在Eclipse插件开发中打开FileChooser

Question

我正在创建一个Eclipse-Plugin，我需要打开文件系统浏览器，选择一个文件然后对所选文件执行某些计算。

我尝试过使用JFileChooser（我有一个打开文件浏览器的类）。从我的处理程序类调用这个特定的类，一旦选择了文件（从文件选择器），我需要返回String（文件位置）并对其执行计算。但是，我遇到了这方面的困难，因为我在关闭JFrame后无法执行此计算。

public class FileChooserDialog {

       private JFrame mainFrame;
       private JLabel headerLabel;
       private JLabel statusLabel;
       private JPanel controlPanel;
       private String selectedFile = null;

       public FileChooserDialog(){
          prepareGUI();
          clickExtractFile();
       }

       public String getSelectedFile(){
           return selectedFile;
       }

       private void prepareGUI(){
          mainFrame = new JFrame("choose a file");
          mainFrame.setSize(400,400);
          mainFrame.setLayout(new GridLayout(3, 1));

          headerLabel = new JLabel("", JLabel.CENTER);   
          headerLabel.setText("Choose a valid zip file to extract");
          statusLabel = new JLabel("",JLabel.CENTER);    

          statusLabel.setSize(350,100);
          statusLabel.setText("No file selected");

          controlPanel = new JPanel();
          controlPanel.setLayout(new FlowLayout());

          mainFrame.add(headerLabel);
          mainFrame.add(controlPanel);
          mainFrame.add(statusLabel);
          mainFrame.setVisible(true); 
       }

       private void clickExtractFile(){
          final JFileChooser  fileDialog = new JFileChooser();
          JButton showFileDialogButton = new JButton("Open File");
          showFileDialogButton.addActionListener(new ActionListener() {
             @Override
             public void actionPerformed(ActionEvent e) {
                int returnVal = fileDialog.showOpenDialog(mainFrame);
                if (returnVal == JFileChooser.APPROVE_OPTION) {
                   File file = fileDialog.getSelectedFile();
                   statusLabel.setText(file.getAbsolutePath());
                }
                else{
                   statusLabel.setText("No file selected");           
                }      
             }
          });
          JButton extractButton = new JButton("Choose This");

          extractButton.addActionListener(new ActionListener() {
             @Override
             public void actionPerformed(ActionEvent e) {
                 selectedFile = statusLabel.getText();

                 if (selectedFile.equals("No file selected")){
                     selectedFile = "No file";
                     System.out.println("nahh nothing selected"); 
                 }else{
                     System.out.println("yes a file was selected. This was: " + selectedFile); 
                 }
                 mainFrame.dispatchEvent(new WindowEvent(mainFrame, WindowEvent.WINDOW_CLOSING));
             }
          });

          controlPanel.add(showFileDialogButton);
          controlPanel.add(extractButton);
          mainFrame.setVisible(true); 
       }

处理程序类：

public class Handler extends AbstractHandler{
        FileChooserDialog chooser = new FileChooserDialog();

        chooser.clickExtractFile();

        //I only want this to be called once the Jframe is closed (i.e. after user clicks "Choose This"
        fileChosen = chooseZip.getSelectedFile();
}

然而，当fileChosen = chooseZip.getSelectedFile();被执行时，我收到一个nullpointer异常，因为它甚至在关闭JFrame之前就被执行了。

例外：

org.eclipse.e4.core.di.InjectionException: java.lang.NullPointerException
    at org.eclipse.e4.core.internal.di.MethodRequestor.execute(MethodRequestor.java:62)
    at org.eclipse.e4.core.internal.di.InjectorImpl.invokeUsingClass(InjectorImpl.java:247)
    at org.eclipse.e4.core.internal.di.InjectorImpl.invoke(InjectorImpl.java:229)
    at org.eclipse.e4.core.contexts.ContextInjectionFactory.invoke(ContextInjectionFactory.java:132)
    at org.eclipse.e4.core.commands.internal.HandlerServiceHandler.execute(HandlerServiceHandler.java:149)
    at org.eclipse.core.commands.Command.executeWithChecks(Command.java:499)
    at org.eclipse.core.commands.ParameterizedCommand.executeWithChecks(ParameterizedCommand.java:508)
    at org.eclipse.e4.core.commands.internal.HandlerServiceImpl.executeHandler(HandlerServiceImpl.java:210)
    at org.eclipse.e4.ui.workbench.renderers.swt.HandledContributionItem.executeItem(HandledContributionItem.java:825)
    at org.eclipse.e4.ui.workbench.renderers.swt.HandledContributionItem.handleWidgetSelection(HandledContributionItem.java:701)
    at org.eclipse.e4.ui.workbench.renderers.swt.HandledContributionItem.access$6(HandledContributionItem.java:685)
    at org.eclipse.e4.ui.workbench.renderers.swt.HandledContributionItem$4.handleEvent(HandledContributionItem.java:613)
    at org.eclipse.swt.widgets.EventTable.sendEvent(EventTable.java:84)

我该如何解决这个问题？