Add doc & tests to Jaxb2XmlEncoder for collections

Issue: SPR-16363

spring-web/src/test/java/org/springframework/http/codec/xml/Jaxb2XmlEncoderTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -17,10 +17,13 @@
 package org.springframework.http.codec.xml;
 
 import java.nio.charset.StandardCharsets;
+import java.util.Arrays;
 import java.util.Collections;
+import java.util.List;
 
 import org.junit.Test;
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.core.ResolvableType;
@@ -36,6 +39,10 @@ import static org.junit.Assert.assertThat;
 import static org.junit.Assert.assertTrue;
 import static org.xmlunit.matchers.CompareMatcher.isSimilarTo;
 
+import javax.xml.bind.annotation.XmlElement;
+import javax.xml.bind.annotation.XmlElements;
+import javax.xml.bind.annotation.XmlRootElement;
+
 /**
  * @author Sebastien Deleuze
  * @author Arjen Poutsma
@@ -66,8 +73,8 @@ public class Jaxb2XmlEncoderTests extends AbstractDataBufferAllocatingTestCase {
 	}
 
 	@Test
-	public void encode() throws Exception {
-		Flux<Pojo> source = Flux.just(new Pojo("foofoo", "barbar"), new Pojo("foofoofoo", "barbarbar"));
+	public void encode() {
+		Mono<Pojo> source = Mono.just(new Pojo("foofoo", "barbar"));
 		Flux<DataBuffer> output = this.encoder.encode(source, this.bufferFactory,
 				ResolvableType.forClass(Pojo.class),
 				MediaType.APPLICATION_XML, Collections.emptyMap());
@@ -84,8 +91,78 @@ public class Jaxb2XmlEncoderTests extends AbstractDataBufferAllocatingTestCase {
 						DataBufferUtils.release(dataBuffer);
 					}
 				})
-				.expectComplete()
-				.verify();
+				.verifyComplete();
+	}
+
+	@Test
+	public void encodeElementsWithCommonType() {
+		Mono<Container> source = Mono.just(new Container());
+		Flux<DataBuffer> output = this.encoder.encode(source, this.bufferFactory,
+				ResolvableType.forClass(Pojo.class),
+				MediaType.APPLICATION_XML, Collections.emptyMap());
+
+		StepVerifier.create(output)
+				.consumeNextWith(dataBuffer -> {
+					try {
+						String s = DataBufferTestUtils
+								.dumpString(dataBuffer, StandardCharsets.UTF_8);
+						assertThat(s, isSimilarTo("<?xml version='1.0' encoding='UTF-8' standalone='yes'?>" +
+								"<container><foo><name>name1</name></foo><bar><title>title1</title></bar></container>"));
+					}
+					finally {
+						DataBufferUtils.release(dataBuffer);
+					}
+				})
+				.verifyComplete();
+	}
+
+
+	public static class Model {}
+
+	public static class Foo extends Model {
+
+		private String name;
+
+		public Foo(String name) {
+			this.name = name;
+		}
+
+		public String getName() {
+			return this.name;
+		}
+
+		public void setName(String name) {
+			this.name = name;
+		}
+	}
+
+	public static class Bar extends Model {
+
+		private String title;
+
+		public Bar(String title) {
+			this.title = title;
+		}
+
+		public String getTitle() {
+			return title;
+		}
+
+		public void setTitle(String title) {
+			this.title = title;
+		}
+	}
+
+	@XmlRootElement
+	public static class Container {
+
+		@XmlElements({
+				@XmlElement(name="foo", type=Foo.class),
+				@XmlElement(name="bar", type=Bar.class)
+		})
+		public List<Model> getElements() {
+			return Arrays.asList(new Foo("name1"), new Bar("title1"));
+		}
 	}
 
 }