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 object17      *18      * @param obj an object whose methods may have ActionListenerFor annotations19      */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 :yellowButton30                 }31             }32         } catch (Exception e) {33             e.printStackTrace();34         }35     }36 37     /**38      * Adds an action listener that calls a given method39      * @param source the event source th which an action listener is added40      * @param param the implicit parameter of the method that the listener calls41      * @param m the method that the listener calls42      * @throws Exception43      */44     public static void addListener(Object source,final Object param,final Method m)throws Exception{45 //        yellowButton.addActionListener(new ActionListener() {46 //            @Override47 //            public void actionPerformed(ActionEvent e) {48 //                doSomething();49 //            }50 //        });51         InvocationHandler handler = new InvocationHandler() {52             @Override53             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 P74011  */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。