Question

我的java Web浏览器应用程序不会显示来自Internet的网页，如：

http://www.google.com

输入正确的网址时，最后会显示

提供的例外情况

无法加载页面

我的应用程序代码中有什么问题？

请帮我找出并解决问题。

 import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.util.*;
import javax.swing.*;
import javax.swing.event.*;
import javax.swing.text.html.*;

// The Mini Web Browser.
public class MiniBrowser extends JFrame
implements HyperlinkListener
{
// These are the buttons for iterating through the page list.
private JButton backButton, forwardButton;

// Page location text field.
private JTextField locationTextField;

// Editor pane for displaying pages.
private JEditorPane displayEditorPane;

// Browser's list of pages that have been visited.
private ArrayList pageList = new ArrayList();

// Constructor for Mini Web Browser.
public MiniBrowser()
{
// Set application title.
super("Mini Browser");

// Set window size.
setSize(640, 480);

// Handle closing events.
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
actionExit();
}
});

// Set up file menu.
JMenuBar menuBar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
fileMenu.setMnemonic(KeyEvent.VK_F);
JMenuItem fileExitMenuItem = new JMenuItem("Exit",
KeyEvent.VK_X);
fileExitMenuItem.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionExit();
}
});
fileMenu.add(fileExitMenuItem);
menuBar.add(fileMenu);
setJMenuBar(menuBar);

// Set up button panel.
JPanel buttonPanel = new JPanel();
backButton = new JButton("< Back");
backButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionBack();
}
});
backButton.setEnabled(false);
buttonPanel.add(backButton);
forwardButton = new JButton("Forward >");
forwardButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionForward();
}
});
forwardButton.setEnabled(false);
buttonPanel.add(forwardButton);
locationTextField = new JTextField(35);
locationTextField.addKeyListener(new KeyAdapter() {
public void keyReleased(KeyEvent e) {
if (e.getKeyCode() == KeyEvent.VK_ENTER) {
actionGo();
}
}
});
buttonPanel.add(locationTextField);
JButton goButton = new JButton("GO");
goButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionGo();
}
});
buttonPanel.add(goButton);

// Set up page display.
displayEditorPane = new JEditorPane();
displayEditorPane.setContentType("text/html");
displayEditorPane.setEditable(false);
displayEditorPane.addHyperlinkListener(this);

getContentPane().setLayout(new BorderLayout());
getContentPane().add(buttonPanel, BorderLayout.NORTH);
getContentPane().add(new JScrollPane(displayEditorPane),
BorderLayout.CENTER);
}

// Exit this program.
private void actionExit() {
System.exit(0);
}

// Go back to the page viewed before the current page.
private void actionBack() {
URL currentUrl = displayEditorPane.getPage();
int pageIndex = pageList.indexOf(currentUrl.toString());
try {
showPage(
new URL((String) pageList.get(pageIndex - 1)), false);
}
catch (Exception e) {}
}

// Go forward to the page viewed after the current page.
private void actionForward() {
URL currentUrl = displayEditorPane.getPage();
int pageIndex = pageList.indexOf(currentUrl.toString());
try {
showPage(
new URL((String) pageList.get(pageIndex + 1)), false);
}
catch (Exception e) {}
}

// Load and show the page specified in the location text field.
private void actionGo() {
URL verifiedUrl = verifyUrl(locationTextField.getText());
if (verifiedUrl != null) {
showPage(verifiedUrl, true);
} else {
showError("Invalid URL");
}
}

// Show dialog box with error message.
private void showError(String errorMessage) {
JOptionPane.showMessageDialog(this, errorMessage,
"Error", JOptionPane.ERROR_MESSAGE);
}

// Verify URL format.
private URL verifyUrl(String url) {
// Only allow HTTP URLs.
if (!url.toLowerCase().startsWith("http://"))
return null;

// Verify format of URL.
URL verifiedUrl = null;
try {
verifiedUrl = new URL(url);
} catch (Exception e) {
return null;
}

return verifiedUrl;
}

/* Show the specified page and add it to
the page list if specified. */
private void showPage(URL pageUrl, boolean addToList)
{
// Show hour glass cursor while crawling is under way.
setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));

try {
// Get URL of page currently being displayed.
URL currentUrl = displayEditorPane.getPage();

// Load and display specified page.
displayEditorPane.setPage(pageUrl);

// Get URL of new page being displayed.
URL newUrl = displayEditorPane.getPage();

// Add page to list if specified.
if (addToList) {
int listSize = pageList.size();
if (listSize > 0) {
int pageIndex =
pageList.indexOf(currentUrl.toString());
if (pageIndex < listSize - 1) {
for (int i = listSize - 1; i > pageIndex; i--) {
pageList.remove(i);
}
}
}
pageList.add(newUrl.toString());
}

// Update location text field with URL of current page.
locationTextField.setText(newUrl.toString());

// Update buttons based on the page being displayed.
updateButtons();
}
catch (Exception e)
{
// Show error messsage.
showError("Unable to load page");
}
finally
{
// Return to default cursor.
setCursor(Cursor.getDefaultCursor());
}
}

/* Update back and forward buttons based on
the page being displayed. */
private void updateButtons() {
if (pageList.size() < 2) {
backButton.setEnabled(false);
forwardButton.setEnabled(false);
} else {
URL currentUrl = displayEditorPane.getPage();
int pageIndex = pageList.indexOf(currentUrl.toString());
backButton.setEnabled(pageIndex > 0);
forwardButton.setEnabled(
pageIndex < (pageList.size() - 1));
}
}

// Handle hyperlink's being clicked.
public void hyperlinkUpdate(HyperlinkEvent event) {
HyperlinkEvent.EventType eventType = event.getEventType();
if (eventType == HyperlinkEvent.EventType.ACTIVATED) {
if (event instanceof HTMLFrameHyperlinkEvent) {
HTMLFrameHyperlinkEvent linkEvent =
(HTMLFrameHyperlinkEvent) event;
HTMLDocument document =
(HTMLDocument) displayEditorPane.getDocument();
document.processHTMLFrameHyperlinkEvent(linkEvent);
} else {
showPage(event.getURL(), true);
}
}
}

// Run the Mini Browser.
public static void main(String[] args) {
MiniBrowser browser = new MiniBrowser();
browser.show();
}
}

Answer 1

似乎在输入url之前调用showPage（）会导致抛出NullPointerException。因此，您可能希望更改何时/如何调用showPage（）和/或向showPage（）添加一些额外的空值检查。只做空检查应该可以解决问题：

private void showPage(URL pageUrl, boolean addToList) {
    // Show hour glass cursor while crawling is under way.
    if (pageUrl == null) {
        return;
    }
    setCursor(Cursor.getPredefinedCursor(Cursor.WAIT_CURSOR));

    try {
        // Get URL of page currently being displayed.
        URL currentUrl = displayEditorPane.getPage();

        // Load and display specified page.
        displayEditorPane.setPage(pageUrl);

        // Get URL of new page being displayed.
        URL newUrl = displayEditorPane.getPage();

        // Add page to list if specified.
        if (newUrl != null && addToList) {
            int listSize = pageList.size();
            if (listSize > 0) {
                int pageIndex = pageList.indexOf(currentUrl.toString());
                if (pageIndex < listSize - 1) {
                    for (int i = listSize - 1; i > pageIndex; i--) {
                        pageList.remove(i);
                    }
                }
            }
            pageList.add(newUrl.toString());
        }

        // Update location text field with URL of current page.
        if (newUrl != null) {
            locationTextField.setText(newUrl.toString());
        }


        // Update buttons based on the page being displayed.
        updateButtons();
    } catch (Exception e) {
        // Show error messsage.
        e.printStackTrace();
        showError("Unable to load page");
    } finally {
        // Return to default cursor.
        setCursor(Cursor.getDefaultCursor());
    }
}

Java Web浏览器连接问题

1 个答案: