Java自定义注解

Posted 2020-11-01

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。

 1 package cn.sp.chapter10.annotation;
 2 
 3 import javax.swing.*;
 4 import java.awt.*;
 5 
 6 /**
 7  * Created by 2YSP on 2017/12/16.
 8  */
 9 public class ButtonFrame extends JFrame {
10 
11     private static final int DEFAULT_WIDTH = 300;
12     private static final int DEFAULT_HEIGHT = 200;
13 
14     private JPanel panel;//面板
15     private JButton yellowButton;
16     private JButton blueButton;
17     private JButton redButton;
18 
19     public ButtonFrame(){
20         setSize(DEFAULT_WIDTH,DEFAULT_HEIGHT);
21         panel = new JPanel();
22         add(panel);
23         yellowButton = new JButton("Yellow");
24         blueButton = new JButton("blue");
25         redButton = new JButton("red");
26 
27         panel.add(yellowButton);
28         panel.add(blueButton);
29         panel.add(redButton);
30 
31         ActionListenerInstaller.processAnnotations(this);
32     }
33 
34     @ActionListenerFor(source = "yellowButton")
35     public void yellowBackground(){
36         yellowButton.setBackground(Color.yellow);
37     }
38 
39     @ActionListenerFor(source = "blueButton")
40     public void blueBackground(){
41         blueButton.setBackground(Color.blue);
42     }
43 
44     @ActionListenerFor(source = "redButton")
45     public void redBackground(){
46         redButton.setBackground(Color.red);
47     }
48 }

 1 package cn.sp.chapter10.annotation;
 2 
 3 import java.awt.event.ActionListener;
 4 import java.lang.reflect.Field;
 5 import java.lang.reflect.InvocationHandler;
 6 import java.lang.reflect.Method;
 7 import java.lang.reflect.Proxy;
 8 
 9 /**
10  * Created by 2YSP on 2017/12/16.
11  * 分析注解以及安装行为监听器
12  */
13 public class ActionListenerInstaller {
14 
15     /**
16      * Process all ActionListenerFor annotation in the given object
17      *
18      * @param obj an object whose methods may have ActionListenerFor annotations
19      */
20     public static void processAnnotations(Object obj) {
21         try {
22             //获取类
23             Class<?> c1 = obj.getClass();
24             for (Method m : c1.getDeclaredMethods()) {
25                 ActionListenerFor a = m.getAnnotation(ActionListenerFor.class);//获取注解
26                 if (a != null) {
27                     Field f = c1.getDeclaredField(a.source());
28                     f.setAccessible(true);//字段私有的 添加权限
29                     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
30                 }
31             }
32         } catch (Exception e) {
33             e.printStackTrace();
34         }
35     }
36 
37     /**
38      * Adds an action listener that calls a given method
39      * @param source the event source th which an action listener is added
40      * @param param the implicit parameter of the method that the listener calls
41      * @param m the method that the listener calls
42      * @throws Exception
43      */
44     public static void addListener(Object source,final Object param,final Method m)throws Exception{
45 //        yellowButton.addActionListener(new ActionListener() {
46 //            @Override
47 //            public void actionPerformed(ActionEvent e) {
48 //                doSomething();
49 //            }
50 //        });
51         InvocationHandler handler = new InvocationHandler() {
52             @Override
53             public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
54                 return m.invoke(param);//执行将按钮背景变色的方法
55             }
56         };
57         //创建代理对象
58         Object listener = Proxy.newProxyInstance(null,new Class[]{java.awt.event.ActionListener.class},handler);
59         Method adder = source.getClass().getMethod("addActionListener", ActionListener.class);
60         adder.invoke(source,listener);
61     }
62 }

 1 package cn.sp.chapter10.annotation;
 2 
 3 import java.lang.annotation.ElementType;
 4 import java.lang.annotation.Retention;
 5 import java.lang.annotation.RetentionPolicy;
 6 import java.lang.annotation.Target;
 7 
 8 /**
 9  * Created by 2YSP on 2017/12/16.
10  * 自定义注解 P740
11  */
12 @Target(ElementType.METHOD)
13 @Retention(RetentionPolicy.RUNTIME)
14 public @interface ActionListenerFor {
15     String source();
16 }

 1 package cn.sp.chapter10.annotation;
 2 
 3 /**
 4  * Created by 2YSP on 2017/12/16.
 5  */
 6 public class test {
 7 
 8     public static void main(String[] args) {
 9         ButtonFrame frame = new ButtonFrame();
10         frame.setVisible(true);
11     }
12 }

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。