接口调用实现类&& 为什么Autowired定义在接口上

package edu.cqu.interfaceTest;

import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Date;

import javax.swing.JOptionPane;
import javax.swing.Timer;

public class TimeTest  {
    public static void main(String args[]) {
    
        ActionListener listener = new TimePrinter();
        Timer t = new Timer(10000,listener);
        t.start();
        JOptionPane.showMessageDialog(null,"退出程序吗？");
        System.exit(0);
    }
}

class TimePrinter implements ActionListener{
    @Override
    public void actionPerformed(ActionEvent arg0) {
        // TODO Auto-generated method stub
        Date now = new Date();
        System.out.println("现在时间时:" + now);
        Toolkit.getDefaultToolkit().beep();
    }
}

// 告诉spring mvc这是一个控制器类
@Controller
@RequestMapping("")
public class CategoryController {
    @Autowired
    CategoryService categoryService;

    @RequestMapping("listCategory")
    public ModelAndView listCategory(){
        ModelAndView mav = new ModelAndView();
        List<Category> cs= categoryService.list();