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:

@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：

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);

	}

}

　　

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();

}

　　

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);

	}

}