根据用于源和目标的路径复制文件/目录时出现 java.lang.NullPointerException 错误

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 3 年前。

复制文件时，我遇到 NullPointerException，具体取决于我用作源和目标的路径。源和目标的一个版本提供了成功的复制，而另一个版本则出错。

我试图理解为什么路径差异会导致错误。

这可以工作并创建目录和文件。

String sourcePath = "I:\\MB\\inkjet\\ABC"; 
String destPath = "C:\\inkjet\\";

这会出错并给出 NullPointerException。

String sourcePath = "I:\\MB\\inkjet\\ABC"; 
String destPath = "C:\\inkjet\\ABC";

不同之处在于目标路径上添加了 ABC。

public class DetailsPanel extends JPanel {

   private EventListenerList listenerList = new EventListenerList();

   public DetailsPanel() {

      Dimension size = getPreferredSize();
      size.width = 250;
      setPreferredSize(size);

      setBorder(BorderFactory.createTitledBorder("Inkjet files to copy"));

      JLabel clientLabel = new JLabel("Client acronym: ");
      JLabel jobLabel = new JLabel("Job number: ");

      final JTextField clientField = new JTextField(10);
      final JTextField jobField = new JTextField(10);

      JButton addBtn = new JButton("Copy");

      addBtn.addActionListener(
         new ActionListener() {

            public void actionPerformed(ActionEvent e) {        
               String client = clientField.getText();           
               clientField.setText("");                 

               String job = jobField.getText();
               jobField.setText("");                

               String text = client + ": " + job + "\n";

               try {
                  fileCopy(client, job);
               } catch (IOException e1) {
               // TODO Auto-generated catch block
                  e1.printStackTrace();
               }

            }

         //*****************************************test

            @SuppressWarnings("unused")
            public void fileCopy(String nClient, String nJob) throws IOException {

               //String sourcePath = "I:\\MB\\FileTest\\" + nClient + "\\" + nJob; 
               //String destPath = "C:\\MB\\FileTest\\" + nClient + "\\" + nJob;


               //this works
               //String sourcePath = "I:\\MB\\inkjet\\ABC"; 
               //String destPath = "C:\\inkjet\\";

              //this doesn't work
               String sourcePath = "I:\\MB\\inkjet\\ABC"; 
               String destPath = "C:\\inkjet\\ABC";


               fireDetailEvent(new DetailEvent(this, "Source path:  " + sourcePath + "\n"));
               fireDetailEvent(new DetailEvent(this, "Destination path:  " + destPath + "\n"));

               File source = new File(sourcePath);  
               File dest = new File(destPath);  

               //Call to method copyUsingStream
               long start = System.nanoTime(); //start recording how much time the copy takes.
               copyUsingStream(source, dest);  //method to copy the directory/files.

               fireDetailEvent(new DetailEvent(this, "Time taken to copy the file(s): "+(System.nanoTime() -start) + " nanoseconds" + "\n"));

            }

            /**
               The copyUsingStream method is a recursive method to copy folders and files from one location to another.
            */

            private void copyUsingStream(File source, File dest) throws IOException {   

               if (!source.isDirectory()){ 
                  // If source is a file -> copy it to the new folder
                  InputStream inStream = null;
                  OutputStream outStream = null;
                  try {
                     inStream = new FileInputStream(source);
                     outStream = new FileOutputStream(dest);
                     byte[] buffer = new byte[1024];
                     int length;

                     while ((length = inStream.read(buffer)) > 0) {
                        outStream.write(buffer, 0, length);
                     }
                  } finally {
                     inStream.close();
                     outStream.close();

                     fireDetailEvent(new DetailEvent(this, "File copied from " + source + " to " + dest + " successfully" + "\n"));
                  } 


               } else {

                  //If a directory -> create the directory inside the new destination
                  //List all contents


                  if (!dest.exists()) {
                     dest.mkdir();

                     fireDetailEvent(new DetailEvent(this, "Directory copied from " + source + " to " + dest + " successfully" + "\n"));


                  }


                  String folder_contents[] = source.list();

                  for (String file : folder_contents) {

                     File srcFile = new File(source, file);
                     File destFile = new File(dest, file);

                     copyUsingStream(srcFile, destFile);

                  }

               }

            } //end method copyUsingStream          



         //*****************************************test


         });

      setLayout(new GridBagLayout());

      GridBagConstraints gc = new GridBagConstraints();

    //// First column //////////////////////////////////////

      gc.anchor = GridBagConstraints.LINE_END;
      gc.weightx = 0.5;
      gc.weighty = 0.5;

      gc.gridx = 0;
      gc.gridy = 0;

      add(clientLabel, gc);

      gc.gridx = 0;
      gc.gridy = 1;
      add(jobLabel, gc);

    //// second column /////////////////////////////////////

      gc.anchor = GridBagConstraints.LINE_START;
      gc.gridx = 1;
      gc.gridy = 0;
      add(clientField, gc);

      gc.gridx = 1;
      gc.gridy = 1;
      add(jobField, gc);

    //// final row /////////////////////////////////////////
      gc.weighty = 10;

      gc.anchor = GridBagConstraints.FIRST_LINE_START;
      gc.gridx = 1;
      gc.gridy = 2;
      add(addBtn, gc);

   }

   public void fireDetailEvent(DetailEvent event) {
      Object[] listeners = listenerList.getListenerList();

      for(int i=0; i < listeners.length; i += 2) {
         if(listeners[i] == DetailListener.class) {
            ((DetailListener)listeners[i+1]).detailEventOccurred(event);
         }
      }
   }

   public void addDetailListener(DetailListener listener) {
      listenerList.add(DetailListener.class, listener);
   }

   public void removeDetailListener(DetailListener listener) {
      listenerList.remove(DetailListener.class, listener);
   }

}

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at DetailsPanel$1.copyUsingStream(DetailsPanel.java:125) at DetailsPanel$1.copyUsingStream(DetailsPanel.java:153) at DetailsPanel$1.copyUsingStream(DetailsPanel.java:153) at DetailsPanel$1.fileCopy(DetailsPanel.java:98) at DetailsPanel$1.actionPerformed(DetailsPanel.java:52) at java.desktop/javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1967) at java.desktop/javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2308) at java.desktop/javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:405) at java.desktop/javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:262) at java.desktop/javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:279) at java.desktop/java.awt.Component.processMouseEvent(Component.java:6636) at java.desktop/javax.swing.JComponent.processMouseEvent(JComponent.java:3342) at java.desktop/java.awt.Component.processEvent(Component.java:6401) at java.desktop/java.awt.Container.processEvent(Container.java:2263) at java.desktop/java.awt.Component.dispatchEventImpl(Component.java:5012) at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2321) at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844) at java.desktop/java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4918) at java.desktop/java.awt.LightweightDispatcher.processMouseEvent(Container.java:4547) at java.desktop/java.awt.LightweightDispatcher.dispatchEvent(Container.java:4488) at java.desktop/java.awt.Container.dispatchEventImpl(Container.java:2307) at java.desktop/java.awt.Window.dispatchEventImpl(Window.java:2762) at java.desktop/java.awt.Component.dispatchEvent(Component.java:4844) at java.desktop/java.awt.EventQueue.dispatchEventImpl(EventQueue.java:772) at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:721) at java.desktop/java.awt.EventQueue$4.run(EventQueue.java:715) at java.base/java.security.AccessController.doPrivileged(AccessController.java:391) at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85) at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:95) at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:745) at java.desktop/java.awt.EventQueue$5.run(EventQueue.java:743) at java.base/java.security.AccessController.doPrivileged(AccessController.java:391) at java.base/java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:85) at java.desktop/java.awt.EventQueue.dispatchEvent(EventQueue.java:742) at java.desktop/java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:203) at java.desktop/java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:124) at java.desktop/java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:113) at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:109) at java.desktop/java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101) at java.desktop/java.awt.EventDispatchThread.run(EventDispatchThread.java:90)

第125行的错误是outStream.close()；线。

          } finally {
             inStream.close();
             outStream.close();

             fireDetailEvent(new DetailEvent(this, "File copied from " + source + " to " + dest + " successfully" + "\n"));
          } 


       } else {

Answer 1

顺便说一句，在关闭流之前，您需要确定流是否为空。

if(inStream != null ) inStream.close();
if(outStream != null ) outStream.close();

并添加 catch 来看看发生了什么。

catch(Exception e) {
  e.printStackTrace();
}
finally {
                     inStream.close();
                     outStream.close();

                     fireDetailEvent(new DetailEvent(this, "File copied from " + source + " to " + dest + " successfully" + "\n"));
                  }