Workaround for WFLY-3474 NullPointerException

Prior to this commit, the ServletResponseHttpHeaders.get method
would throw an NPE when used under Wildfly 8.0.0.Final and 8.1.0.Final.
This can be traced to WFLY-3474, which throws an NPE when calling
HttpServletResponse.getHeaders("foo") and that header has not
been defined prior to that.
This would cause NPE being thrown by AbstractSockJsService when
checking for CORS HTTP headers in the server HTTP response.

This commit surrounds that method call in AbstractSockJsService and
guards against this issue.

Issue: SPR-11919
(cherry picked from commit 24cdefb)

spring-websocket/src/main/java/org/springframework/web/socket/sockjs/support/AbstractSockJsService.java

@@ -44,7 +44,6 @@ import org.springframework.util.StringUtils;
 import org.springframework.web.socket.WebSocketHandler;
 import org.springframework.web.socket.sockjs.SockJsException;
 import org.springframework.web.socket.sockjs.SockJsService;
-import org.springframework.web.util.UriComponentsBuilder;
 
 /**
  * An abstract base class for {@link SockJsService} implementations that provides SockJS
@@ -102,16 +101,20 @@ public abstract class AbstractSockJsService implements SockJsService {
 	}
 
 	/**
-	 * Transports which don't support cross-domain communication natively (e.g.
-	 * "eventsource", "htmlfile") rely on serving a simple page (using the
-	 * "foreign" domain) from an invisible iframe. Code run from this iframe
-	 * doesn't need to worry about cross-domain issues since it is running from
-	 * a domain local to the SockJS server. The iframe does need to load the
-	 * SockJS javascript client library and this option allows configuring that url.
-	 * For more details see the reference documentation.
-	 *
+	 * Transports with no native cross-domain communication (e.g. "eventsource",
+	 * "htmlfile") must get a simple page from the "foreign" domain in an invisible
+	 * iframe so that code in the iframe can run from  a domain local to the SockJS
+	 * server. Since the iframe needs to load the SockJS javascript client library,
+	 * this property allows specifying where to load it from.
 	 * <p>By default this is set to point to
 	 * "https://d1fxtkz8shb9d2.cloudfront.net/sockjs-0.3.4.min.js".
+	 * However, it can also be set to point to a URL served by the application.
+	 * <p>Note that it's possible to specify a relative URL in which case the URL
+	 * must be relative to the iframe URL. For example assuming a SockJS endpoint
+	 * mapped to "/sockjs", and resulting iframe URL "/sockjs/iframe.html", then the
+	 * the relative URL must start with "../../" to traverse up to the location
+	 * above the SockJS mapping. In case of a prefix-based Servlet mapping one more
+	 * traversal may be needed.
 	 */
 	public void setSockJsClientLibraryUrl(String clientLibraryUrl) {
 		this.clientLibraryUrl = clientLibraryUrl;
@@ -148,16 +151,13 @@ public abstract class AbstractSockJsService implements SockJsService {
 	 * clients with a "cookie_needed" boolean property that indicates whether the use of a
 	 * JSESSIONID cookie is required for the application to function correctly, e.g. for
 	 * load balancing or in Java Servlet containers for the use of an HTTP session.
-	 *
 	 * <p>This is especially important for IE 8,9 that support XDomainRequest -- a modified
 	 * AJAX/XHR -- that can do requests across domains but does not send any cookies. In
 	 * those cases, the SockJS client prefers the "iframe-htmlfile" transport over
 	 * "xdr-streaming" in order to be able to send cookies.
-	 *
 	 * <p>The SockJS protocol also expects a SockJS service to echo back the JSESSIONID
 	 * cookie when this property is set to true. However, when running in a Servlet
 	 * container this is not necessary since the container takes care of it.
-	 *
 	 * <p>The default value is "true" to maximize the chance for applications to work
 	 * correctly in IE 8,9 with support for cookies (and the JSESSIONID cookie in
 	 * particular). However, an application can choose to set this to "false" if
@@ -358,14 +358,18 @@ public abstract class AbstractSockJsService implements SockJsService {
 
 
 	protected void addCorsHeaders(ServerHttpRequest request, ServerHttpResponse response, HttpMethod... httpMethods) {
-
 		HttpHeaders requestHeaders = request.getHeaders();
 		HttpHeaders responseHeaders = response.getHeaders();
 
-		// Perhaps a CORS Filter has already added this?
-		if (!CollectionUtils.isEmpty(responseHeaders.get("Access-Control-Allow-Origin"))) {
-			logger.debug("Skip adding CORS headers, response already contains \"Access-Control-Allow-Origin\"");
-			return;
+		try {
+			// Perhaps a CORS Filter has already added this?
+			if (!CollectionUtils.isEmpty(responseHeaders.get("Access-Control-Allow-Origin"))) {
+				logger.debug("Skip adding CORS headers, response already contains \"Access-Control-Allow-Origin\"");
+				return;
+			}
+		}
+		catch (NullPointerException npe) {
+			// See SPR-11919 and https://issues.jboss.org/browse/WFLY-3474
 		}
 
 		String origin = requestHeaders.getFirst("origin");
@@ -434,6 +438,7 @@ public abstract class AbstractSockJsService implements SockJsService {
 		}
 	};
 
+
 	private final SockJsRequestHandler iframeHandler = new SockJsRequestHandler() {
 
 		private static final String IFRAME_CONTENT =

spring-websocket/src/test/java/org/springframework/web/socket/sockjs/support/SockJsServiceTests.java

@@ -23,6 +23,7 @@ import org.junit.Test;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.http.server.ServerHttpResponse;
+import org.springframework.http.server.ServletServerHttpResponse;
 import org.springframework.scheduling.TaskScheduler;
 import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;
 import org.springframework.web.socket.AbstractHttpRequestTests;
@@ -30,6 +31,10 @@ import org.springframework.web.socket.WebSocketHandler;
 import org.springframework.web.socket.sockjs.SockJsException;
 
 import static org.junit.Assert.*;
+import static org.mockito.Mockito.*;
+
+import javax.servlet.ServletOutputStream;
+import javax.servlet.http.HttpServletResponse;
 
 /**
  * Test fixture for {@link AbstractSockJsService}.
@@ -106,6 +111,21 @@ public class SockJsServiceTests extends AbstractHttpRequestTests {
 		assertEquals("foobar:123", this.servletResponse.getHeader("Access-Control-Allow-Origin"));
 	}
 
+	// SPR-11919
+
+	@Test
+	public void handleInfoGetWildflyNPE() throws Exception {
+		HttpServletResponse mockResponse = mock(HttpServletResponse.class);
+		ServletOutputStream ous = mock(ServletOutputStream.class);
+		when(mockResponse.getHeaders("Access-Control-Allow-Origin")).thenThrow(NullPointerException.class);
+		when(mockResponse.getOutputStream()).thenReturn(ous);
+		this.response = new ServletServerHttpResponse(mockResponse);
+
+		handleRequest("GET", "/echo/info", HttpStatus.OK);
+
+		verify(mockResponse, times(1)).getOutputStream();
+	}
+
 	@Test
 	public void handleInfoOptions() throws Exception {