Don't throw NPE when serving webjar directories

Prior to this change, serving resources with ResourceHttpRequestHandler
could result in NPE when requesting an existing folder located in a JAR.

This commit swallows those exceptions, as it is not possible to foresee
those cases without reading the actual resource. This result in a HTTP
200 response with a zero Content-Length instead of a HTTP 500 internal
exception.

Issue: SPR-13620

Author: bclozel

spring-webmvc/src/main/java/org/springframework/web/servlet/resource/ResourceHttpRequestHandler.java

@@ -410,6 +410,9 @@ protected void writeContent(HttpServletResponse response, Resource resource) thr
 			try {
 				StreamUtils.copy(in, response.getOutputStream());
 			}
+			catch (NullPointerException ex) {
+				// ignore, see SPR-13620
+			}
 			finally {
 				try {
 					in.close();

spring-webmvc/src/test/java/org/springframework/web/servlet/resource/ResourceHttpRequestHandlerTests.java

@@ -310,6 +310,20 @@ public void writeContentNotClosingInputStream() throws Exception {
 		assertEquals(0, this.response.getContentLength());
 	}
 
+	// SPR-13620
+	@Test
+	public void writeContentInputStreamThrowingNullPointerException() throws Exception {
+		Resource resource = mock(Resource.class);
+		InputStream in = mock(InputStream.class);
+		given(resource.getInputStream()).willReturn(in);
+		given(in.read(any())).willThrow(NullPointerException.class);
+
+		this.handler.writeContent(this.response, resource);
+
+		assertEquals(200, this.response.getStatus());
+		assertEquals(0, this.response.getContentLength());
+	}
+
 
 	private long headerAsLong(String responseHeaderName) {
 		return Long.valueOf(this.response.getHeader(responseHeaderName));