自动将焦点设置在JTextField上

Question

无法找到答案（或者说我可能没有恰当地提问）。

如何在我的JFrame启动时立即将焦点设置在JTextField textfield1上？ 所谓的焦点是指用户可以立即开始输入而不必单击JTextField。

我尝试了textfield1.setCaretPosition(0)和textfield1.moveCaretPosition(0)，但都没有起作用。

此外，在单击提交按钮后，焦点仍然留在提交按钮上（即使在调用上述两个方法之后）。

- Cody

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请参见JComponent.requestFocusInWindow()。 - Andrew Thompson

你尝试过 textField1.requestFocusInWindow(); 吗？ - paulsm4

可能是Fullscreen Swing frames not focusing correctly in Linux (Windows is fine)的重复问题。 - Andrew Thompson

谢谢，我尝试在谷歌上搜索，但是没有想到“focus”是正确的词。 - Cody

Answer 1

在Swing中，有许多方法可以控制焦点，但在您的情况下，最简单的方法可能是直接使用JTextField#requestFocusInWindow

虽然不是主题，但您可能需要查看如何使用焦点子系统

- MadProgrammer

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当我的JFrame被初始化时，是否应该包装成invokeLater？ - mKorbel

Answer 2

比直接请求初始组件的焦点更有趣的是使用自定义的FocusTraversalPolicy实现通用机制来查找/配置默认值。它的api中已经有了基本概念，包括返回初始/默认组件的方法，例如：

```java public abstract Component getDefaultComponent(Container aContainer) ```

返回要聚焦的默认组件。当进入以aContainer为根的新焦点遍历周期时，该组件将是第一个接收焦点的组件。

默认实现返回循环中的第一个组件，自定义实现可能会查找自定义指示器，例如客户端属性。简单的用法是将该策略设置为具有内容窗格中标记的框架，类似于：

```java frame.setFocusTraversalPolicyProvider(true); frame.setFocusTraversalPolicy(new MyCustomPolicy()); ```

// a custom policy checking for the property
FocusTraversalPolicy policy = new LayoutFocusTraversalPolicy() {

    @Override
    public Component getDefaultComponent(Container aContainer) {
        if (aContainer instanceof JComponent) {
            JComponent parent = (JComponent) aContainer;
            if (parent.getClientProperty("defaultFocus") instanceof Component) {
                return (Component) parent.getClientProperty("defaultFocus");
            }
        }
        if (aContainer instanceof RootPaneContainer) {
            RootPaneContainer root = (RootPaneContainer) aContainer;
            JComponent parent = (JComponent) root.getContentPane();
            if (parent.getClientProperty("defaultFocus") instanceof Component) {
                return (Component) parent.getClientProperty("defaultFocus");
            }

        }
        return super.getDefaultComponent(aContainer);
    }

};
JFrame frame = ... // create  and fill
JTextField field = new JTextField(20);
frame.add(field, BorderLayout.SOUTH);
// set the client property 
((JComponent) frame.getContentPane()).putClientProperty("defaultFocus", field);
// set the custom policy
frame.setFocusTraversalPolicy(policy);

更加优雅的方法是默认使用此策略 - 可以通过将其安装为KeyboardFocusManager的默认策略来实现。有了这个，使用将会变得更简单：

• 在应用程序生命周期的早期安装自定义默认值
• 标记初始焦点组件（无需手动设置框架策略）

代码：

// very early in the application code
initializeDefaultFocusTraversalPolicy();

// default in any frame
JFrame frame = ... // create  and fill
JTextField field = new JTextField(20);
frame.add(field, BorderLayout.SOUTH);
// set the client property 
((JComponent) frame.getContentPane())
    .putClientProperty(DelegatingFocusTraversalPolicy.DEFAULT_FOCUS_KEY, field);

如果没有一个小技巧，Swing就不会是Swing:

• 窗口的默认策略在创建时就被内部设置了 - 因此键盘焦点管理器必须在创建框架之前进行配置
• UIManager对于管理器的默认设置有自己的想法，事实上，在创建非常第一个框架时无条件地配置它（这将覆盖我们自己的默认设置）

解决方法是创建一个虚拟框架，然后设置策略：

public static void initializeDefaultFocusTraversalPolicy() {
    // a custom default policy is overwritten by the UIManager the
    // very first time it sees a top-level container created
    // it does so  unconditionally depending on a flag in LAFState
    // see UIManager.maybeInitializeFocusPolicy
    // so we tricks him into doing it for a fake frame
    new JFrame();
    // and set our custom default afterwards
    FocusTraversalPolicy p = KeyboardFocusManager.getCurrentKeyboardFocusManager()
         .getDefaultFocusTraversalPolicy();
    KeyboardFocusManager.getCurrentKeyboardFocusManager()
         .setDefaultFocusTraversalPolicy(new CustomFocusTraversalPolicy(p));
}

/**
 * Basically the same custom policy as the ad-hoc above, just with
 * delegating 
 */
public static class CustomFocusTraversalPolicy extends FocusTraversalPolicy {
    public static final String DEFAULT_FOCUS_KEY = "defaultFocus";
    private FocusTraversalPolicy delegate;

    public CustomFocusTraversalPolicy(FocusTraversalPolicy delegate) {
        this.delegate = Contract.asNotNull(delegate, "the delegate must not be null");
    }

    @Override
    public Component getDefaultComponent(Container container) {
        if (container instanceof JComponent) {
            JComponent parent = (JComponent) container;
            if (parent.getClientProperty(DEFAULT_FOCUS_KEY) instanceof Component) {
                return (Component) parent.getClientProperty(DEFAULT_FOCUS_KEY);
            }
        }
        if (container instanceof RootPaneContainer) {
            RootPaneContainer root = (RootPaneContainer) container;
            JComponent parent = (JComponent) root.getContentPane();
            if (parent.getClientProperty(DEFAULT_FOCUS_KEY) instanceof Component) {
                return (Component) parent.getClientProperty(DEFAULT_FOCUS_KEY);
            }

        }
        return delegate.getDefaultComponent(container);
    }
    @Override
    public Component getComponentAfter(Container aContainer,
            Component aComponent) {
        return delegate.getComponentAfter(aContainer, aComponent);
    }

    @Override
    public Component getComponentBefore(Container aContainer,
            Component aComponent) {
        return delegate.getComponentBefore(aContainer, aComponent);
    }

    @Override
    public Component getFirstComponent(Container aContainer) {
        return delegate.getFirstComponent(aContainer);
    }

    @Override
    public Component getLastComponent(Container aContainer) {
        return delegate.getLastComponent(aContainer);
    }
}

- kleopatra