AspectJ中的类型间声明(成员注入)

在上一篇博客初窥AspectJ中,我们提到AspectJ给java提供了三种新的结构,pointcut,advice以及inter-type declaration(ITD),而且我们通过一个简单的Demo介绍了如何使用pointcut和advice。而本文将介绍inter-type declaration是什么,可以做什么,最后同样会通过一个Demo来介绍如何使用。后文将主要用ITD来表示inter-type declaration。

本文中Demo的代码可以在github aspect-demo中找到。

ITD与成员注入

inter-type declaration (ITD),翻译成中文是类型间声明。即使看到中文翻译,相信大家还是一头雾水,不知所云,所以我不是很喜欢对一些英文名字,尤其是技术名字进行生硬的翻译,这只会增加大家的理解负担。其实,换一种说法可能更好理解,member introduction(成员注入),其目的就是通过aspect的方式,在现有的类中注入一些新的成员变量或者成员方法。通过aspect,我们可以向一个类中注入如下成员:

  • 成员变量(final或者非final)
  • 方法
  • 构造函数

除了往类里面添加内容,aspect还可以修改java中的interface(接口),实现在现有接口中注入:

  • 方法的默认实现
  • 非final的域

通过ITD注入的成员的访问修饰符可以是:

  • private: 通过private声明的私有成员属于目标类,但是呢,只对aspect脚本可见,而对目标类不可见;
  • public: 声明为public的成员对所有类和apsect都可见;
  • default package protected:这里的包可见性是相对于aspect所在的包,而不是相对于目标类所在的包。

inter-type declaration示例

在编写aspect之前,先准备一个简单的java类:

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package cc.databus.aspect.intertype;

public class Point {
private int x;
private int y;

public Point(int x, int y) {
this.x = x;
this.y = y;
}

public int getX() {
return x;
}

public void setX(int x) {
this.x = x;
}

public int getY() {
return y;
}

public void setY(int y) {
this.y = y;
}
}

有了这个基础类,下面来看看如何通过aspect修改这个类实现的接口,成员变量以及成员方法。这里是我们的aspect代码:

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package cc.databus.aspect.intertype;

public aspect PointAspect {
// creates a new interface named HasName
private interface HasName{}
// make class Ppint implements HashName
declare parents: Point implements HasName;
// make HasName has a field named name
private String HasName.name;
// make HasName has a method getName() and default implemented
public String HasName.getName() {
return name;
}

// make HasName has a method named setName and default
public void HasName.setName(String name) {
this.name = name;
}

// add a field named created to class Point
// with default value 0
long Point.created = 0;


// add a field named lastUpdated to class Point
// with default value 0
private long Point.lastUpdated = 0;


// add a private method setUpdated()
private void Point.setUpdated() {
this.lastUpdated = System.currentTimeMillis();
}

// implement toString() for Point
// include the fields added in the aspect file
public String Point.toString() {
return String.format(
"Point: {name=%s, x=%d; y=%d, created=%d, updated=%d}",
getName(), getX(), getY(), created, lastUpdated);
}

// pointcut the constructor, and set the value for created
after() returning(Point p) : call(Point.new(..)) && !within(PointAspect) {
System.out.println(thisJoinPointStaticPart);
System.out.println("Set created");
p.created = System.currentTimeMillis();
}

// define a pointcut for setX and setY
pointcut update(Point p): target(p) && call(void Point.set*(..));

// make the lastUpdated updated every time
// setX or setY invoked
after(Point p): update(p) && !within(PointAspect) {
System.out.println("set updated for Point due to " + thisJoinPointStaticPart);
p.setUpdated();
}
}

在上面的aspect文件中,我们首先定义了一个接口,并且让Point类实现该接口,且给该新接口加了一个成员变量(name)并实现了对应的setter/getter:

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// creates a new interface named HasName
private interface HasName{}
// make class Ppint implements HashName
declare parents: Point implements HasName;
// make HasName has a field named name
private String HasName.name;
// make HasName has a method getName() and default implemented
public String HasName.getName() {
return name;
}

// make HasName has a method named setName and default
public void HasName.setName(String name) {
this.name = name;
}

随后,我们给Point类加了两个成员变量,并实现了两个成员方法。其中,实现toString()接口的时候,我们把通过aspect注入的成员变量也都包含在结果里面:

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// add a field named created to class Point
// with default value 0
long Point.created = 0;

// add a field named lastUpdated to class Point
// with default value 0
private long Point.lastUpdated = 0;

// add a private method setUpdated()
private void Point.updated() {
this.lastUpdated = System.currentTimeMillis();
}

// implement toString() for Point
// include the fields added in the aspect file
public String Point.toString() {
return String.format(
"Point: {name=%s, x=%d; y=%d, created=%d, updated=%d}",
getName(), getX(), getY(), created, lastUpdated);
}

最后,我们加了两个pointcut一级advice,分别实现在调用Point构造函数之后为created的赋值,以及调用setX(int), set(int)以及setName(string)的时候更新lastUpdated成员变量(这里使用!within(PointAspect)排除掉在aspect脚本里面调用set*的情况):

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// pointcut the constructor, and set the value for created
after() returning(Point p) : call(Point.new(..)) && !within(PointAspect) {
System.out.println(thisJoinPointStaticPart);
System.out.println("Set created");
p.created = System.currentTimeMillis();
}

// define a pointcut for setX and setY
pointcut update(Point p): target(p) && call(void Point.set*(..));

// make the lastUpdated updated every time
// setX or setY invoked
after(Point p): update(p) && !within(PointAspect) {
System.out.println("set updated for Point due to " + thisJoinPointStaticPart);
p.setUpdated();
}

同样,我们可以新建一个单元测试类来进行测试:

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package cc.databus.aspect.intertype;

import org.junit.Test;

public class TestPointAspect {

@Test
public void test() {
Point point = new Point(1,1);
point.setName("test");
point.setX(12);
point.setY(123);
System.out.println(point);
}
}

运行测试,我们能看到如下结果:

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call(cc.databus.aspect.intertype.Point(int, int))
Set created
set updated for Point due to call(void cc.databus.aspect.intertype.Point.setName(String))
set updated for Point due to call(void cc.databus.aspect.intertype.Point.setX(int))
set updated for Point due to call(void cc.databus.aspect.intertype.Point.setY(int))
Point: {name=test, x=12; y=123, created=1536153649547, updated=1536153649548}

可以看到,通过aspect注入的成员对象和成员方法都是工作的。

总结

ITD着实是一个强大的功能,能够方便给现有类注入新的功能。但是,笔者认为使用这种方法相对容易出错,尤其在大项目的情况下,如果通过大量的aspect脚本来实现功能,相信对后期的维护是一个很大的挑战。所以,我建议在没有spring这种框架做支撑的情况下,不要大量的使用这种方法为项目造血。

Reference

  1. Advanced AspectJ Part II : Inter-type declaration
  2. Inter-type declarations
git恢复本地误删除的分支 初窥AspectJ

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