Use SingletonSupplier for XStream instance in XStreamMarshaller

Closes gh-25017

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

@@ -84,6 +84,7 @@
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
+import org.springframework.util.function.SingletonSupplier;
 import org.springframework.util.xml.StaxUtils;
 
 /**
@@ -187,8 +188,7 @@ public class XStreamMarshaller extends AbstractMarshaller implements BeanClassLo
 
 	private ClassLoader beanClassLoader = new CompositeClassLoader();
 
-	@Nullable
-	private volatile XStream xstream;
+	private final SingletonSupplier<XStream> xstream = SingletonSupplier.of(this::buildXStream);
 
 
 	/**
@@ -407,12 +407,12 @@ public void setBeanClassLoader(ClassLoader classLoader) {
 
 	@Override
 	public void afterPropertiesSet() {
-		this.xstream = buildXStream();
+		// no-op due to use of SingletonSupplier for the XStream field.
 	}
 
 	/**
 	 * Build the native XStream delegate to be used by this marshaller,
-	 * delegating to {@link #constructXStream()}, {@link #configureXStream}
+	 * delegating to {@link #constructXStream}, {@link #configureXStream},
 	 * and {@link #customizeXStream}.
 	 */
 	protected XStream buildXStream() {
@@ -617,21 +617,11 @@ protected void customizeXStream(XStream xstream) {
 	 * <p><b>NOTE: This method has been marked as final as of Spring 4.0.</b>
 	 * It can be used to access the fully configured XStream for marshalling
 	 * but not configuration purposes anymore.
-	 * <p>As of Spring Framework 5.2.7, creation of the {@link XStream} instance
+	 * <p>As of Spring Framework 5.1.16, creation of the {@link XStream} instance
 	 * returned by this method is thread safe.
 	 */
 	public final XStream getXStream() {
-		XStream xs = this.xstream;
-		if (xs == null) {
-			synchronized (this) {
-				xs = this.xstream;
-				if (xs == null) {
-					xs = buildXStream();
-					this.xstream = xs;
-				}
-			}
-		}
-		return xs;
+		return this.xstream.obtain();
 	}