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

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  1. 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
  2. //com/spark/system/applicationContext.xml等价于"file:com/spark/system/applicationContext.xml"
  3. ac1.getBean("beanId");
  4.  
  5. //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
  6. //com/spark/system/applicationContext.xml 等价于"classpath:com/spark/system/applicationContext.xml"
  7.  ac2.getBean("beanId"); 
  8.
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。


  1.  public void getBean(HttpServletRequest req,HttpSession se)
  2.    {
  3. //       se.getServletContext() 也可以
  4.         WebApplicationContext wac=(WebApplicationContext)req.getServletContext().getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
  5.      wac.getBean("");
  6.     }
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:

  1. import org.springframework.web.context.support.WebApplicationContextUtils;
  2. ApplicationContext ac1 = WebApplicationContextUtils.getRequiredWebApplicationContext(ServletContext sc)
  3. ApplicationContext ac2 = WebApplicationContextUtils.getWebApplicationContext(ServletContext sc)
  4. ac1.getBean("beanId");
  5. 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:

  1. import org.springframework.web.context.WebApplicationContext;
  2. import org.springframework.web.context.support.WebApplicationObjectSupport;
  3.  
  4. public class ApplicationContextUtils extends WebApplicationObjectSupport{
  5.  
  6.   public  WebApplicationContext isgetWebApplicationContext(){
  7.         return super.getWebApplicationContext();
  8.  }
  9.     
  10. }


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)

  1. /*** Eclipse Class Decompiler, copyright (c) 2012 cnfree ([email protected]) ***/
  2. package org.springframework.web.context.support;
  3.  
  4. import java.io.File;
  5. import javax.servlet.ServletContext;
  6. import org.springframework.context.ApplicationContext;
  7. import org.springframework.context.support.ApplicationObjectSupport;
  8. import org.springframework.web.context.ServletContextAware;
  9. import org.springframework.web.context.WebApplicationContext;
  10. import org.springframework.web.util.WebUtils;
  11.  
  12. public abstract class WebApplicationObjectSupport <span style="color:#FF0000;">extends ApplicationObjectSupport</span> implements ServletContextAware{
  13.  
  14.   private ServletContext servletContext;
  15.  
  16.    public final void setServletContext(ServletContext servletContext){
  17.        if (servletContext != this.servletContext){
  18.          this.servletContext = servletContext;
  19.          if (servletContext != null) initServletContext(servletContext);
  20.      }
  21.    }
  22.  
  23.   protected boolean isContextRequired(){
  24.       return true;
  25.     }
  26.  
  27.   protected void initApplicationContext(ApplicationContext context){
  28.       super.initApplicationContext(context);
  29.         if ((this.servletContext == null)
  30.              && (context instanceof WebApplicationContext)){
  31.            this.servletContext = ((WebApplicationContext)context)
  32.                     .getServletContext();
  33.            if (this.servletContext != null)
  34.               initServletContext(this.servletContext);
  35.       }
  36.    }
  37.  
  38.   protected void initServletContext(ServletContext servletContext){}
  39.  
  40.  <span style="color:#FF0000;">protected final WebApplicationContext getWebApplicationContext()
  41.             throws IllegalStateException{
  42.      ApplicationContext ctx = getApplicationContext();
  43.        if (ctx instanceof WebApplicationContext){ return ((WebApplicationContext)getApplicationContext()); }
  44.      if (isContextRequired()){ throw new IllegalStateException(
  45.                 "WebApplicationObjectSupport instance [" + this
  46.                       + "] does not run in a WebApplicationContext but in: "
  47.                      + ctx); }
  48.        return null;
  49.     }</span>
  50.  
  51.  protected final ServletContext getServletContext()
  52.           throws IllegalStateException{
  53.      if (this.servletContext != null){ return this.servletContext; }
  54.        ServletContext servletContext = getWebApplicationContext()
  55.               .getServletContext();
  56.        if ((servletContext == null) && (isContextRequired())){ throw new IllegalStateException(
  57.                 "WebApplicationObjectSupport instance ["
  58.                        + this
  59.                         + "] does not run within a ServletContext. Make sure the object is fully configured!"); }
  60.       return servletContext;
  61.     }
  62.  
  63.   protected final File getTempDir() throws IllegalStateException{
  64.        return WebUtils.getTempDir(getServletContext());
  65.   }
  66. }

下面是ApplicationObjectSupport源码 

  1. /*** Eclipse Class Decompiler, copyright (c) 2012 cnfree ([email protected]) ***/
  2. package org.springframework.context.support;
  3.  
  4. import org.apache.commons.logging.Log;
  5. import org.apache.commons.logging.LogFactory;
  6. import org.springframework.beans.BeansException;
  7. import org.springframework.context.ApplicationContext;
  8. import org.springframework.context.ApplicationContextAware;
  9. import org.springframework.context.ApplicationContextException;
  10.  
  11. public abstract class ApplicationObjectSupport implements
  12.       ApplicationContextAware{
  13.  
  14.    protected final Log logger;
  15.  
  16.     private ApplicationContext applicationContext;
  17.  
  18.    private MessageSourceAccessor messageSourceAccessor;
  19.  
  20.  public ApplicationObjectSupport(){
  21.         this.logger = LogFactory.getLog(super.getClass());
  22.   }
  23.  
  24.   public final void setApplicationContext(ApplicationContext context)
  25.            throws BeansException{
  26.         if ((context == null) && (!(isContextRequired()))){
  27.          this.applicationContext = null;
  28.          this.messageSourceAccessor = null;
  29.       }
  30.        else if (this.applicationContext == null){
  31.           if (!(requiredContextClass().isInstance(context))){ throw new ApplicationContextException(
  32.                     "Invalid application context: needs to be of type ["
  33.                            + requiredContextClass().getName() + "]"); }
  34.            this.applicationContext = context;
  35.             this.messageSourceAccessor = new MessageSourceAccessor(context);
  36.             initApplicationContext(context);
  37.         }
  38.        else if (this.applicationContext != context){ throw new ApplicationContextException(
  39.               "Cannot reinitialize with different application context: current one is ["
  40.                      + this.applicationContext + "], passed-in one is ["
  41.                       + context + "]"); }
  42.     }
  43.  
  44.   protected boolean isContextRequired(){
  45.       return false;
  46.    }
  47.  
  48.   protected Class requiredContextClass(){
  49.        return ApplicationContext.class;
  50.   }
  51.  
  52.   protected void initApplicationContext(ApplicationContext context)
  53.            throws BeansException{
  54.         initApplicationContext();
  55.    }
  56.  
  57.   protected void initApplicationContext() throws BeansException{}
  58.  
  59.   public final ApplicationContext getApplicationContext()
  60.          throws IllegalStateException{
  61.      if ((this.applicationContext == null) && (isContextRequired())){ throw new IllegalStateException(
  62.              "ApplicationObjectSupport instance [" + this
  63.                      + "] does not run in an ApplicationContext"); }
  64.         return this.applicationContext;
  65.  }
  66.  
  67.   protected final MessageSourceAccessor getMessageSourceAccessor()
  68.             throws IllegalStateException{
  69.      if ((this.messageSourceAccessor == null) && (isContextRequired())){ throw new IllegalStateException(
  70.               "ApplicationObjectSupport instance [" + this
  71.                      + "] does not run in an ApplicationContext"); }
  72.         return this.messageSourceAccessor;
  73.   }
  74. }

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:

  1.   ResourcePatternResolver resolver=new PathMatchingResourcePatternResolver();
  2.         Resource resource=resolver.getResource("classpath:com/**/beans.xml");
  3.        BeanFactory bf=new XmlBeanFactory(resource);
  4.        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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