org.springframework.web.SpringServletContainerInitializer Maven / Gradle / Ivy
/*
* Copyright 2002-2020 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.web;
import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.ServiceLoader;
import java.util.Set;
import javax.servlet.ServletContainerInitializer;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.annotation.HandlesTypes;
import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.lang.Nullable;
import org.springframework.util.ReflectionUtils;
/**
* Servlet 3.0 {@link ServletContainerInitializer} designed to support code-based
* configuration of the servlet container using Spring's {@link WebApplicationInitializer}
* SPI as opposed to (or possibly in combination with) the traditional
* {@code web.xml}-based approach.
*
* Mechanism of Operation
* This class will be loaded and instantiated and have its {@link #onStartup}
* method invoked by any Servlet 3.0-compliant container during container startup assuming
* that the {@code spring-web} module JAR is present on the classpath. This occurs through
* the JAR Services API {@link ServiceLoader#load(Class)} method detecting the
* {@code spring-web} module's {@code META-INF/services/javax.servlet.ServletContainerInitializer}
* service provider configuration file. See the
*
* JAR Services API documentation as well as section 8.2.4 of the Servlet 3.0
* Final Draft specification for complete details.
*
* In combination with {@code web.xml}
* A web application can choose to limit the amount of classpath scanning the Servlet
* container does at startup either through the {@code metadata-complete} attribute in
* {@code web.xml}, which controls scanning for Servlet annotations or through an
* {@code } element also in {@code web.xml}, which controls which
* web fragments (i.e. jars) are allowed to perform a {@code ServletContainerInitializer}
* scan. When using this feature, the {@link SpringServletContainerInitializer}
* can be enabled by adding "spring_web" to the list of named web fragments in
* {@code web.xml} as follows:
*
*
* <absolute-ordering>
* <name>some_web_fragment</name>
* <name>spring_web</name>
* </absolute-ordering>
*
*
* Relationship to Spring's {@code WebApplicationInitializer}
* Spring's {@code WebApplicationInitializer} SPI consists of just one method:
* {@link WebApplicationInitializer#onStartup(ServletContext)}. The signature is intentionally
* quite similar to {@link ServletContainerInitializer#onStartup(Set, ServletContext)}:
* simply put, {@code SpringServletContainerInitializer} is responsible for instantiating
* and delegating the {@code ServletContext} to any user-defined
* {@code WebApplicationInitializer} implementations. It is then the responsibility of
* each {@code WebApplicationInitializer} to do the actual work of initializing the
* {@code ServletContext}. The exact process of delegation is described in detail in the
* {@link #onStartup onStartup} documentation below.
*
* General Notes
* In general, this class should be viewed as supporting infrastructure for
* the more important and user-facing {@code WebApplicationInitializer} SPI. Taking
* advantage of this container initializer is also completely optional: while
* it is true that this initializer will be loaded and invoked under all Servlet 3.0+
* runtimes, it remains the user's choice whether to make any
* {@code WebApplicationInitializer} implementations available on the classpath. If no
* {@code WebApplicationInitializer} types are detected, this container initializer will
* have no effect.
*
* Note that use of this container initializer and of {@code WebApplicationInitializer}
* is not in any way "tied" to Spring MVC other than the fact that the types are shipped
* in the {@code spring-web} module JAR. Rather, they can be considered general-purpose
* in their ability to facilitate convenient code-based configuration of the
* {@code ServletContext}. In other words, any servlet, listener, or filter may be
* registered within a {@code WebApplicationInitializer}, not just Spring MVC-specific
* components.
*
*
This class is neither designed for extension nor intended to be extended.
* It should be considered an internal type, with {@code WebApplicationInitializer}
* being the public-facing SPI.
*
*
See Also
* See {@link WebApplicationInitializer} Javadoc for examples and detailed usage
* recommendations.
*
* @author Chris Beams
* @author Juergen Hoeller
* @author Rossen Stoyanchev
* @since 3.1
* @see #onStartup(Set, ServletContext)
* @see WebApplicationInitializer
*/
@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
/**
* Delegate the {@code ServletContext} to any {@link WebApplicationInitializer}
* implementations present on the application classpath.
*
Because this class declares @{@code HandlesTypes(WebApplicationInitializer.class)},
* Servlet 3.0+ containers will automatically scan the classpath for implementations
* of Spring's {@code WebApplicationInitializer} interface and provide the set of all
* such types to the {@code webAppInitializerClasses} parameter of this method.
*
If no {@code WebApplicationInitializer} implementations are found on the classpath,
* this method is effectively a no-op. An INFO-level log message will be issued notifying
* the user that the {@code ServletContainerInitializer} has indeed been invoked but that
* no {@code WebApplicationInitializer} implementations were found.
*
Assuming that one or more {@code WebApplicationInitializer} types are detected,
* they will be instantiated (and sorted if the @{@link
* org.springframework.core.annotation.Order @Order} annotation is present or
* the {@link org.springframework.core.Ordered Ordered} interface has been
* implemented). Then the {@link WebApplicationInitializer#onStartup(ServletContext)}
* method will be invoked on each instance, delegating the {@code ServletContext} such
* that each instance may register and configure servlets such as Spring's
* {@code DispatcherServlet}, listeners such as Spring's {@code ContextLoaderListener},
* or any other Servlet API componentry such as filters.
* @param webAppInitializerClasses all implementations of
* {@link WebApplicationInitializer} found on the application classpath
* @param servletContext the servlet context to be initialized
* @see WebApplicationInitializer#onStartup(ServletContext)
* @see AnnotationAwareOrderComparator
*/
@Override
public void onStartup(@Nullable Set> webAppInitializerClasses, ServletContext servletContext)
throws ServletException {
List initializers = Collections.emptyList();
if (webAppInitializerClasses != null) {
initializers = new ArrayList<>(webAppInitializerClasses.size());
for (Class waiClass : webAppInitializerClasses) {
// Be defensive: Some servlet containers provide us with invalid classes,
// no matter what @HandlesTypes says...
if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
try {
initializers.add((WebApplicationInitializer)
ReflectionUtils.accessibleConstructor(waiClass).newInstance());
}
catch (Throwable ex) {
throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
}
}
}
}
if (initializers.isEmpty()) {
servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
return;
}
servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
AnnotationAwareOrderComparator.sort(initializers);
for (WebApplicationInitializer initializer : initializers) {
initializer.onStartup(servletContext);
}
}
}