Test that double registration of ServletContextInitializers does not happen

Closes gh-4902

spring-boot/src/test/java/org/springframework/boot/web/servlet/ServletContextInitializerBeansTests.java

@@ -0,0 +1,121 @@
+/*
+ * Copyright 2012-2017 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.web.servlet;
+
+import java.io.IOException;
+
+import javax.servlet.Filter;
+import javax.servlet.FilterChain;
+import javax.servlet.FilterConfig;
+import javax.servlet.ServletContext;
+import javax.servlet.ServletException;
+import javax.servlet.ServletRequest;
+import javax.servlet.ServletResponse;
+import javax.servlet.http.HttpServlet;
+
+import org.junit.Test;
+
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.context.annotation.Bean;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ServletContextInitializerBeans}.
+ *
+ * @author Andy Wilkinson
+ */
+public class ServletContextInitializerBeansTests {
+
+	private ConfigurableApplicationContext context;
+
+	@Test
+	public void servletThatImplementsServletContextInitializerIsOnlyRegisteredOnce() {
+		load(ServletConfiguration.class);
+		ServletContextInitializerBeans initializerBeans = new ServletContextInitializerBeans(
+				this.context.getBeanFactory());
+		assertThat(initializerBeans.size()).isEqualTo(1);
+		assertThat(initializerBeans.iterator()).hasOnlyElementsOfType(TestServlet.class);
+	}
+
+	@Test
+	public void filterThatImplementsServletContextInitializerIsOnlyRegisteredOnce() {
+		load(FilterConfiguration.class);
+		ServletContextInitializerBeans initializerBeans = new ServletContextInitializerBeans(
+				this.context.getBeanFactory());
+		assertThat(initializerBeans.size()).isEqualTo(1);
+		assertThat(initializerBeans.iterator()).hasOnlyElementsOfType(TestFilter.class);
+	}
+
+	private void load(Class<?> configuration) {
+		this.context = new AnnotationConfigApplicationContext(configuration);
+	}
+
+	static class ServletConfiguration {
+
+		@Bean
+		public TestServlet testServlet() {
+			return new TestServlet();
+		}
+
+	}
+
+	static class FilterConfiguration {
+
+		@Bean
+		public TestFilter testFilter() {
+			return new TestFilter();
+		}
+
+	}
+
+	static class TestServlet extends HttpServlet implements ServletContextInitializer {
+
+		@Override
+		public void onStartup(ServletContext servletContext) throws ServletException {
+
+		}
+
+	}
+
+	static class TestFilter implements Filter, ServletContextInitializer {
+
+		@Override
+		public void onStartup(ServletContext servletContext) throws ServletException {
+
+		}
+
+		@Override
+		public void init(FilterConfig filterConfig) throws ServletException {
+
+		}
+
+		@Override
+		public void doFilter(ServletRequest request, ServletResponse response,
+				FilterChain chain) throws IOException, ServletException {
+
+		}
+
+		@Override
+		public void destroy() {
+
+		}
+
+	}
+
+}