InputStream's "close()" method throwing NullPointerException

Question

I'm learning Java and trying to program a simple FTP application with an interface to transfer files between my PC and a server. I created the server on my own PC to test it.

I'm using the class FTPClient (org.apache.commons.net.ftp) to create the client (*FTPClient ftpClient = new FTPClient()*), JFileChooser as the client's interface (*JFileChooser fc_client = new JFileChooser*) and a JList as the server's interface (*JList list_server = new JList(new DefaultListModel()*).

I placed a JButton on the interface to transfer a file from the server to the client. It seems it works fine (is downloads the file I select on the server's file list), but it throws a NullPointerException when closing the InputStream after transferring a second file (it doesn't throw anything with the first file I transfer) and successive files.

This is the code for the ActionListener I added to the button:

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);
            }

        }
        }

    });

And here the info about the exception:

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)

The first line (com.mycompany.ftpclientproject.ClientInterface$4.actionPerformed(ClientInterface.java:174)) is referred to iStream.close().

Do you have any idea about what could be happening? Thank you.

It would be easier to help you if we knew which list is #174 — Grzegorz Oledzki, May 22 '17 at 11:03

Steve Smith · Answer 1 · 2017-05-22T11:10:30.903

1

It's not the close() method throwing the error, it's your call to an object that doesn't exists: iStream is null. Wrap it in a null check before calling methods on it, e.g.

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

I would also check your Logger output to see why it's null.

edited May 22 '17 at 11:10

answered May 22 '17 at 11:04

Steve Smith

2,244
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22

It is still throwing a NullPointerException but now on `FileUtils.copyInputStreamToFile(iStream, f)`. I realized I was wrong, it doesn't transfer the file successfully (as I wrote in the initial post). After checking it, I realized the size of the incorrectly transferred files is 0 bytes. – NHarvat May 22 '17 at 11:18
Just add the nullcheck to after the line `iStream = ftpClient.retrieve...`. If no `iStream` is created, you can't do anything with it. – Steve Smith May 22 '17 at 11:21
That is the problem, the stream is not created for the second file I want to transfer and successive files, I don't understand why... I checked the String I put as an argument for `ftpClient.retrieveFileStream(String path)` and it is ok, so it should be created. I also have a button to upload files, it's programmed similarly to this one and works fine... – NHarvat May 22 '17 at 13:56
What happens if you try the "second" file first? If that fails, it means your path must be wrong. – Steve Smith May 22 '17 at 14:25
It doesn't matter which one is the first or seconds file, it always fails for the second one. – NHarvat May 22 '17 at 14:59

InputStream's "close()" method throwing NullPointerException

1 Answers1