Allow StaxEventItemReader to auto-detect the input file encoding

Before this commit, it was not possible to pass a null encoding
to the StaxEventItemReader, which prevents the XML event reader
to auto-detect the file encoding.

This commits makes the encoding setter more lenient by accepting
a null value.

Resolves #4101

Author: fmbenhassine

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2006-2022 the original author or authors.
+ * Copyright 2006-2023 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.
@@ -142,10 +142,10 @@ public void setXmlInputFactory(XMLInputFactory xmlInputFactory) {
 
 	/**
 	 * Set encoding to be used for the input file. Defaults to {@link #DEFAULT_ENCODING}.
-	 * @param encoding the encoding to be used
+	 * @param encoding the encoding to be used. Can be {@code null}, in which case, the
+	 * XML event reader will attempt to auto-detect the encoding from tht input file.
 	 */
-	public void setEncoding(String encoding) {
-		Assert.notNull(encoding, "The encoding must not be null");
+	public void setEncoding(@Nullable String encoding) {
 		this.encoding = encoding;
 	}
 
@@ -239,7 +239,8 @@ protected void doOpen() throws Exception {
 		}
 
 		inputStream = resource.getInputStream();
-		eventReader = xmlInputFactory.createXMLEventReader(inputStream, this.encoding);
+		eventReader = this.encoding != null ? xmlInputFactory.createXMLEventReader(inputStream, this.encoding)
+				: xmlInputFactory.createXMLEventReader(inputStream);
 		fragmentReader = new DefaultFragmentEventReader(eventReader);
 		noInput = false;
 

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2017-2020 the original author or authors.
+ * Copyright 2017-2023 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.
@@ -26,6 +26,7 @@
 
 import org.springframework.batch.item.xml.StaxEventItemReader;
 import org.springframework.core.io.Resource;
+import org.springframework.lang.Nullable;
 import org.springframework.oxm.Unmarshaller;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -194,12 +195,13 @@ public StaxEventItemReaderBuilder<T> xmlInputFactory(XMLInputFactory xmlInputFac
 
 	/**
 	 * Encoding for the input file. Defaults to
-	 * {@link StaxEventItemReader#DEFAULT_ENCODING}.
+	 * {@link StaxEventItemReader#DEFAULT_ENCODING}. Can be {@code null}, in which case
+	 * the XML event reader will attempt to auto-detect the encoding from tht input file.
 	 * @param encoding String encoding algorithm
 	 * @return the current instance of the builder
 	 * @see StaxEventItemReader#setEncoding(String)
 	 */
-	public StaxEventItemReaderBuilder<T> encoding(String encoding) {
+	public StaxEventItemReaderBuilder<T> encoding(@Nullable String encoding) {
 		this.encoding = encoding;
 
 		return this;

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2008-2022 the original author or authors.
+ * Copyright 2008-2023 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.
@@ -19,6 +19,7 @@
 import org.hamcrest.Matchers;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+
 import org.springframework.batch.item.ExecutionContext;
 import org.springframework.batch.item.ItemCountAware;
 import org.springframework.batch.item.ItemStreamException;
@@ -36,8 +37,10 @@
 import javax.xml.namespace.QName;
 import javax.xml.stream.FactoryConfigurationError;
 import javax.xml.stream.XMLEventReader;
+import javax.xml.stream.XMLInputFactory;
 import javax.xml.stream.XMLStreamException;
 import javax.xml.stream.events.EndElement;
+import javax.xml.stream.events.StartDocument;
 import javax.xml.stream.events.StartElement;
 import javax.xml.stream.events.XMLEvent;
 import javax.xml.transform.Source;
@@ -58,6 +61,9 @@
 import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
 
 /**
  * Tests for {@link StaxEventItemReader}.
@@ -159,6 +165,37 @@ void testCustomEncoding() throws Exception {
 		source.close();
 	}
 
+	@Test
+	void testNullEncoding() throws Exception {
+		// given
+		XMLEventReader eventReader = mock(XMLEventReader.class);
+		when(eventReader.peek()).thenReturn(mock(StartDocument.class));
+
+		Resource resource = mock(Resource.class);
+		InputStream inputStream = mock(InputStream.class);
+		when(resource.getInputStream()).thenReturn(inputStream);
+		when(resource.isReadable()).thenReturn(true);
+		when(resource.exists()).thenReturn(true);
+		XMLInputFactory xmlInputFactory = mock(XMLInputFactory.class);
+		when(xmlInputFactory.createXMLEventReader(inputStream)).thenReturn(eventReader);
+
+		StaxEventItemReader<Object> reader = new StaxEventItemReader<>();
+		reader.setUnmarshaller(new MockFragmentUnmarshaller());
+		reader.setFragmentRootElementName(FRAGMENT_ROOT_ELEMENT);
+		reader.setResource(resource);
+		reader.setEncoding(null);
+		reader.setStrict(false);
+		reader.setXmlInputFactory(xmlInputFactory);
+		reader.afterPropertiesSet();
+
+		// when
+		reader.open(new ExecutionContext());
+
+		// then
+		verify(xmlInputFactory).createXMLEventReader(inputStream);
+		reader.close();
+	}
+
 	@Test
 	void testItemCountAwareFragment() throws Exception {
 		StaxEventItemReader<ItemCountAwareFragment> source = createNewItemCountAwareInputSource();