关于spring获取webApplication.getBean多种途径和简单解释

ApplicationContext ac1 = new FileSystemXmlApplicationContext("com/spark/system/applicationContext.xml");//如果配置文件放在文件系统的目录下则优先使用该方式
//com/spark/system/applicationContext.xml等价于"file:com/spark/system/applicationContext.xml"
ac1.getBean("beanId");

//ApplicationContext ac2=new ClassPathXmlApplicationContext("com/spark/system/applicationContext.xml");//如果配置文件在类路径下则优先使用该方式
//com/spark/system/applicationContext.xml 等价于"classpath:com/spark/system/applicationContext.xml"
 ac2.getBean("beanId"); 

说明：

这种方式适用于采用Spring框架的独立应用程序，需要程序通过配置文件手工初始化Spring的情况。

	public void getBean(HttpServletRequest req,HttpSession se)
	{
//		se.getServletContext() 也可以
		WebApplicationContext wac=(WebApplicationContext)req.getServletContext().getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
		wac.getBean("");
	}

说明:此种方式正是我们下面所提到的WebApplicationContextUtils 工具类中getWebApplicationContext(ServletContext sc) 方法的内部实现，以下方式是通过spring 提供的WebApplicationContextUtils 工具类获取WebApplicationContext

方式一:

import org.springframework.web.context.support.WebApplicationContextUtils;
ApplicationContext ac1 = WebApplicationContextUtils.getRequiredWebApplicationContext(ServletContext sc)
ApplicationContext ac2 = WebApplicationContextUtils.getWebApplicationContext(ServletContext sc)
ac1.getBean("beanId");
ac2.getBean("beanId"); 

说明：
这种方式适合于采用Spring框架的B/S系统，通过ServletContext对象获取ApplicationContext对象，然后在通过它获取需要的类实例。
上面两个工具方式的区别是，前者在获取失败时抛出异常，后者返回null。

方式二:

import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.context.support.WebApplicationObjectSupport;

public class ApplicationContextUtils extends WebApplicationObjectSupport{

	public  WebApplicationContext isgetWebApplicationContext(){
		return super.getWebApplicationContext();
	}
	
}

继承自抽象类WebApplicationObjectSupport
说明：
抽象类WebApplicationObjectSupport 继承自ApplicationObjectSupport提供getApplicationContext()方法，可以方便的获取到ApplicationContext。Spring初始化时，会通过该ApplicationObjectSupport 的setApplicationContext(ApplicationContext context)方法将ApplicationContext 对象注入。当然直接继承ApplicationObjectSupport自己实现也可以，既然spring 提供了更方便的抽象工具类WebApplicationObjectSupport 建议使用它。以免出现问题

下面看WebApplicationObjectSupport关键源码(红色部分)

/*** Eclipse Class Decompiler, copyright (c) 2012 cnfree ([email protected]) ***/
package org.springframework.web.context.support;

import java.io.File;
import javax.servlet.ServletContext;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ApplicationObjectSupport;
import org.springframework.web.context.ServletContextAware;
import org.springframework.web.context.WebApplicationContext;
import org.springframework.web.util.WebUtils;

public abstract class WebApplicationObjectSupport <span style="color:#FF0000;">extends ApplicationObjectSupport</span> implements ServletContextAware{

	private ServletContext servletContext;

	public final void setServletContext(ServletContext servletContext){
		if (servletContext != this.servletContext){
			this.servletContext = servletContext;
			if (servletContext != null) initServletContext(servletContext);
		}
	}

	protected boolean isContextRequired(){
		return true;
	}

	protected void initApplicationContext(ApplicationContext context){
		super.initApplicationContext(context);
		if ((this.servletContext == null)
				&& (context instanceof WebApplicationContext)){
			this.servletContext = ((WebApplicationContext)context)
					.getServletContext();
			if (this.servletContext != null)
				initServletContext(this.servletContext);
		}
	}

	protected void initServletContext(ServletContext servletContext){}

	<span style="color:#FF0000;">protected final WebApplicationContext getWebApplicationContext()
			throws IllegalStateException{
		ApplicationContext ctx = getApplicationContext();
		if (ctx instanceof WebApplicationContext){ return ((WebApplicationContext)getApplicationContext()); }
		if (isContextRequired()){ throw new IllegalStateException(
				"WebApplicationObjectSupport instance [" + this
						+ "] does not run in a WebApplicationContext but in: "
						+ ctx); }
		return null;
	}</span>

	protected final ServletContext getServletContext()
			throws IllegalStateException{
		if (this.servletContext != null){ return this.servletContext; }
		ServletContext servletContext = getWebApplicationContext()
				.getServletContext();
		if ((servletContext == null) && (isContextRequired())){ throw new IllegalStateException(
				"WebApplicationObjectSupport instance ["
						+ this
						+ "] does not run within a ServletContext. Make sure the object is fully configured!"); }
		return servletContext;
	}

	protected final File getTempDir() throws IllegalStateException{
		return WebUtils.getTempDir(getServletContext());
	}
}

下面是ApplicationObjectSupport源码

/*** Eclipse Class Decompiler, copyright (c) 2012 cnfree ([email protected]) ***/
package org.springframework.context.support;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ApplicationContextException;

public abstract class ApplicationObjectSupport implements
		ApplicationContextAware{

	protected final Log logger;

	private ApplicationContext applicationContext;

	private MessageSourceAccessor messageSourceAccessor;

	public ApplicationObjectSupport(){
		this.logger = LogFactory.getLog(super.getClass());
	}

	public final void setApplicationContext(ApplicationContext context)
			throws BeansException{
		if ((context == null) && (!(isContextRequired()))){
			this.applicationContext = null;
			this.messageSourceAccessor = null;
		}
		else if (this.applicationContext == null){
			if (!(requiredContextClass().isInstance(context))){ throw new ApplicationContextException(
					"Invalid application context: needs to be of type ["
							+ requiredContextClass().getName() + "]"); }
			this.applicationContext = context;
			this.messageSourceAccessor = new MessageSourceAccessor(context);
			initApplicationContext(context);
		}
		else if (this.applicationContext != context){ throw new ApplicationContextException(
				"Cannot reinitialize with different application context: current one is ["
						+ this.applicationContext + "], passed-in one is ["
						+ context + "]"); }
	}

	protected boolean isContextRequired(){
		return false;
	}

	protected Class requiredContextClass(){
		return ApplicationContext.class;
	}

	protected void initApplicationContext(ApplicationContext context)
			throws BeansException{
		initApplicationContext();
	}

	protected void initApplicationContext() throws BeansException{}

	public final ApplicationContext getApplicationContext()
			throws IllegalStateException{
		if ((this.applicationContext == null) && (isContextRequired())){ throw new IllegalStateException(
				"ApplicationObjectSupport instance [" + this
						+ "] does not run in an ApplicationContext"); }
		return this.applicationContext;
	}

	protected final MessageSourceAccessor getMessageSourceAccessor()
			throws IllegalStateException{
		if ((this.messageSourceAccessor == null) && (isContextRequired())){ throw new IllegalStateException(
				"ApplicationObjectSupport instance [" + this
						+ "] does not run in an ApplicationContext"); }
		return this.messageSourceAccessor;
	}
}

通过源码很容易看得出spring做的这两次封装是如何获取到WebApplicationContext的  当然自己也可以实现底层接口自己封装。

比如:继承自抽象类ApplicationObjectSupport,抽象类ApplicationObjectSupport提供getApplicationContext()方法，可以方便的获取到ApplicationContext。Spring初始化时，会通过该抽象类的setApplicationContext(ApplicationContext context)方法将ApplicationContext 对象注入。

再比如:实现接口ApplicationContextAware,实现该接口的setApplicationContext(ApplicationContext context)方法，并保存ApplicationContext 对象。Spring初始化时，会通过该方法将ApplicationContext 对象注入。

以上方法适合不同的情况，请根据具体情况选用相应的方法。
这里值得提一点的是，系统中用到上述方法的类实际上就于Spring框架紧密耦合在一起了，因为这些类是知道它们是运行在Spring框架上的，因此，系统中，应该尽量的减少这类应用，使系统尽可能的独立于当前运行环境，尽量通过DI的方式获取需要的服务提供者。

PS:除了通过applicationContext来手动获取getBean("beanId")之外，还可以通过beanfactory工厂的.getBean("beanId")获取Bean 实例

例如:

	ResourcePatternResolver resolver=new PathMatchingResourcePatternResolver();
		Resource resource=resolver.getResource("classpath:com/**/beans.xml");
		BeanFactory bf=new XmlBeanFactory(resource);
		bf.getBean("beanId");

有待研究 通过BeanFactory.getBean和ApplicationContext.getBean 的异同解释，和利弊以及使用情况。志同道合的同志可随时留言讨论，小弟欢迎大家一起学习

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