JAVA高级特性 - 注解

注解是插入到代码中用于某种工具处理的标签。这些标签可以在源码层次上进行操作，或者可以处理编译器将其纳入到注解类文件中。

注解不会改变对程序的编译方式。Java编译器会对包含注解和不包含注解的代码生成相同的虚拟机指令。

在java中，注解通常使用@Annotation来表示，注解通常用于框架的设计，配合反射，从而减少代码量，明确业务逻辑。

我们最为常见的注解是@Override和@Test这种注解，前者用于标明方法为实现某接口，后者则常用于JUNIT.

下面给出个注解的实例定义:

?

1 2 3 4 5 6

@Target(ElementType.Method) @Retention(RetentionPolicy.RUNTIME) public @interface Test {     long timeout() default 0L; }

@Interface声明创建了一个真正的java接口。处理注解的工具将接受实现了该接口的对象。

注解Target和Retention是元注解，它们注解了注解，即标识了Test注解是一个只能用在方法上的注解，并且当类文件载入虚拟机的时候，仍然可以保留下来。

注解的使用通常要配合反射使用，下面给出例子：

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62

package com.inspur.jiyq.corejava.annotation;   import java.awt.Color;   import javax.swing.JButton; import javax.swing.JFrame; import javax.swing.JPanel;   public class ButtonFrame extends JFrame{     private static final long serialVersionUID = 1L;           public static final int DEFAULT_WIDTH = 300;     public static final int DEFAULT_HEIGHT = 200;           private JPanel panel;     private JButton yellowButton;     private JButton blueButton;     private JButton redButton;           public ButtonFrame()     {         setTitle("ButtonTest");         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)     {         ButtonFrame frame = new ButtonFrame();         frame.setVisible(true);     } }

　　

?

1 2 3 4 5 6 7 8 9 10 11 12

package com.inspur.jiyq.corejava.annotation;   import java.lang.annotation.ElementType; import java.lang.annotation.Retention; import java.lang.annotation.RetentionPolicy; import java.lang.annotation.Target;   @Target(ElementType.METHOD) @Retention(RetentionPolicy.RUNTIME) public @interface ActionListenerFor {     String source(); }

　　

?

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55

package com.inspur.jiyq.corejava.annotation;   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;   /**  * 用于分析注解及新增行为监听器  */ public class ActionListenerInstaller {     /**      * Process all ActionListenerFor annotations in the given object      *      * @param obj      *            an object whose methods may have ActionListnerFor annotations      */     public static void processAnnotations(Object obj) {         try {             Class<?> cl = obj.getClass();             for (Method m : cl.getDeclaredMethods()) {                 ActionListenerFor a = m.getAnnotation(ActionListenerFor.class);                 if (a != null) {                     Field f = cl.getDeclaredField(a.source());                     f.setAccessible(true);                     addListener(f.get(obj), obj, m);                 }             }         } catch (Exception e) {             e.printStackTrace();         }     }       public static void addListener(Object source, final Object param,             final Method m) throws NoSuchMethodException, SecurityException,             IllegalAccessException, IllegalArgumentException,             InvocationTargetException {         InvocationHandler 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[] { java.awt.event.ActionListener.class }, handler);         Method adder = source.getClass().getMethod("addActionListener",                 ActionListener.class);           adder.invoke(source, listener);     } }