《Java核心技术 卷II》注解简介

8.3.1 注解简介

每个注解都必须通过一个注解接口进行定义。@interface

接口中的方法和注解中的元素相对应。@Test(timeout="10000")

元注解：Target，Retention。

8.3.2 示例：注解事件处理器，重点(工作中有价值)

组装事件源监听器是个苦差事: myButton.addActionListener(()->doSomething());

用注解免除苦差事:@ActionListenerFor(source="myButton") void doSomething(){...}

package 第8章脚本编译与注解.使用注解;

import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import java.lang.annotation.Retention;
import java.lang.annotation.Target;

@Retention(RUNTIME)
@Target(METHOD)
public @interface ActionListenerFor {
    String source();
}

package 第8章脚本编译与注解.使用注解;

import java.awt.event.ActionListener;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;

public class ActionListenerInstaller {
    
    public static void processAnnotations(Object obj) {
        
        Class<?> cl = obj.getClass();
        for (Method m : cl.getDeclaredMethods()) {
            ActionListenerFor a = m.getAnnotation(ActionListenerFor.class);
            if(a != null) {
                try {
                    Field f = cl.getDeclaredField(a.source());
                    f.setAccessible(true);
                    addListener(f.get(obj), obj, m);
                } catch (NoSuchFieldException | SecurityException e) {
                    e.printStackTrace();
                } catch (IllegalArgumentException e) {
                    e.printStackTrace();
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                } catch (NoSuchMethodException e) {
                    e.printStackTrace();
                } catch (InvocationTargetException e) {
                    e.printStackTrace();
                }
                
            }
        }
        
    }
    
    public static void addListener(Object source, final Object param, final Method m) throws NoSuchMethodException, SecurityException, IllegalAccessException, InvocationTargetException {
        var handler = new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                return m.invoke(param);
            }
        };
        Object listener = Proxy.newProxyInstance(null,
            new Class[] {ActionListener.class}, handler);
        Method adder = source.getClass().getMethod("addActionListener", ActionListener.class);
        adder.invoke(source, listener);
    }
    
}

package 第8章脚本编译与注解.使用注解;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Toolkit;

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

public class ButtonFrame extends JFrame{
    private static final int DEFAULT_WIDTH = 300;
    private static final int DEFAULT_HEIGHT = 200;
    
    private JPanel panel;
    private JButton yellowButton;
    private JButton blueButton;
    private JButton redButton;
    
    public ButtonFrame() {
        setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
        panel = new JPanel();
        add(panel);
        
        yellowButton = new JButton("Yellow");
        blueButton = new JButton("Blue");
        redButton = new JButton("Red");
        
        panel.add(yellowButton);
        panel.add(blueButton);
        panel.add(redButton);
        
        ActionListenerInstaller.processAnnotations(this);
    }
    
    @ActionListenerFor(source = "yellowButton")
    public void yellowBackground() {
        panel.setBackground(Color.YELLOW);
    }
    @ActionListenerFor(source = "blueButton")
    public void blueBackground() {
        panel.setBackground(Color.BLUE);
    }
    @ActionListenerFor(source = "redButton")
    public void redBackground() {
        panel.setBackground(Color.RED);
    }

    public static void main(String[] args) {
        var frame = new ButtonFrame();
        //初始打开为屏幕中央
        Toolkit toolkit = Toolkit.getDefaultToolkit();
        Dimension screenSize = toolkit.getScreenSize();
        int screenWidth = (int) screenSize.getWidth();
        int screenHeight = (int) screenSize.getHeight();
        //计算窗口位置
        int x = (screenWidth - frame.getWidth())/2;
        int y = (screenHeight - frame.getHeight())/2;
        frame.setLocation(x, y);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
    
}