Web中使用普通类得到applicationContext

用一个类来存放applicationContext：
public class ContextHolder {
  private final static ContextHolder instance = new ContextHolder();
  private ApplicationContext ac;
  private ContextHolder() {
  }
  public static ContextHolder getInstance() {
    return instance;
  }
  public synchronized void setApplicationContext(ApplicationContext ac) {
    this.ac = ac;
  }
  public ApplicationContext getApplicationContext() {
    return ac;
  }  
}

然后写一个servlet，继承自org.springframework.web.context.ContextLoaderServlet，并配置web.xml，让它在tomcat启动时自动运行。然后在它的init方法中，加入如下的代码：
WebApplicationContext context = WebApplicationContextUtils.
    getWebApplicationContext(this.getServletContext());
ContextHolder.getInstance().setApplicationContext(context);