在前面《[AspectJ] 明明白白AspectJ (1) 》中用例子说明了AspectJ的确是代码生成器。
现在，同样的方式，将官方提供的例子“Bean Example”（File -->New -->Project -->AspectJ -->AspectJ Examples --> Bean Example）简单剖析一下。Bean Example主要是利用AOP来实现属性改变通知，效果像观察者模式来关注bean的属性，但它是用java.beans.PropertyChangeSupport来实现的。如果你对java.beans.PropertyChangeSupport用法不了解，可以参考我以前的一篇随笔《[java 拾遗篇] JavaBean实现约束属性简单例 》。
Bean Example的源码有三个文件，分别是Point.java , BoundPoint.aj , Demo.java
Point.java
package bean;
class Point 
{
protected int x = 0;
protected int y = 0;
/** *//**
* Return the X coordinate
*/
public int getX(){
return x;
}
/** *//**
* Return the y coordinate
*/
public int getY(){
return y;
}
/** *//**
* Set the x and y coordinates
*/
public void setRectangular(int newX, int newY){
setX(newX);
setY(newY);
}
/** *//**
* Set the X coordinate
*/
public void setX(int newX) {
x = newX;
}
/** *//**
* set the y coordinate
*/
public void setY(int newY) {
y = newY;
}
/** *//**
* Move the point by the specified x and y offset
*/
public void offset(int deltaX, int deltaY){
setRectangular(x + deltaX, y + deltaY);
}
/** *//**
* Make a string of this
*/
public String toString(){
return "(" + getX() + ", " + getY() + ")" ;
}
}
BoundPoint.aj
package bean;
import java.beans.*;
import java.io.Serializable;
/**//*
* Add bound properties and serialization to point objects
*/
aspect BoundPoint {
/**//*
* privately introduce a field into Point to hold the property
* change support object. `this' is a reference to a Point object.
*/
private PropertyChangeSupport Point.support = new PropertyChangeSupport(this);
/**//*
* Introduce the property change registration methods into Point.
* also introduce implementation of the Serializable interface.
*/
public void Point.addPropertyChangeListener(PropertyChangeListener listener){
support.addPropertyChangeListener(listener);
}
public void Point.addPropertyChangeListener(String propertyName,
PropertyChangeListener listener){
support.addPropertyChangeListener(propertyName, listener);
}
public void Point.removePropertyChangeListener(String propertyName,
PropertyChangeListener listener) {
support.removePropertyChangeListener(propertyName, listener);
}
public void Point.removePropertyChangeListener(PropertyChangeListener listener) {
support.removePropertyChangeListener(listener);
}
public void Point.hasListeners(String propertyName) {
support.hasListeners(propertyName);
}
declare parents: Point implements Serializable;
/** *//**
* Pointcut describing the set<property> methods on Point.
* (uses a wildcard in the method name)
*/
pointcut setter(Point p): call(void Point.set*(*)) && target(p);
/** *//**
* Advice to get the property change event fired when the
* setters are called. It's around advice because you need
* the old value of the property.
*/
void around(Point p): setter(p) {
String propertyName =
thisJoinPointStaticPart.getSignature().getName().substring("set".length());
int oldX = p.getX();
int oldY = p.getY();
proceed(p);
if (propertyName.equals("X")){
firePropertyChange(p, propertyName, oldX, p.getX());
} else {
firePropertyChange(p, propertyName, oldY, p.getY());
}
}
/**//*
* Utility to fire the property change event.
*/
void firePropertyChange(Point p,
String property,
double oldval,
double newval) {
p.support.firePropertyChange(property,
new Double(oldval),
new Double(newval));
}
}
Demo.java
package bean;
import java.beans.*;
import java.io.*;
public class Demo implements PropertyChangeListener {
static final String fileName = "test.tmp";
/** *//**
* when Demo is playing the listener role,
* this method reports that a propery has changed
*/
public void propertyChange(PropertyChangeEvent e){
System.out.println("Property " + e.getPropertyName() + " changed from " +
e.getOldValue() + " to " + e.getNewValue() );
}
/** *//**
* main: test the program
*/
public static void main(String[] args){
Point p1 = new Point();
p1.addPropertyChangeListener(new Demo());
System.out.println("p1 =" + p1);
p1.setRectangular(5,2);
System.out.println("p1 =" + p1);
p1.setX( 6 );
p1.setY( 3 );
System.out.println("p1 =" + p1);
p1.offset(6,4);
System.out.println("p1 =" + p1);
save(p1, fileName);
Point p2 = (Point) restore(fileName);
System.out.println("Had: " + p1);
System.out.println("Got: " + p2);
}
/** *//**
* Save a serializable object to a file
*/
static void save(Serializable p, String fn){
try {
System.out.println("Writing to file: " + p);
FileOutputStream fo = new FileOutputStream(fn);
ObjectOutputStream so = new ObjectOutputStream(fo);
so.writeObject(p);
so.flush();
} catch (Exception e) {
System.out.println(e);
System.exit(1);
}
}
/** *//**
* Restore a serializable object from the file
*/
static Object restore(String fn){
try {
Object result;
System.out.println("Reading from file: " + fn);
FileInputStream fi = new FileInputStream(fn);
ObjectInputStream si = new ObjectInputStream(fi);
return si.readObject();
} catch (Exception e) {
System.out.println(e);
System.exit(1);
}
return null;
}
}
跑跑Demo输出的结果：
p1 =(0, 0)
Property X changed from 0.0 to 5.0
Property Y changed from 0.0 to 2.0
p1 =(5, 2)
Property X changed from 5.0 to 6.0
Property Y changed from 2.0 to 3.0
p1 =(6, 3)
Property X changed from 6.0 to 12.0
Property Y changed from 3.0 to 7.0
p1 =(12, 7)
Writing to file: (12, 7)
Reading from file: test.tmp
Had: (12, 7)
Got: (12, 7)
好，跟上次一样，将编译的代码反编来对照一下。看看AspectJ根据“规则”（aspect的定义）修改了代码的哪些地方，给你一个明白！
三个文件：Point.class , BoundPoint.class , Demo.class
代码上，我添加了一些注释，反编译出来的文件般都没什么注释的。
Point.class
package bean;
import java.beans.PropertyChangeListener;
import java.beans.PropertyChangeSupport;
import java.io.Serializable;
import org.aspectj.lang.Signature;
import org.aspectj.runtime.internal.AroundClosure;
import org.aspectj.runtime.reflect.Factory;
//BoundPoint.aj 中的声明“declare parents: Point implements Serializable;”使Point实现了Serializable
public class Point implements Serializable
{
Point()
{
//为Point添加向关注者提供属性改变通知的支持，target为this（自身）
BoundPoint.ajc$interFieldInit$bean_BoundPoint$bean_Point$support(this);
x = 0;
y = 0;
}
public int getX()
{
return x;
}
public int getY()
{
return y;
}
public void setRectangular(int newX, int newY)
{
int i = newX;
Point point = this;
//原来的setX(newX)函数被定义为pointcut了，在aspectj是代码生成器，这里就难免不作变动咯，呵呵，setX(newX)前后的通知
//但是有点奇怪，同下面定义的setX_aroundBody1$advice方法签名不一致哦，有待研究
setX_aroundBody1$advice(this, point, i, BoundPoint.aspectOf(), point, null, ajc$tjp_0);
int j = newY;
Point point1 = this;
//setY(newY)前后的通知
setY_aroundBody3$advice(this, point1, j, BoundPoint.aspectOf(), point1, null, ajc$tjp_1);
}
public void setX(int newX)
{
x = newX;
}
public void setY(int newY)
{
y = newY;
}
public void offset(int deltaX, int deltaY)
{
setRectangular(x + deltaX, y + deltaY);
}
public String toString()
{
return (new StringBuilder("(")).append(getX()).append(", ").append(getY()).append(")").toString();
}
/** *//**
* 改变属性（x）值
* @param point
* @param point1
* @param i
*/
private static final void setX_aroundBody0(Point point, Point point1, int i)
{
point1.setX(i);
}
/** *//**
* 调用setX(newX)前后的通知,想一想，因为Point实现属性改变通知支持，那么当改变属性（用setter函数），用aop实现的，
* 那么setX(newX)的代码就会被修改。属性还是要改便的呀，所以“setX_aroundBody0(this, s1, ajc_aroundClosure);”的作用
* 就是