前面的文章介绍了用 WebApplicationInitializer 或者 AbstractAnnotationConfigDispatcherServletInitializer 来代替 web.xml 。
我们学 java web 的时候就知道，servlet 容器会自动加载 web.xml。
那么，疑问就来了，WebApplicationInitializer 或者 AbstractAnnotationConfigDispatcherServletInitializer 既然替代了 web.xml，那应该也会被 servlet 容器加载，是不是这样呢？答案是：是的。
而完成这个加载工作的是 SpringServletContainerInitializer 类。看下它的源码：
java
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;
import org.springframework.lang.Nullable;
import org.springframework.util.ReflectionUtils;
/**
* 本类继承了 servlet 3.0 的 ServletContainerInitializer 接口。
* 下面这段话的意思是：
* servlet 3.0 的 ServletContainerInitializer 接口是用来让 servlet 容器支持基于编码的配置。
* 在这个过程中要结合 spring 的 WebApplicationInitializer。
* 而 spring 的 WebApplicationInitializer 可以替换 web.xml 也可以结合 web.xml 一起使用。
*
* 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.
*
* 当 servlet 容器启动的时候，如果累路径下有 spring-web 的话，这个类会被加载，并且 onStartup 方法会被调用。
* 这背后的原理在于 java 的 SPI 机制。
* spring-web 模块下有个 META-INF/services/javax.servlet.ServletContainerInitializer 文件，
* 其内容就是当前类的完全限定名。
*
* <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>
*
* SpringServletContainerInitializer 可以看做一个简单的代理，真正的工作还是交给了 WebApplicationInitializer 。
*
* <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.
*
* 注意：不要扩展这个类，也不要继承这个类。
* 这个类应该被视为内部类型，不对外。
* 对外的是 WebApplicationInitializer。
*
* <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 {
{
System.out.println("@HandlesTypes(WebApplicationInitializer.class) 这个注解表明了当前类 SpringServletContainerInitializer "
+ " 需要处理的类型是 WebApplicationInitializer ");
}
/**
* 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 issued 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(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
throws ServletException {
servletContext.log("SpringServletContainerInitializer 利用了 java 的 SPI 机制");
List<WebApplicationInitializer> initializers = new LinkedList<>();
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)
ReflectionUtils.accessibleConstructor(waiClass).newInstance());
}
catch (Throwable ex) {
throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
}
}
}
}
System.out.println("SpringServletContainerInitializer 在其 onStartup 方法中检测类路径下的 WebApplicationInitializer（spring的） ");
if (initializers.isEmpty()) {
servletContext.log("在类路径下没有检测到 spring 的 WebApplicationInitializer。------- No Spring WebApplicationInitializer types detected on classpath");
return;
}
System.out.println("在类路径下检测到了 " + initializers.size() + " 个 spring 的 WebApplicationInitializer。");
servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
AnnotationAwareOrderComparator.sort(initializers);
System.out.println("依次调用 WebApplicationInitializer 的 onStartup 方法");
for (WebApplicationInitializer initializer : initializers) {
System.out.println("调用 " + initializer.getClass().getName() + " 的 onStartup 方法");
initializer.onStartup(servletContext);
}
System.out.println("至此，通过 servlet、java spi 成功引导了 spring ");
}
}关于这个类有几个需要注意的点：
- 它实现了 servlet 3.0 的
javax.servlet.ServletContainerInitializer接口 - 它加了个
@HandlesTypes(WebApplicationInitializer.class)注解 - 它实现了
javax.servlet.ServletContainerInitializer接口中定义的onStartup方法
总结下来。SpringServletContainerInitializer 这个类实现了 servlet 3.0 的 javax.servlet.ServletContainerInitializer 接口，这就决定了在 servlet 容器启动的时候 onStartUp 方法会被自动触发，而在 onStartUp 方法内部 WebApplicationInitializer 的 onStartUp 方法被调用。这就是 WebApplicationInitializer 被容器带起来的过程。
这里只是说了个大概，要想非常清楚，还请自己研究下 servlet 3.0 的 javax.servlet.ServletContainerInitializer 接口。