严重：将上下文初始化事件发送到类listener.DomListener的侦听器实例的异常java.lang.NullPointerException

Question

初始化上下文时，我正在尝试将XML文件解析为DOM。但是我收到以下错误：

  

严重：将上下文初始化事件发送给侦听器的异常   类listener.DomListener的实例java.lang.NullPointerException

这是我的代码

public class DomListener implements ServletContextListener {

private final String FILEPATH = "WEB-INF/studentAccounts.xml";
private ServletContext context;

@Override
public void contextInitialized(ServletContextEvent sce) {
    System.out.println("Context Initialized");
    context = sce.getServletContext();
    try {
        String realPath = context.getRealPath("/");
        String filePath = realPath + FILEPATH;
        Document doc = parseDomFromFile(filePath);
        context.setAttribute("DOC", (Object) doc);

    } catch (ParserConfigurationException ex) {
        Logger.getLogger(DomListener.class.getName()).log(Level.SEVERE, null, ex);
    } catch (SAXException ex) {
        Logger.getLogger(DomListener.class.getName()).log(Level.SEVERE, null, ex);
    } catch (IOException ex) {
        Logger.getLogger(DomListener.class.getName()).log(Level.SEVERE, null, ex);
    }
}

@Override
public void contextDestroyed(ServletContextEvent sce) {
    System.out.println("Context Destroyed");
}
}

我在NullPointerException行上有一个try{，但我不知道为什么会发生:(任何人都可以帮忙吗？