Java中无法添加键盘监听

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;

import javax.swing.JButton;
import javax.swing.JFrame;

public class USBMain {

public static void main(String[] args) {

USBMain u = new USBMain();

u.launchFrame();

// 开启盘符检查线程

new CheckRootThread().start();

}

// 界面

private void launchFrame() {

final JFrame frame = new JFrame();

frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

frame.setLocation(450, 250);

JButton hide = new JButton("点击隐藏窗口");

// 点击按钮后隐藏窗口事件监听

hide.addActionListener(new ActionListener() {

public void actionPerformed(ActionEvent e) {

frame.setVisible(false);

}

});

frame.add(hide);

frame.pack();

frame.setVisible(true);

frame.addKeyListener(new KeyAdapter() {

@Override
public void keyPressed(KeyEvent e) {
char charA = e.getKeyChar();
System.out.println("你按了《" + charA + "》键");
}

});
}

}

试了好几种方法，就是没有效果

KeyboardFocusManager manager = KeyboardFocusManager.getCurrentKeyboardFocusManager();
        manager.addKeyEventPostProcessor(new KeyEventPostProcessor() {
            @Override
            public boolean postProcessKeyEvent(KeyEvent e) {
                if (e.getID() == KeyEvent.KEY_PRESSED) {
                    System.out.println(e.getKeyChar());
                }
                return false;
            }
        });

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