JFileChooser fails

Question

In my code at a certain point i have this code

void selectRoot() {
        JFileChooser ch = new JFileChooser();
        ch.showOpenDialog((JFrame)this);
    }

This throws that Exception:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at java.io.Win32FileSystem.normalize(Win32FileSystem.java:164)
    at java.io.Win32FileSystem.getUserPath(Win32FileSystem.java:296)
    at java.io.Win32FileSystem.resolve(Win32FileSystem.java:312)
    at java.io.File.getAbsolutePath(File.java:501)
    at sun.awt.shell.Win32ShellFolder2.<init>(Win32ShellFolder2.java:291)
    at sun.awt.shell.Win32ShellFolderManager2.createShellFolderFromRelativePIDL(Win32ShellFolderManager2.java:66)
    at sun.awt.shell.Win32ShellFolderManager2.createShellFolder(Win32ShellFolderManager2.java:56)
    at sun.awt.shell.Win32ShellFolderManager2.getRecent(Win32ShellFolderManager2.java:114)
    at sun.awt.shell.Win32ShellFolderManager2.get(Win32ShellFolderManager2.java:251)
    at sun.awt.shell.ShellFolder.get(ShellFolder.java:227)
    at sun.swing.WindowsPlacesBar.<init>(WindowsPlacesBar.java:64)
    at com.sun.java.swing.plaf.windows.WindowsFileChooserUI.updateUseShellFolder(WindowsFileChooserUI.java:505)
    at com.sun.java.swing.plaf.windows.WindowsFileChooserUI.installComponents(WindowsFileChooserUI.java:196)
    at javax.swing.plaf.basic.BasicFileChooserUI.installUI(BasicFileChooserUI.java:136)
    at com.sun.java.swing.plaf.windows.WindowsFileChooserUI.installUI(WindowsFileChooserUI.java:129)
    at javax.swing.JComponent.setUI(JComponent.java:673)
    at javax.swing.JFileChooser.updateUI(JFileChooser.java:1763)
    at javax.swing.JFileChooser.setup(JFileChooser.java:360)
    at javax.swing.JFileChooser.<init>(JFileChooser.java:333)
    at javax.swing.JFileChooser.<init>(JFileChooser.java:286)
    at my.pack.Main.selectRoot(Main.java:184)

Looking in the source of java.io.Win32FileSystem.normalize() it seems the (current) path is null:

   public String normalize(String path) {
        int n = path.length(); // Line: 164
        char slash = this.slash;
        char altSlash = this.altSlash;

If i run selectRoot() from another point in the program it runs fine ...

Update

Guys that is really a good one for the great book of programing pitfalls !

In my original code there is a JTextField that holds the actual path and get initialize that way:

root = new JTextField();
root.setToolTipText("Root folder");
root.setText(System.clearProperty("user.dir"));

System.clearProperty() clears the system property (to null) and returns the old value (arrrghhhhhhh). clearProperty() probably slipped in from auto-completion.

Thank you all. Will give some up votes to you ...

Answer 1

Make sure to run showOpenDialog from EDT. If you call this method from another thread, it can cause all sorts of errors:

SwingUtilities.invokeLater(new Runnable() {
    public void run() {
        JFileChooser ch = new JFileChooser();
        ch.showOpenDialog((JFrame)this);
    }
});