Additionally check for null on registring Servlets and Filters

See gh-482

spring-boot/src/main/java/org/springframework/boot/context/embedded/FilterRegistrationBean.java

@@ -230,7 +230,14 @@ public class FilterRegistrationBean extends RegistrationBean {
 	@Override
 	public void onStartup(ServletContext servletContext) throws ServletException {
 		Assert.notNull(this.filter, "Filter must not be null");
-		configure(servletContext.addFilter(getOrDeduceName(this.filter), this.filter));
+		String name = getOrDeduceName(this.filter);
+		FilterRegistration.Dynamic added = servletContext.addFilter(name, this.filter);
+		if (added == null) {
+			logger.info("Filter " + name
+					+ " was not registered (possibly already registered?)");
+			return;
+		}
+		configure(added);
 	}
 
 	/**

spring-boot/src/main/java/org/springframework/boot/context/embedded/ServletRegistrationBean.java

@@ -26,6 +26,7 @@ import javax.servlet.Servlet;
 import javax.servlet.ServletContext;
 import javax.servlet.ServletException;
 import javax.servlet.ServletRegistration;
+import javax.servlet.ServletRegistration.Dynamic;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
@@ -156,8 +157,15 @@ public class ServletRegistrationBean extends RegistrationBean {
 	@Override
 	public void onStartup(ServletContext servletContext) throws ServletException {
 		Assert.notNull(this.servlet, "Servlet must not be null");
-		logger.info("Mapping servlet: '" + getServletName() + "' to " + this.urlMappings);
-		configure(servletContext.addServlet(getServletName(), this.servlet));
+		String name = getServletName();
+		logger.info("Mapping servlet: '" + name + "' to " + this.urlMappings);
+		Dynamic added = servletContext.addServlet(name, this.servlet);
+		if (added == null) {
+			logger.info("Servlet " + name
+					+ " was not registered (possibly already registered?)");
+			return;
+		}
+		configure(added);
 	}
 
 	/**

spring-boot/src/test/java/org/springframework/boot/context/embedded/ServletRegistrationBeanTests.java

@@ -38,6 +38,7 @@ import org.mockito.MockitoAnnotations;
 import static org.mockito.BDDMockito.given;
 import static org.mockito.Matchers.anyObject;
 import static org.mockito.Matchers.anyString;
+import static org.mockito.Mockito.times;
 import static org.mockito.Mockito.verify;
 
 /**
@@ -79,6 +80,16 @@ public class ServletRegistrationBeanTests {
 		verify(this.registration).addMapping("/*");
 	}
 
+	@Test
+	public void startupWithDoubleRegistration() throws Exception {
+		ServletRegistrationBean bean = new ServletRegistrationBean(this.servlet);
+		given(this.servletContext.addServlet(anyString(), (Servlet) anyObject()))
+				.willReturn(null);
+		bean.onStartup(this.servletContext);
+		verify(this.servletContext).addServlet("mockServlet", this.servlet);
+		verify(this.registration, times(0)).setAsyncSupported(true);
+	}
+
 	@Test
 	public void startupWithSpecifiedValues() throws Exception {
 		ServletRegistrationBean bean = new ServletRegistrationBean();