1.在javax.servlet.Filter中直接获取
ServletContext context = config.getServletContext();
2.在HttpServlet中直接获取
this.getServletContext()
3.在其他方法中，通过HttpRequest获得
request.getSession().getServletContext();
4.在struts2中还有常见的两种方法获得
获取HttpServletRequest / HttpSession / ServletContext / HttpServletResponse对象
方法一，通过ServletActionContext.类直接获取：
HttpServletRequest request = ServletActionContext.getRequest();
request.getSession();
ServletContext servletContext = ServletActionContext.getServletContext();
HttpServletResponse response = ServletActionContext.getResponse();
方法二，实现指定接口，由struts框架运行时注入：
public class HelloWorldAction implements ServletRequestAware, ServletResponseAware, ServletContextAware{
private HttpServletRequest request;
private ServletContext servletContext;
private HttpServletResponse response;
public void setServletRequest(HttpServletRequest req) {
this.request=req;
}
public void setServletResponse(HttpServletResponse res) {
this.response=res;
}
public void setServletContext(ServletContext ser) {
this.servletContext=ser;
}
}