ServletRequestMethodArgumentResolver passes null references through (again)

Issue: SPR-15214
(cherry picked from commit fc11321)

Author: jhoeller

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ServletRequestMethodArgumentResolver.java

@@ -100,31 +100,31 @@ public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer m
 		}
 		else if (HttpSession.class.isAssignableFrom(paramType)) {
 			HttpSession session = request.getSession();
-			if (!paramType.isInstance(session)) {
+			if (session != null && !paramType.isInstance(session)) {
 				throw new IllegalStateException(
 						"Current session is not of type [" + paramType.getName() + "]: " + session);
 			}
 			return session;
 		}
 		else if (InputStream.class.isAssignableFrom(paramType)) {
 			InputStream inputStream = request.getInputStream();
-			if (!paramType.isInstance(inputStream)) {
+			if (inputStream != null && !paramType.isInstance(inputStream)) {
 				throw new IllegalStateException(
 						"Request input stream is not of type [" + paramType.getName() + "]: " + inputStream);
 			}
 			return inputStream;
 		}
 		else if (Reader.class.isAssignableFrom(paramType)) {
 			Reader reader = request.getReader();
-			if (!paramType.isInstance(reader)) {
+			if (reader != null && !paramType.isInstance(reader)) {
 				throw new IllegalStateException(
 						"Request body reader is not of type [" + paramType.getName() + "]: " + reader);
 			}
 			return reader;
 		}
 		else if (Principal.class.isAssignableFrom(paramType)) {
 			Principal userPrincipal = request.getUserPrincipal();
-			if (!paramType.isInstance(userPrincipal)) {
+			if (userPrincipal != null && !paramType.isInstance(userPrincipal)) {
 				throw new IllegalStateException(
 						"Current user principal is not of type [" + paramType.getName() + "]: " + userPrincipal);
 			}

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ServletRequestMethodArgumentResolverTests.java

@@ -50,25 +50,27 @@
  */
 public class ServletRequestMethodArgumentResolverTests {
 
-	private final ServletRequestMethodArgumentResolver resolver = new ServletRequestMethodArgumentResolver();
-
-	private Method method;
+	private ServletRequestMethodArgumentResolver resolver;
 
 	private ModelAndViewContainer mavContainer;
 
+	private MockHttpServletRequest servletRequest;
+
 	private ServletWebRequest webRequest;
 
-	private MockHttpServletRequest servletRequest;
+	private Method method;
 
 
 	@Before
-	public void setUp() throws Exception {
-		method = getClass().getMethod("supportedParams", ServletRequest.class, MultipartRequest.class,
-				HttpSession.class, Principal.class, Locale.class, InputStream.class, Reader.class,
-				WebRequest.class, TimeZone.class, ZoneId.class, HttpMethod.class);
+	public void setup() throws Exception {
+		resolver = new ServletRequestMethodArgumentResolver();
 		mavContainer = new ModelAndViewContainer();
 		servletRequest = new MockHttpServletRequest("GET", "");
 		webRequest = new ServletWebRequest(servletRequest, new MockHttpServletResponse());
+
+		method = getClass().getMethod("supportedParams", ServletRequest.class, MultipartRequest.class,
+				HttpSession.class, Principal.class, Locale.class, InputStream.class, Reader.class,
+				WebRequest.class, TimeZone.class, ZoneId.class, HttpMethod.class);
 	}
 
 
@@ -112,6 +114,15 @@ public String getName() {
 		assertSame("Invalid result", principal, result);
 	}
 
+	@Test
+	public void principalAsNull() throws Exception {
+		MethodParameter principalParameter = new MethodParameter(method, 3);
+		assertTrue("Principal not supported", resolver.supportsParameter(principalParameter));
+
+		Object result = resolver.resolveArgument(principalParameter, null, webRequest, null);
+		assertNull("Invalid result", result);
+	}
+
 	@Test
 	public void locale() throws Exception {
 		Locale locale = Locale.ENGLISH;