Register @WebListeners in a way that allows them to register components

Previously, @WebListeners were discovered via custom component scanning
and then registered programmatically via the ServletContext. The servlet
spec requires any ServletContextListener registered in this manner to be
prohibited from programatically configuring servlets, filters, and
listeners. This left us not strictly complying with the servlet spec
as a ServletContextListener registered via a @weblistener annotation
should be able to programatically configure other components.

This commit updates WebListenerHandler to register each @weblistener
component directly with Jetty, Tomcat, or Undertow rather than via the
ServletContext API. This ensure that any @WebListener-annoated
ServletContextListener registered via servlet component scanning is
able to programatically register servlets, filters, and listeners.

There is a small chance that this will be a breaking change for some
users:

1. The ServletListenerRegistrationBeans that were previously defined
   for each @weblistener will now be
   WebListenerHandler.WebListenerRegistrars
2. Each @WebListener-annotated class will now be instantiated by
   Jetty, Tomcat, or Undertow. Jetty and Tomcat both require the class
   to be public and have a public default constructor. Previously,
   a package-private class or default constructor could be used as the
   instantiation was performed by Spring Framework. Undertow is not
   affected as it can instantiate a package-private type.

Author: wilkinsona

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/ServletWebServerFactoryAutoConfiguration.java

@@ -17,13 +17,15 @@
 package org.springframework.boot.autoconfigure.web.servlet;
 
 import java.util.function.Supplier;
+import java.util.stream.Collectors;
 
 import javax.servlet.DispatcherType;
 import javax.servlet.ServletRequest;
 
 import org.springframework.beans.BeansException;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
+import org.springframework.beans.factory.ObjectProvider;
 import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.beans.factory.support.RootBeanDefinition;
@@ -38,6 +40,7 @@
 import org.springframework.boot.web.server.ErrorPageRegistrarBeanPostProcessor;
 import org.springframework.boot.web.server.WebServerFactoryCustomizerBeanPostProcessor;
 import org.springframework.boot.web.servlet.FilterRegistrationBean;
+import org.springframework.boot.web.servlet.WebListenerRegistrar;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
@@ -69,8 +72,10 @@
 public class ServletWebServerFactoryAutoConfiguration {
 
 	@Bean
-	public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties) {
-		return new ServletWebServerFactoryCustomizer(serverProperties);
+	public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(ServerProperties serverProperties,
+			ObjectProvider<WebListenerRegistrar> webListenerRegistrars) {
+		return new ServletWebServerFactoryCustomizer(serverProperties,
+				webListenerRegistrars.orderedStream().collect(Collectors.toList()));
 	}
 
 	@Bean

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/servlet/ServletWebServerFactoryCustomizer.java

@@ -16,9 +16,13 @@
 
 package org.springframework.boot.autoconfigure.web.servlet;
 
+import java.util.Collections;
+import java.util.List;
+
 import org.springframework.boot.autoconfigure.web.ServerProperties;
 import org.springframework.boot.context.properties.PropertyMapper;
 import org.springframework.boot.web.server.WebServerFactoryCustomizer;
+import org.springframework.boot.web.servlet.WebListenerRegistrar;
 import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
 import org.springframework.core.Ordered;
 
@@ -37,8 +41,16 @@ public class ServletWebServerFactoryCustomizer
 
 	private final ServerProperties serverProperties;
 
+	private final Iterable<WebListenerRegistrar> webListenerRegistrars;
+
 	public ServletWebServerFactoryCustomizer(ServerProperties serverProperties) {
+		this(serverProperties, Collections.emptyList());
+	}
+
+	public ServletWebServerFactoryCustomizer(ServerProperties serverProperties,
+			List<WebListenerRegistrar> webListenerRegistrars) {
 		this.serverProperties = serverProperties;
+		this.webListenerRegistrars = webListenerRegistrars;
 	}
 
 	@Override
@@ -62,6 +74,9 @@ public void customize(ConfigurableServletWebServerFactory factory) {
 		map.from(this.serverProperties::getServerHeader).to(factory::setServerHeader);
 		map.from(this.serverProperties.getServlet()::getContextParameters).to(factory::setInitParameters);
 		map.from(this.serverProperties.getShutdown()).to(factory::setShutdown);
+		for (WebListenerRegistrar registrar : this.webListenerRegistrars) {
+			registrar.register(factory);
+		}
 	}
 
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/jetty/JettyServletWebServerFactory.java

@@ -27,6 +27,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.EventListener;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Set;
@@ -46,8 +47,11 @@
 import org.eclipse.jetty.server.session.FileSessionDataStore;
 import org.eclipse.jetty.server.session.SessionHandler;
 import org.eclipse.jetty.servlet.ErrorPageErrorHandler;
+import org.eclipse.jetty.servlet.ListenerHolder;
+import org.eclipse.jetty.servlet.ServletHandler;
 import org.eclipse.jetty.servlet.ServletHolder;
 import org.eclipse.jetty.servlet.ServletMapping;
+import org.eclipse.jetty.servlet.Source;
 import org.eclipse.jetty.util.resource.JarResource;
 import org.eclipse.jetty.util.resource.Resource;
 import org.eclipse.jetty.util.resource.ResourceCollection;
@@ -336,6 +340,7 @@ protected Configuration[] getWebAppContextConfigurations(WebAppContext webAppCon
 		configurations.add(getServletContextInitializerConfiguration(webAppContext, initializers));
 		configurations.add(getErrorPageConfiguration());
 		configurations.add(getMimeTypeConfiguration());
+		configurations.add(new WebListenersConfiguration(getWebListenerClassNames()));
 		configurations.addAll(getConfigurations());
 		return configurations.toArray(new Configuration[0]);
 	}
@@ -604,4 +609,40 @@ public String[] list() {
 
 	}
 
+	/**
+	 * {@link AbstractConfiguration} to apply {@code @WebListener} classes.
+	 */
+	private static class WebListenersConfiguration extends AbstractConfiguration {
+
+		private final Set<String> classNames;
+
+		WebListenersConfiguration(Set<String> webListenerClassNames) {
+			this.classNames = webListenerClassNames;
+		}
+
+		@Override
+		public void configure(WebAppContext context) throws Exception {
+			ServletHandler servletHandler = context.getServletHandler();
+			for (String className : this.classNames) {
+				configure(context, servletHandler, className);
+			}
+		}
+
+		private void configure(WebAppContext context, ServletHandler servletHandler, String className)
+				throws ClassNotFoundException {
+			ListenerHolder holder = servletHandler.newListenerHolder(new Source(Source.Origin.ANNOTATION, className));
+			holder.setHeldClass(loadClass(context, className));
+			servletHandler.addListener(holder);
+		}
+
+		@SuppressWarnings("unchecked")
+		private Class<? extends EventListener> loadClass(WebAppContext context, String className)
+				throws ClassNotFoundException {
+			ClassLoader classLoader = context.getClassLoader();
+			classLoader = (classLoader != null) ? classLoader : getClass().getClassLoader();
+			return (Class<? extends EventListener>) classLoader.loadClass(className);
+		}
+
+	}
+
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/tomcat/TomcatServletWebServerFactory.java

@@ -383,6 +383,9 @@ protected void configureContext(Context context, ServletContextInitializer[] ini
 		}
 		configureSession(context);
 		new DisableReferenceClearingContextCustomizer().customize(context);
+		for (String webListenerClassName : getWebListenerClassNames()) {
+			context.addApplicationListener(webListenerClassName);
+		}
 		for (TomcatContextCustomizer customizer : this.tomcatContextCustomizers) {
 			customizer.customize(context);
 		}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/undertow/UndertowServletWebServerFactory.java

@@ -25,6 +25,7 @@
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
+import java.util.EventListener;
 import java.util.HashMap;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -47,6 +48,7 @@
 import io.undertow.servlet.Servlets;
 import io.undertow.servlet.api.DeploymentInfo;
 import io.undertow.servlet.api.DeploymentManager;
+import io.undertow.servlet.api.ListenerInfo;
 import io.undertow.servlet.api.MimeMapping;
 import io.undertow.servlet.api.ServletContainerInitializerInfo;
 import io.undertow.servlet.api.ServletStackTraces;
@@ -330,6 +332,7 @@ private DeploymentManager createManager(ServletContextInitializer... initializer
 		deployment.setEagerFilterInit(this.eagerFilterInit);
 		deployment.setPreservePathOnForward(this.preservePathOnForward);
 		configureMimeMappings(deployment);
+		configureWebListeners(deployment);
 		for (UndertowDeploymentInfoCustomizer customizer : this.deploymentInfoCustomizers) {
 			customizer.customize(deployment);
 		}
@@ -350,6 +353,22 @@ private DeploymentManager createManager(ServletContextInitializer... initializer
 		return manager;
 	}
 
+	private void configureWebListeners(DeploymentInfo deployment) {
+		for (String className : getWebListenerClassNames()) {
+			try {
+				deployment.addListener(new ListenerInfo(loadWebListenerClass(className)));
+			}
+			catch (ClassNotFoundException ex) {
+				throw new IllegalStateException("Failed to load web listener class '" + className + "'", ex);
+			}
+		}
+	}
+
+	@SuppressWarnings("unchecked")
+	private Class<? extends EventListener> loadWebListenerClass(String className) throws ClassNotFoundException {
+		return (Class<? extends EventListener>) getServletClassLoader().loadClass(className);
+	}
+
 	private boolean isZeroOrLess(Duration timeoutDuration) {
 		return timeoutDuration == null || timeoutDuration.isZero() || timeoutDuration.isNegative();
 	}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/WebListenerHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -38,9 +38,25 @@ class WebListenerHandler extends ServletComponentHandler {
 	@Override
 	protected void doHandle(Map<String, Object> attributes, AnnotatedBeanDefinition beanDefinition,
 			BeanDefinitionRegistry registry) {
-		BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(ServletListenerRegistrationBean.class);
-		builder.addPropertyValue("listener", beanDefinition);
-		registry.registerBeanDefinition(beanDefinition.getBeanClassName(), builder.getBeanDefinition());
+		BeanDefinitionBuilder builder = BeanDefinitionBuilder
+				.rootBeanDefinition(ServletComponentWebListenerRegistrar.class);
+		builder.addConstructorArgValue(beanDefinition.getBeanClassName());
+		registry.registerBeanDefinition(beanDefinition.getBeanClassName() + "Registrar", builder.getBeanDefinition());
+	}
+
+	static class ServletComponentWebListenerRegistrar implements WebListenerRegistrar {
+
+		private final String listenerClassName;
+
+		ServletComponentWebListenerRegistrar(String listenerClassName) {
+			this.listenerClassName = listenerClassName;
+		}
+
+		@Override
+		public void register(WebListenerRegistry registry) {
+			registry.addWebListeners(this.listenerClassName);
+		}
+
 	}
 
 }

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/WebListenerRegistrar.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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 javax.servlet.annotation.WebListener;
+
+/**
+ * Interface to be implemented by types that register {@link WebListener @WebListeners}.
+ *
+ * @author Andy Wilkinson
+ * @since 2.4.0
+ */
+public interface WebListenerRegistrar {
+
+	/**
+	 * Register web listeners with the given registry.
+	 * @param registry the web listener registry
+	 */
+	void register(WebListenerRegistry registry);
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/WebListenerRegistry.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2012-2020 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
+ *
+ *      https://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 javax.servlet.annotation.WebListener;
+
+/**
+ * A registry that holds {@link WebListener @WebListeners}.
+ *
+ * @author Andy Wilkinson
+ * @since 2.4.0
+ */
+public interface WebListenerRegistry {
+
+	/**
+	 * Adds web listeners that will be registered with the servlet container.
+	 * @param webListenerClassNames the class names of the web listeners
+	 */
+	void addWebListeners(String... webListenerClassNames);
+
+}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/server/AbstractServletWebServerFactory.java

@@ -23,6 +23,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Locale;
@@ -81,6 +82,8 @@ public abstract class AbstractServletWebServerFactory extends AbstractConfigurab
 
 	private final StaticResourceJars staticResourceJars = new StaticResourceJars();
 
+	private final Set<String> webListenerClassNames = new HashSet<String>();
+
 	/**
 	 * Create a new {@link AbstractServletWebServerFactory} instance.
 	 */
@@ -283,6 +286,15 @@ protected final File getValidSessionStoreDir(boolean mkdirs) {
 		return this.session.getSessionStoreDirectory().getValidDirectory(mkdirs);
 	}
 
+	@Override
+	public void addWebListeners(String... webListenerClassNames) {
+		this.webListenerClassNames.addAll(Arrays.asList(webListenerClassNames));
+	}
+
+	protected final Set<String> getWebListenerClassNames() {
+		return this.webListenerClassNames;
+	}
+
 	/**
 	 * {@link ServletContextInitializer} to apply appropriate parts of the {@link Session}
 	 * configuration.

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/server/ConfigurableServletWebServerFactory.java

@@ -28,6 +28,7 @@
 import org.springframework.boot.web.server.MimeMappings;
 import org.springframework.boot.web.server.WebServerFactoryCustomizer;
 import org.springframework.boot.web.servlet.ServletContextInitializer;
+import org.springframework.boot.web.servlet.WebListenerRegistry;
 
 /**
  * A configurable {@link ServletWebServerFactory}.
@@ -41,7 +42,8 @@
  * @see ServletWebServerFactory
  * @see WebServerFactoryCustomizer
  */
-public interface ConfigurableServletWebServerFactory extends ConfigurableWebServerFactory, ServletWebServerFactory {
+public interface ConfigurableServletWebServerFactory
+		extends ConfigurableWebServerFactory, ServletWebServerFactory, WebListenerRegistry {
 
 	/**
 	 * Sets the context path for the web server. The context should start with a "/"