Java自定义注解

1.什么是注解

注解是那些插入到源代码中使用其他工具可以对其进行处理的标签。下面就是一个注解的使用示例，注解事件处理器。

package cn.sp.chapter10.annotation;

import javax.swing.*;
import java.awt.*;

/**
 * Created by 2YSP on 2017/12/16.
 */
public class ButtonFrame extends JFrame {

    private static final int DEFAULT_WIDTH = 300;
    private static final int DEFAULT_HEIGHT = 200;

    private JPanel panel;//面板
    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(){
        yellowButton.setBackground(Color.yellow);
    }

    @ActionListenerFor(source = "blueButton")
    public void blueBackground(){
        blueButton.setBackground(Color.blue);
    }

    @ActionListenerFor(source = "redButton")
    public void redBackground(){
        redButton.setBackground(Color.red);
    }
}

package cn.sp.chapter10.annotation;

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

/**
 * Created by 2YSP on 2017/12/16.
 * 分析注解以及安装行为监听器
 */
public class ActionListenerInstaller {

    /**
     * Process all ActionListenerFor annotation in the given object
     *
     * @param obj an object whose methods may have ActionListenerFor annotations
     */
    public static void processAnnotations(Object obj) {
        try {
            //获取类
            Class<?> c1 = obj.getClass();
            for (Method m : c1.getDeclaredMethods()) {
                ActionListenerFor a = m.getAnnotation(ActionListenerFor.class);//获取注解
                if (a != null) {
                    Field f = c1.getDeclaredField(a.source());
                    f.setAccessible(true);//字段私有的 添加权限
                    addListener(f.get(obj),obj,m);// field.get(obj) 返回指定对象上此 Field 表示的字段的值 :yellowButton
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /**
     * Adds an action listener that calls a given method
     * @param source the event source th which an action listener is added
     * @param param the implicit parameter of the method that the listener calls
     * @param m the method that the listener calls
     * @throws Exception
     */
    public static void addListener(Object source,final Object param,final Method m)throws Exception{
//        yellowButton.addActionListener(new ActionListener() {
//            @Override
//            public void actionPerformed(ActionEvent e) {
//                doSomething();
//            }
//        });
        InvocationHandler handler = new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                return m.invoke(param);//执行将按钮背景变色的方法
            }
        };
        //创建代理对象
        Object listener = Proxy.newProxyInstance(null,new Class[]{java.awt.event.ActionListener.class},handler);
        Method adder = source.getClass().getMethod("addActionListener", ActionListener.class);
        adder.invoke(source,listener);
    }
}

package cn.sp.chapter10.annotation;

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

/**
 * Created by 2YSP on 2017/12/16.
 * 自定义注解 P740
 */
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface ActionListenerFor {
    String source();
}

package cn.sp.chapter10.annotation;

/**
 * Created by 2YSP on 2017/12/16.
 */
public class test {

    public static void main(String[] args) {
        ButtonFrame frame = new ButtonFrame();
        frame.setVisible(true);
    }
}

运行后点击三个按钮可以分别改变其背景色，

@Target和

@Retention是元注解，它们注解了ActionListenerFor注解。

2.注解语法

modify @interface AnnotationName
{
　　elementDeclartion1
　　elementDeclartion2
　　.......
}

其中每个元素声明都可以是以下其中一种形式：

type elementName();

 或 

type elementName() default value;

所有注解都隐式地扩展自java.lang.annotation.Annotation接口。