Clarified that getBody() never returns null

As the only place that historically differed, HttpComponents(Async)ClientHttpResponse returns an empty stream instead of null now.

Issue: SPR-13563
(cherry picked from commit ca60d79)

spring-web/src/main/java/org/springframework/http/HttpInputMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2010 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -23,7 +23,8 @@ import java.io.InputStream;
  * Represents an HTTP input message, consisting of {@linkplain #getHeaders() headers}
  * and a readable {@linkplain #getBody() body}.
  *
- * <p>Typically implemented by an HTTP request on the server-side, or a response on the client-side.
+ * <p>Typically implemented by an HTTP request handle on the server side,
+ * or an HTTP response handle on the client side.
  *
  * @author Arjen Poutsma
  * @since 3.0
@@ -32,7 +33,7 @@ public interface HttpInputMessage extends HttpMessage {
 
 	/**
 	 * Return the body of the message as an input stream.
-	 * @return the input stream body
+	 * @return the input stream body (never {@code null})
 	 * @throws IOException in case of I/O Errors
 	 */
 	InputStream getBody() throws IOException;

spring-web/src/main/java/org/springframework/http/HttpMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2009 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -17,8 +17,8 @@
 package org.springframework.http;
 
 /**
- * Represents the base interface for HTTP request and response messages. Consists of {@link HttpHeaders}, retrievable
- * via {@link #getHeaders()}.
+ * Represents the base interface for HTTP request and response messages.
+ * Consists of {@link HttpHeaders}, retrievable via {@link #getHeaders()}.
  *
  * @author Arjen Poutsma
  * @since 3.0
@@ -27,7 +27,7 @@ public interface HttpMessage {
 
 	/**
 	 * Return the headers of this message.
-	 * @return a corresponding HttpHeaders object
+	 * @return a corresponding HttpHeaders object (never {@code null})
 	 */
 	HttpHeaders getHeaders();
 

spring-web/src/main/java/org/springframework/http/HttpOutputMessage.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2010 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -23,7 +23,8 @@ import java.io.OutputStream;
  * Represents an HTTP output message, consisting of {@linkplain #getHeaders() headers}
  * and a writable {@linkplain #getBody() body}.
  *
- * <p>Typically implemented by an HTTP request on the client-side, or a response on the server-side.
+ * <p>Typically implemented by an HTTP request handle on the client side,
+ * or an HTTP response handle on the server side.
  *
  * @author Arjen Poutsma
  * @since 3.0
@@ -32,7 +33,7 @@ public interface HttpOutputMessage extends HttpMessage {
 
 	/**
 	 * Return the body of the message as an output stream.
-	 * @return the output stream body
+	 * @return the output stream body (never {@code null})
 	 * @throws IOException in case of I/O Errors
 	 */
 	OutputStream getBody() throws IOException;

spring-web/src/main/java/org/springframework/http/client/HttpComponentsClientHttpResponse.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -16,6 +16,7 @@
 
 package org.springframework.http.client;
 
+import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 
@@ -69,7 +70,7 @@ final class HttpComponentsClientHttpResponse extends AbstractClientHttpResponse
 
 	public InputStream getBody() throws IOException {
 		HttpEntity entity = this.httpResponse.getEntity();
-		return entity != null ? entity.getContent() : null;
+		return (entity != null ? entity.getContent() : new ByteArrayInputStream(new byte[0]));
 	}
 
 	public void close() {