访问文件夹中的文件时出现java NullPointerException

Question

我试图列出文件夹中的文件，但它会产生空指针异常 这是密码

String url=req.getRequestURI();
String path=session.getServletContext().getRealPath(url);
File folder = new File(path);
      File[] listOfFiles = folder.listFiles();

    for (int i = 0; i < listOfFiles.length; i++) {//this line is producing nullpointer exception
      if (listOfFiles[i].isFile()) {
        System.out.println("File " + listOfFiles[i].getName());
      } else if (listOfFiles[i].isDirectory()) {
        System.out.println("Directory " + listOfFiles[i].getName());
      }
    }

打印路径时，会给出正确的路径： 像D:\apache-tomcat-8.5.24\webapps\JaasLogin\JaasLogin\user-files。 我是java新手 上下文路径：/jaasLogin 但当我像"D:\\apache-tomcat-8.5.24\\webapps\\JaasLogin\\JaasLogin\\user-files"一样直接给出路径时，它会起作用

我得到的输出

 org.apache.catalina.core.StandardWrapperValve.invoke Servlet.service() for servlet [userfileserv] in context with path [/JaasLogin] threw exception
 java.lang.NullPointerException
    at myPackage.UserFileServ.doGet(UserFileServ.java:31)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:635)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)

Answer 1

正如Javadoc for File.listFiles()所述：

Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

因此，为了防止NPE，采取以下措施：

    File[] listOfFiles = folder.listFiles();

    if (listOfFiles != null) {
        for (int i = 0; i < listOfFiles.length; i++) {
          if (listOfFiles[i].isFile()) {
            System.out.println("File " + listOfFiles[i].getName());
          } else if (listOfFiles[i].isDirectory()) {
            System.out.println("Directory " + listOfFiles[i].getName());
          }
        }
    }
    else {
        System.out.println("Could not read " + folder.getName());
    }