从目录中读取多个文件时出现NullPointerException

Question

当我运行以下代码时，我得到一个NullPointerException。 此方法用于扫描目录并检索其中的所有文件（.java），然后计算NLOC和NLC。

所以这是代码：

public void listFilesForFolder(final File folder) throws IOException {
    List<String> lines;
    for (File fileEntry : folder.listFiles()) {  // <<--- exception here
      if (fileEntry.isDirectory()) {
        listFilesForFolder(fileEntry);
      } else {
        if (fileEntry.isFile()) {
          temp = fileEntry.getName();
          if ((temp.substring(temp.lastIndexOf('.') + 1, temp.length()).toLowerCase()).equals("java")) {
            fileName = folder.getAbsolutePath() + "\\" + fileEntry.getName();
            try {
              lines = getLines(fileName);
              totalCommentCount += calcComments(lines);
              totalLOC += calcLOC(lines);

            } catch (IOException e) {
              System.out.println("File does not exist!" + e);
            }
          }
        }
      }
    }
    StringBuilder output = new StringBuilder();
    output.append("Comment Lines :").append(totalCommentCount);
    output.append("\n");
    output.append("Lines of Code (LOC) :").append(totalLOC);
    txtDisplayResults.setText(output.toString());
 }

这是错误消息（我使用netbeans作为IDE）：

23-Jul-2013 13:17:53 scaj.GUI listFilesForFolder
SEVERE: null
java.lang.NullPointerException
    at scaj.GUI.listFilesForFolder(GUI.java:61)
    at scaj.GUI.listFilesForFolder(GUI.java:68)
    at scaj.GUI.bScanActionPerformed(GUI.java:326)
    at scaj.GUI.access$100(GUI.java:17)
    at scaj.GUI$2.actionPerformed(GUI.java:235)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
    at java.awt.Component.processMouseEvent(Component.java:6038)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3260)
    at java.awt.Component.processEvent(Component.java:5803)
    at java.awt.Container.processEvent(Container.java:2058)
    at java.awt.Component.dispatchEventImpl(Component.java:4410)
    at java.awt.Container.dispatchEventImpl(Container.java:2116)
    at java.awt.Component.dispatchEvent(Component.java:4240)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4322)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3986)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3916)
    at java.awt.Container.dispatchEventImpl(Container.java:2102)
    at java.awt.Window.dispatchEventImpl(Window.java:2429)
    at java.awt.Component.dispatchEvent(Component.java:4240)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)

Answer 1

folder.listFiles()返回null，当folder不是目录（在这种情况下是这样）或发生IO错误时。 Web服务器是否可能没有访问某些文件夹的正确权限？ 或者它们无法访问（它们通过网络连接）。

将nullcheck添加到folder.listFiles()并打印出它返回null的文件夹。 这应该可以帮助您确定它发生的原因。 如果找不到原因，请在问题中添加正在发生的文件夹。

Answer 2

您的folder.listFiles（）返回null。 listFiles（）方法：

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