究 Spring 3.1之無web.xml式 基於代碼配置的servlet3.0應用

  你們應該都已經知道Spring 3.1對無web.xml式基於代碼配置的servlet3.0應用。經過spring的api或是網絡上高手們的博文，也必定很快就學會而且加到本身的應用中去了。PS：若是還沒，也能夠小小參考一下鄙人的上一篇文章＜＜探 Spring 3.1之無web.xml式 基於代碼配置的servlet3.0應用＞＞。

通過一天的深度research, 我瞭解，理解以及重現了springframework的那一小段代碼。

OK,第一步，入手點，WebApplicationInitializer接口。由於咱們只需實現這個接口覆寫它的一個方法，就能夠作到配置web.xml一樣的功效。看它的源碼，其實看和不看沒什麼兩樣：

package org.springframework.web;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
public interface WebApplicationInitializer {
void onStartup(ServletContext servletContext) throws ServletException;
}

package org.springframework.web;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
public interface WebApplicationInitializer {
	void onStartup(ServletContext servletContext) throws ServletException;
}

就這麼點兒，有效代碼5行，弄地我一頭霧水，就是一個普通接口，聲明瞭一個方法。連註解都沒有，server是怎麼找到實現了它的類的？若是這樣，何不找我定義的其它接口（的實現類完成配置工做）呢。可見如今java的解耦技術，真使人汗顏。
第二步，這個接口旁邊(同包)有個SpringServletContainerInitializer, 看下它是何方神聖吧：

package org.springframework.web;

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

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
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;
}

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

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

}

package org.springframework.web;

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

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {
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;
		}

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

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

}