  * @author Michael Minella
  * @author Glenn Renfro
+ * @author Parikshit Dutta
  * @since 4.0
  */
 public class StaxEventItemReaderBuilder<T> {
 
+	protected Log logger = LogFactory.getLog(getClass());
+
 	private boolean strict = true;
 
 	private Resource resource;
@@ -199,10 +205,13 @@ public StaxEventItemReaderBuilder<T> xmlInputFactory(XMLInputFactory xmlInputFac
 	 * @return a new instance of the {@link StaxEventItemReader}
 	 */
 	public StaxEventItemReader<T> build() {
-		Assert.notNull(this.resource, "A resource is required.");
-
 		StaxEventItemReader<T> reader = new StaxEventItemReader<>();
 
+		if (this.resource == null) {
+			logger.debug("The resource is null. This is only a valid scenario when " +
+					"injecting resource later as in when using the MultiResourceItemReader");
+		}
+
 		if (this.saveState) {
 			Assert.state(StringUtils.hasText(this.name), "A name is required when saveState is set to true.");
 		}

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/xml/builder/StaxEventItemReaderBuilderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2019 the original author or authors.
+ * Copyright 2017-2020 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.
@@ -32,11 +32,13 @@
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.fail;
 
 /**
  * @author Michael Minella
  * @author Mahmoud Ben Hassine
+ * @author Parikshit Dutta
  */
 public class StaxEventItemReaderBuilderTests {
 
@@ -55,13 +57,6 @@ public void setUp() {
 
 	@Test
 	public void testValidation() {
-		try {
-			new StaxEventItemReaderBuilder<Foo>().build();
-			fail("Validation of the missing resource failed");
-		}
-		catch (IllegalArgumentException ignore) {
-		}
-
 		try {
 			new StaxEventItemReaderBuilder<Foo>()
 					.resource(this.resource)
@@ -85,6 +80,16 @@ public void testValidation() {
 		}
 	}
 
+	@Test
+	public void testBuildWithoutProvidingResource() {
+		StaxEventItemReader<Foo> reader = new StaxEventItemReaderBuilder<Foo>()
+				.name("fooReader")
+				.addFragmentRootElements("foo")
+				.build();
+
+		assertNotNull(reader);
+	}
+
 	@Test
 	public void testConfiguration() throws Exception {
 		Jaxb2Marshaller unmarshaller = new Jaxb2Marshaller();