Suppress unwanted output to System.err when pretty printing content

Previously, when the XML pretty printer attempted to pretty print
the content of a request or response it would output an error to
System.err if the content was not valid XML. While, benign, this
output was distracting.

This commit updates the XML pretty printer to suppress its output to
System.err by configuring it with an ErrorHandler and an ErrorListener
that swallow any errors of which they are notified.

Closes gh-153

spring-restdocs-core/src/main/java/org/springframework/restdocs/operation/preprocess/PrettyPrintingContentModifier.java

@@ -23,13 +23,23 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
+import javax.xml.parsers.ParserConfigurationException;
+import javax.xml.parsers.SAXParser;
+import javax.xml.parsers.SAXParserFactory;
+import javax.xml.transform.ErrorListener;
 import javax.xml.transform.OutputKeys;
 import javax.xml.transform.Transformer;
+import javax.xml.transform.TransformerException;
 import javax.xml.transform.TransformerFactory;
+import javax.xml.transform.sax.SAXSource;
 import javax.xml.transform.stream.StreamResult;
-import javax.xml.transform.stream.StreamSource;
 
 import org.springframework.http.MediaType;
+import org.xml.sax.ErrorHandler;
+import org.xml.sax.InputSource;
+import org.xml.sax.SAXException;
+import org.xml.sax.SAXParseException;
+import org.xml.sax.XMLReader;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.SerializationFeature;
@@ -47,12 +57,14 @@ public class PrettyPrintingContentModifier implements ContentModifier {
 
 	@Override
 	public byte[] modifyContent(byte[] originalContent, MediaType contentType) {
-		for (PrettyPrinter prettyPrinter : PRETTY_PRINTERS) {
-			try {
-				return prettyPrinter.prettyPrint(originalContent).getBytes();
-			}
-			catch (Exception ex) {
-				// Continue
+		if (originalContent.length > 0) {
+			for (PrettyPrinter prettyPrinter : PRETTY_PRINTERS) {
+				try {
+					return prettyPrinter.prettyPrint(originalContent).getBytes();
+				}
+				catch (Exception ex) {
+					// Continue
+				}
 			}
 		}
 		return originalContent;
@@ -74,10 +86,61 @@ public class PrettyPrintingContentModifier implements ContentModifier {
 					"4");
 			transformer.setOutputProperty(OutputKeys.DOCTYPE_PUBLIC, "yes");
 			StringWriter transformed = new StringWriter();
-			transformer.transform(new StreamSource(new ByteArrayInputStream(original)),
+			transformer.setErrorListener(new SilentErrorListener());
+			transformer.transform(createSaxSource(original),
 					new StreamResult(transformed));
 			return transformed.toString();
 		}
+
+		private SAXSource createSaxSource(byte[] original)
+				throws ParserConfigurationException, SAXException {
+			SAXParserFactory parserFactory = SAXParserFactory.newInstance();
+			SAXParser parser = parserFactory.newSAXParser();
+			XMLReader xmlReader = parser.getXMLReader();
+			xmlReader.setErrorHandler(new SilentErrorHandler());
+			SAXSource xmlInput = new SAXSource(xmlReader, new InputSource(
+					new ByteArrayInputStream(original)));
+			return xmlInput;
+		}
+
+		private static final class SilentErrorListener implements ErrorListener {
+
+			@Override
+			public void warning(TransformerException exception)
+					throws TransformerException {
+
+			}
+
+			@Override
+			public void error(TransformerException exception) throws TransformerException {
+
+			}
+
+			@Override
+			public void fatalError(TransformerException exception)
+					throws TransformerException {
+
+			}
+
+		}
+
+		private static final class SilentErrorHandler implements ErrorHandler {
+
+			@Override
+			public void warning(SAXParseException exception) throws SAXException {
+
+			}
+
+			@Override
+			public void error(SAXParseException exception) throws SAXException {
+
+			}
+
+			@Override
+			public void fatalError(SAXParseException exception) throws SAXException {
+
+			}
+		}
 	}
 
 	private static final class JsonPrettyPrinter implements PrettyPrinter {