java - InputStream 的 "close()"方法抛出 NullPointerException

Question

我正在学习 Java，并尝试编写一个简单的 FTP 应用程序，该应用程序具有在我的 PC 和服务器之间传输文件的接口(interface)。我在自己的电脑上创建了服务器来测试它。

我正在使用类 FTPClient (org.apache.commons.net.ftp)创建客户端(*FTPClient ftpClient = new FTPClient()*) , JFileChooser 作为客户端接口(interface) (*JFileChooser fc_client = new JFileChooser*)和一个 JList 作为服务器的接口(interface) (*JList list_server = new JList(new DefaultListModel()*).

我在界面上放置了一个 JButton，用于将文件从服务器传输到客户端。看起来它工作正常(下载我在服务器文件列表中选择的文件)，但是在传输第二个文件(它不会与我传输的第一个文件一起抛出任何内容)和连续文件后关闭 InputStream 时，它会抛出 NullPointerException .

这是我添加到按钮的 ActionListener 的代码:

download.addActionListener(new ActionListener(){
        @Override
        public void actionPerformed(ActionEvent ae) {
            InputStream iStream = null;
            try {
                iStream = ftpClient.retrieveFileStream(ftpClient.printWorkingDirectory() + "/" + list_server.getSelectedValue().toString());
                File f = new File(fc_client.getCurrentDirectory().getAbsolutePath() + "/" + list_server.getSelectedValue().toString());
                FileUtils.copyInputStreamToFile(iStream, f);
                fc_client.updateUI();

            } catch (FileNotFoundException ex) {
                Logger.getLogger(ClientInterface.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IOException ex) {
                Logger.getLogger(ClientInterface.class.getName()).log(Level.SEVERE, null, ex);
            } finally {
                try {
                    iStream.close();
                } catch (IOException ex) {
                    Logger.getLogger(ClientInterface.class.getName()).log(Level.SEVERE, null, ex);
            }

        }
        }

    });

这里是有关异常的信息:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at com.mycompany.ftpclientproject.ClientInterface$4.actionPerformed(ClientInterface.java:174)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6516)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
at java.awt.Component.processEvent(Component.java:6281)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4872)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4698)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4698)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:747)
at java.awt.EventQueue.access$300(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:706)
at java.awt.EventQueue$3.run(EventQueue.java:704)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:720)
at java.awt.EventQueue$4.run(EventQueue.java:718)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:717)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

第一行(com.mycompany.ftpclientproject.ClientInterface$4.actionPerformed(ClientInterface.java:174)) is referred to iStream.close().

你知道会发生什么吗？谢谢。

Answer 1

这不是 close() 方法引发错误，而是您对不存在的对象的调用:iStream 为 null。在调用它的方法之前将其包装在空检查中，例如

if (iStream != null) {
    iStream.close();
}

我还会检查您的记录器输出以了解其为何为空。