如何使用文件选择器

File choosers provide a GUI for navigating the file system, and then either choosing a file or directory from a list, or entering the name of a file or directory. To display a file chooser, you usually use the JFileChooser API to show a modal dialog containing the file chooser. Another way to present a file chooser is to add an instance of JFileChooser to a container.

Click the Launch button to run JWSFileChooserDemo using Java™ Web Start (download JDK 7 or later). Alternatively, to compile and run the example yourself, consult the example index.

When working with the JWSFileChooserDemo example, be careful not to lose files that you need. Whenever you click the save button and select an existing file, this demo brings up the File Exists dialog box with a request to replace the file. Accepting the request overwrites the file.

The rest of this section discusses how to use the JFileChooser API. A JFileChooser object only presents the GUI for choosing files. Your program is responsible for doing something with the chosen file, such as opening or saving it. Refer to Basic I/O for information on how to read and write files.

The JFileChooser API makes it easy to bring up open and save dialogs. The type of look and feel determines what these standard dialogs look like and how they differ. In the Java look and feel, the save dialog looks the same as the open dialog, except for the title on the dialog's window and the text on the button that approves the operation. Here is a picture of a standard open dialog in the Java look and feel:

Here is a picture of an application called FileChooserDemo that brings up an open dialog and a save dialog.

Bringing up a standard open dialog requires only two lines of code:

//Create a file chooser
final JFileChooser fc = new JFileChooser();
...
//In response to a button click:
int returnVal = fc.showOpenDialog(aComponent);

The argument to the showOpenDialog method specifies the parent component for the dialog. The parent component affects the position of the dialog and the frame that the dialog depends on. For example, the Java look and feel places the dialog directly over the parent component. If the parent component is in a frame, then the dialog is dependent on that frame. This dialog disappears when the frame is minimized and reappears when the frame is maximized.

By default, a file chooser that has not been shown before displays all files in the user's home directory. You can specify the file chooser's initial directory by using one of JFileChooser's other constructors, or you can set the directory with the setCurrentDirectory method.

The call to showOpenDialog appears in the actionPerformed method of the Open a File button's action listener:

public void actionPerformed(ActionEvent e) {
    //Handle open button action.
    if (e.getSource() == openButton) {
        int returnVal = fc.showOpenDialog(FileChooserDemo.this);

        if (returnVal == JFileChooser.APPROVE_OPTION) {
            File file = fc.getSelectedFile();
            //This is where a real application would open the file.
            log.append("Opening: " + file.getName() + "." + newline);
        } else {
            log.append("Open command cancelled by user." + newline);
        }
   } ...
}

The showXxxDialog methods return an integer that indicates whether the user selected a file. Depending on how you use a file chooser, it is often sufficient to check whether the return value is APPROVE_OPTION and then not to change any other value. To get the chosen file (or directory, if you set up the file chooser to allow directory selections), call the getSelectedFile method on the file chooser. This method returns an instance of File.

The example obtains the name of the file and uses it in the log message. You can call other methods on the File object, such as getPath, isDirectory, or exists to obtain information about the file. You can also call other methods such as delete and rename to change the file in some way. Of course, you might also want to open or save the file by using one of the reader or writer classes provided by the Java platform. See Basic I/O for information about using readers and writers to read and write data to the file system.

The example program uses the same instance of the JFileChooser class to display a standard save dialog. This time the program calls showSaveDialog:

int returnVal = fc.showSaveDialog(FileChooserDemo.this);

By using the same file chooser instance to display its open and save dialogs, the program reaps the following benefits:

• The chooser remembers the current directory between uses, so the open and save versions automatically share the same current directory.
• You have to customize only one file chooser, and the customizations apply to both the open and save versions.

Finally, the example program has commented-out lines of code that let you change the file selection mode. For example, the following line of code makes the file chooser able to select only directories, and not files:

fc.setFileSelectionMode(JFileChooser.DIRECTORIES_ONLY);

Another possible selection mode is FILES_AND_DIRECTORIES. The default is FILES_ONLY. The following picture shows an open dialog with the file selection mode set to DIRECTORIES_ONLY. Note that, in the Java look and feel at least, only directories are visible — not files.

If you want to create a file chooser for a task other than opening or saving, or if you want to customize the file chooser, keep reading. We will discuss the following topics:

• Another Example: FileChooserDemo2
• Using a File Chooser for a Custom Task
• Filtering the List of Files
• Customizing the File View
• Providing an Accessory Component
• The File Chooser API
• Examples that Use File Choosers

Another Example: FileChooserDemo2

Let us look at FileChooserDemo2 example, a modified version of the previous demo program that uses more of the JFileChooser API. This example uses a file chooser that has been customized in several ways. Like the original example, the user invokes a file chooser with the push of a button. Here is a picture of the file chooser:

As the figure shows, this file chooser has been customized for a special task (Attach), provides a user-choosable file filter (Just Images), uses a special file view for image files, and has an accessory component that displays a thumbnail sketch of the currently selected image file.

The remainder of this section shows you the code that creates and customizes this file chooser. See the example index for links to all the files required by this example.

Using a File Chooser for a Custom Task

As you have seen, the JFileChooser class provides the showOpenDialog method for displaying an open dialog and the showSaveDialog method for displaying a save dialog.

The class has another method, showDialog, for displaying a file chooser for a custom task in a dialog. In the Java look and feel, the only difference between this dialog and the other file chooser dialogs is the title on the dialog window and the label on the approve button. Here is the code from FileChooserDemo2 that brings up the file chooser dialog for the Attach task:

JFileChooser fc = new JFileChooser();
int returnVal = fc.showDialog(FileChooserDemo2.this, "Attach");

The first argument to the showDialog method is the parent component for the dialog. The second argument is a String object that provides both the title for the dialog window and the label for the approve button.

Once again, the file chooser doesn't do anything with the selected file. The program is responsible for implementing the custom task for which the file chooser was created.

Filtering the List of Files

By default, a file chooser displays all of the files and directories that it detects, except for hidden files. A program can apply one or more file filters to a file chooser so that the chooser shows only some files. The file chooser calls the filter's accept method for each file to determine whether it should be displayed. A file filter accepts or rejects a file based on criteria such as file type, size, ownership, and so on. Filters affect the list of files displayed by the file chooser. The user can enter the name of any file even if it is not displayed.

JFileChooser supports three different kinds of filtering. The filters are checked in the order listed here. For example, an application-controlled filter sees only those files accepted by the built-in filtering.

Built-in filtering

Filtering is set up through specific method calls on a file chooser. Currently the only built-in filter available is for hidden files, such as those whose names begin with period (.) on UNIX systems. By default, hidden files are not shown. Call setFileHidingEnabled(false) to show hidden files.

Application-controlled filtering

The application determines which files are shown. Create a custom subclass of FileFilter, instantiate it, and use the instance as an argument to the setFileFilter method. The installed filter is displayed on the list of user-choosable filters. The file chooser shows only those files that the filter accepts.

User-choosable filtering

The file chooser GUI provides a list of filters that the user can choose from. When the user chooses a filter, the file chooser shows only those files accepted by that filter. FileChooserDemo2 adds a custom file filter to the list of user-choosable filters:

fc.addChoosableFileFilter(new ImageFilter());

By default, the list of user-choosable filters includes the Accept All filter, which enables the user to see all non-hidden files. This example uses the following code to disable the Accept All filter:

fc.setAcceptAllFileFilterUsed(false);

Our custom file filter is implemented in ImageFilter.java and is a subclass of FileFilter. The ImageFilter class implements the getDescription method to return "Just Images" — a string to put in the list of user-choosable filters. ImageFilter also implements the accept method so that it accepts all directories and any file that has a .png, .jpg, .jpeg, .gif, .tif, or .tiff filename extension.

public boolean accept(File f) {
    if (f.isDirectory()) {
        return true;
    }

    String extension = Utils.getExtension(f);
    if (extension != null) {
        if (extension.equals(Utils.tiff) ||
            extension.equals(Utils.tif) ||
            extension.equals(Utils.gif) ||
            extension.equals(Utils.jpeg) ||
            extension.equals(Utils.jpg) ||
            extension.equals(Utils.png)) {
                return true;
        } else {
            return false;
        }
    }

    return false;
}

By accepting all directories, this filter allows the user to navigate around the file system. If the bold lines were omitted from this method, the user would be limited to the directory with which the chooser was initialized.

The preceding code sample uses the getExtension method and several string constants from Utils.java, shown here:

public class Utils {

    public final static String jpeg = "jpeg";
    public final static String jpg = "jpg";
    public final static String gif = "gif";
    public final static String tiff = "tiff";
    public final static String tif = "tif";
    public final static String png = "png";

    /*
     * Get the extension of a file.
     */  
    public static String getExtension(File f) {
        String ext = null;
        String s = f.getName();
        int i = s.lastIndexOf('.');

        if (i > 0 &&  i < s.length() - 1) {
            ext = s.substring(i+1).toLowerCase();
        }
        return ext;
    }
}

Customizing the File View

In the Java look and feel, the chooser's list shows each file's name and displays a small icon that represents whether the file is a true file or a directory. You can customize this file view by creating a custom subclass of FileView and using an instance of the class as an argument to the setFileView method. The example uses an instance of a custom class, implemented in ImageFileView.java, as the file chooser's file view.

fc.setFileView(new ImageFileView());

The ImageFileView class shows a different icon for each type of image accepted by the image filter described previously.

The ImageFileView class overrides the five abstract methods defined in the FileView as follows.

String getTypeDescription(File f)

Returns a description of the file type. Here is ImageFileView's implementation of this method:

public String getTypeDescription(File f) {
    String extension = Utils.getExtension(f);
    String type = null;

    if (extension != null) {
        if (extension.equals(Utils.jpeg) ||
            extension.equals(Utils.jpg)) {
            type = "JPEG Image";
        } else if (extension.equals(Utils.gif)){
            type = "GIF Image";
        } else if (extension.equals(Utils.tiff) ||
                   extension.equals(Utils.tif)) {
            type = "TIFF Image";
        } else if (extension.equals(Utils.png)){
            type = "PNG Image";
        }
    }
    return type;
}

Icon getIcon(File f)

Returns an icon representing the file or its type. Here is ImageFileView's implementation of this method:

public Icon getIcon(File f) {
    String extension = Utils.getExtension(f);
    Icon icon = null;

    if (extension != null) {
        if (extension.equals(Utils.jpeg) ||
            extension.equals(Utils.jpg)) {
            icon = jpgIcon;
        } else if (extension.equals(Utils.gif)) {
            icon = gifIcon;
        } else if (extension.equals(Utils.tiff) ||
                   extension.equals(Utils.tif)) {
            icon = tiffIcon;
        } else if (extension.equals(Utils.png)) {
            icon = pngIcon;
        }
    }
    return icon;
}

String getName(File f)

Returns the name of the file. Most implementations of this method should return null to indicate that the look and feel should figure it out. Another common implementation returns f.getName().

String getDescription(File f)

Returns a description of the file. The intent is to describe individual files more specifically. A common implementation of this method returns null to indicate that the look and feel should figure it out.

Boolean isTraversable(File f)

Returns whether a directory is traversable. Most implementations of this method should return null to indicate that the look and feel should figure it out. Some applications might want to prevent users from descending into a certain type of directory because it represents a compound document. The isTraversable method should never return true for a non-directory.

Providing an Accessory Component

The customized file chooser in FileChooserDemo2 has an accessory component. If the currently selected item is a PNG, JPEG, TIFF, or GIF image, the accessory component displays a thumbnail sketch of the image. Otherwise, the accessory component is empty. Aside from a previewer, probably the most common use for the accessory component is a panel with more controls on it such as checkboxes that toggle between features.

The example calls the setAccessory method to establish an instance of the ImagePreview class, implemented in ImagePreview.java, as the chooser's accessory component:

fc.setAccessory(new ImagePreview(fc));

Any object that inherits from the JComponent class can be an accessory component. The component should have a preferred size that looks good in the file chooser.

The file chooser fires a property change event when the user selects an item in the list. A program with an accessory component must register to receive these events to update the accessory component whenever the selection changes. In the example, the ImagePreview object itself registers for these events. This keeps all the code related to the accessory component together in one class.

Here is the example's implementation of the propertyChange method, which is the method called when a property change event is fired:

//where member variables are declared
File file = null;
...
public void propertyChange(PropertyChangeEvent e) {
    boolean update = false;
    String prop = e.getPropertyName();

    //If the directory changed, don't show an image.
    if (JFileChooser.DIRECTORY_CHANGED_PROPERTY.equals(prop)) {
        file = null;
        update = true;

    //If a file became selected, find out which one.
    } else if (JFileChooser.SELECTED_FILE_CHANGED_PROPERTY.equals(prop)) {
        file = (File) e.getNewValue();
        update = true;
    }

    //Update the preview accordingly.
    if (update) {
        thumbnail = null;
        if (isShowing()) {
            loadImage();
            repaint();
        }
    }
}

If SELECTED_FILE_CHANGED_PROPERTY is the property that changed, this method obtains a File object from the file chooser. The loadImage and repaint methods use the File object to load the image and repaint the accessory component.

The File Chooser API

The API for using file choosers falls into these categories:

• Creating and Showing the File Chooser
• Selecting Files and Directories
• Navigating the File Chooser's List
• Customizing the File Chooser

Examples That Use File Choosers

This table shows the examples that use file choosers and points to where those examples are described.