WebApplicationInitializer

ServletContainInitalizer是Java EE 6中Servlet 3.0的新增接口；它的onStartup方法是一个web应用中我们的代码可以控制到的最早时间点。

它不需要通过web.xml部署描述符来定义，需要在/META-INF/services/javax.servlet.ServletContainerInitializer中列出具体的实现，Servlet容器在启动时会自动扫描加载它们并调用onStartUp方法。但是文件不能放在WAR文件的/META-INF/services中，而是需要放在JAR文件的/META-INF/services中，这样就很不方便。如果你使用spring的话，spring
Framework提供了一个桥接口，在Spring中SpringServletContainerInitializer类实现了ServletContainerInitializer接口，Spring的JAR中列出了SpringServletContainerInitializer，如下。在SpringServletContainerInitializer中会扫描所有WebApplicationInitializer的实现，调用它们的onStartUp方法

如图，位于sping-web.jar/META-INF/services中



SpringServletContainerInitializer源码如下

/*
* Copyright 2002-2014 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.web;

import java.lang.reflect.Modifier;
import java.util.LinkedList;
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;

/**
* 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.
*
* <h2>Mechanism of Operation</h2>
* 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
* <a href="http://download.oracle.com/javase/6/docs/technotes/guides/jar/jar.html#Service%20Provider">
* JAR Services API documentation</a> as well as section <em>8.2.4</em> of the Servlet 3.0
* Final Draft specification for complete details.
*
* <h3>In combination with {@code web.xml}</h3>
* 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 <absolute-ordering>} 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:
*
* <pre class="code">
* {@code
* <absolute-ordering>
* <name>some_web_fragment</name>
* <name>spring_web</name>
* </absolute-ordering>
* }</pre>
*
* <h2>Relationship to Spring's {@code WebApplicationInitializer}</h2>
* 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.
*
* <h2>General Notes</h2>
* In general, this class should be viewed as <em>supporting infrastructure</em> for
* the more important and user-facing {@code WebApplicationInitializer} SPI. Taking
* advantage of this container initializer is also completely <em>optional</em>: 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.
*
* <p>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.
*
* <p>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.
*
* <h2>See Also</h2>
* See {@link WebApplicationInitializer} Javadoc for examples and detailed usage
* recommendations.<p>
*
* @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.
*
* <p>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.
*
* <p>If no {@code WebApplicationInitializer} implementations are found on the
* classpath, this method is effectively a no-op. An INFO-level log message will be
* issu
4000
ed notifying the user that the {@code ServletContainerInitializer} has indeed
* been invoked but that no {@code WebApplicationInitializer} implementations were
* found.
*
* <p>Assuming that one or more {@code WebApplicationInitializer} types are detected,
* they will be instantiated (and <em>sorted</em> 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(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
throws ServletException {

List<WebApplicationInitializer> initializers = new LinkedList<WebApplicationInitializer>();

if (webAppInitializerClasses != null) {
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) 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;
}

AnnotationAwareOrderComparator.sort(initializers);
servletContext.log("Spring WebApplicationInitializers detected on classpath: " + initializers);

for (WebApplicationInitializer initializer : initializers) {
initializer.onStartup(servletContext);
}
}

}