Change default driver in XStreamMarshaller from XppDriver to DomDriver

As explained in commit a247b83cd9, the
XppDriver from XStream relies on the XPP3 library which publishes
javax.xml.namespace.QName as part of its JAR. The QName type is also
published by the java.xml system module in modular JREs (i.e., Java 9
or higher).

This results in a split package between the unnamed module and the
java.xml system module, which the Java Language Specification defines
as illegal (see §6.5.5.2 and §7.4.3).

Most Java compilers do not currently enforce this rule; however, the
Eclipse compiler does. This makes it impossible to use spring-oxm out
of the box in the Eclipse IDE. In addition, if bug JDK-8215739 is fixed
in future versions of other JDK implementations, this rule will affect
any users using spring-oxm with those JDKs.

This commit therefore switches the default driver in XStreamMarshaller
from XppDriver to DomDriver. Users can naturally switch back to the
XppDriver if they wish, since the driver is configurable.

Closes gh-27464

spring-oxm/src/main/java/org/springframework/oxm/xstream/XStreamMarshaller.java

@@ -52,6 +52,7 @@ import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
 import com.thoughtworks.xstream.io.StreamException;
 import com.thoughtworks.xstream.io.naming.NameCoder;
 import com.thoughtworks.xstream.io.xml.CompactWriter;
+import com.thoughtworks.xstream.io.xml.DomDriver;
 import com.thoughtworks.xstream.io.xml.DomReader;
 import com.thoughtworks.xstream.io.xml.DomWriter;
 import com.thoughtworks.xstream.io.xml.QNameMap;
@@ -59,7 +60,6 @@ import com.thoughtworks.xstream.io.xml.SaxWriter;
 import com.thoughtworks.xstream.io.xml.StaxReader;
 import com.thoughtworks.xstream.io.xml.StaxWriter;
 import com.thoughtworks.xstream.io.xml.XmlFriendlyNameCoder;
-import com.thoughtworks.xstream.io.xml.XppDriver;
 import com.thoughtworks.xstream.mapper.CannotResolveClassException;
 import com.thoughtworks.xstream.mapper.Mapper;
 import com.thoughtworks.xstream.mapper.MapperWrapper;
@@ -112,6 +112,11 @@ import org.springframework.util.xml.StaxUtils;
  * Note that {@link XStream} construction has been reworked in 4.0, with the
  * stream driver and the class loader getting passed into XStream itself now.
  *
+ * <p>As of Spring Framework 6.0, the default {@link HierarchicalStreamDriver} is
+ * a {@link DomDriver} that uses the configured {@linkplain #setEncoding(String)
+ * encoding} and {@link #setNameCoder(NameCoder) NameCoder}. The driver can be
+ * changed via {@link #setStreamDriver(HierarchicalStreamDriver)}.
+ *
  * @author Peter Meijer
  * @author Arjen Poutsma
  * @author Juergen Hoeller
@@ -205,7 +210,7 @@ public class XStreamMarshaller extends AbstractMarshaller implements BeanClassLo
 	}
 
 	/**
-	 * Set a XStream {@link HierarchicalStreamDriver} to be used for readers and writers.
+	 * Set an XStream {@link HierarchicalStreamDriver} to be used for readers and writers.
 	 * <p>As of Spring 4.0, this stream driver will also be passed to the {@link XStream}
 	 * constructor and therefore used by streaming-related native API methods themselves.
 	 */
@@ -216,7 +221,7 @@ public class XStreamMarshaller extends AbstractMarshaller implements BeanClassLo
 
 	private HierarchicalStreamDriver getDefaultDriver() {
 		if (this.defaultDriver == null) {
-			this.defaultDriver = new XppDriver();
+			this.defaultDriver = new DomDriver(this.encoding, this.nameCoder);
 		}
 		return this.defaultDriver;
 	}

spring-oxm/src/test/java/org/springframework/oxm/xstream/XStreamMarshallerTests.java

@@ -43,8 +43,6 @@ import com.thoughtworks.xstream.io.HierarchicalStreamWriter;
 import com.thoughtworks.xstream.io.json.JettisonMappedXmlDriver;
 import com.thoughtworks.xstream.io.json.JsonHierarchicalStreamDriver;
 import com.thoughtworks.xstream.io.json.JsonWriter;
-import com.thoughtworks.xstream.io.xml.DomDriver;
-import com.thoughtworks.xstream.io.xml.XmlFriendlyNameCoder;
 import com.thoughtworks.xstream.security.AnyTypePermission;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -86,7 +84,6 @@ class XStreamMarshallerTests {
 		marshaller = new XStreamMarshaller();
 		marshaller.setTypePermissions(AnyTypePermission.ANY);
 		marshaller.setAliases(Collections.singletonMap("flight", Flight.class.getName()));
-		marshaller.setStreamDriver(new DomDriver("UTF-8", new XmlFriendlyNameCoder()));
 		flight.setFlightNumber(42L);
 	}
 

spring-oxm/src/test/java/org/springframework/oxm/xstream/XStreamUnmarshallerTests.java

@@ -30,8 +30,6 @@ import javax.xml.transform.Source;
 import javax.xml.transform.dom.DOMSource;
 import javax.xml.transform.stream.StreamSource;
 
-import com.thoughtworks.xstream.io.xml.DomDriver;
-import com.thoughtworks.xstream.io.xml.XmlFriendlyNameCoder;
 import com.thoughtworks.xstream.security.AnyTypePermission;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -57,7 +55,6 @@ public class XStreamUnmarshallerTests {
 	public void createUnmarshaller() {
 		unmarshaller = new XStreamMarshaller();
 		unmarshaller.setTypePermissions(AnyTypePermission.ANY);
-		unmarshaller.setStreamDriver(new DomDriver("UTF-8", new XmlFriendlyNameCoder()));
 		Map<String, Class<?>> aliases = new HashMap<>();
 		aliases.put("flight", Flight.class);
 		unmarshaller.setAliases(aliases);