Spring Method Replacement

public abstract class AbstractRecorder implements Recorder {
  public abstract void record();;
  public abstract void playback();;
}

The actual implementations of record() and playback() are left abstract (<replaced-method> works just fine with abstract methods). This leaves it up to me to create implementations of these methods to define the mechanisms by which recording and playback are performed.

For example, I could implement record() as recording to tape, CD, or MP3. At the same time, I could implement playback() to play to a speaker, over the radio airwaves, or maybe across a phone line.

If I were to create subclasses to cover all of the possible combinations of record() and playback() implementations mentioned above, I'd end up with at least 9 subclasses of AbstractRecorder (3 recording implementations times 3 playback implementations). If I were to add another recording implementation I'd end up creating 3 more subclasses; one for each of the playback implementations.

But if I were to use Spring's <replaced-method> I'd only need to create one replacer class for each recording implementation and one replacer class for each playback implementation. Then I'd piece together the chosen record-playback combination in Spring's configuration file when I declare the "recorder" bean:

<beans>  
  <bean id="recorder" class="com.habuma.wiring.AbstractRecorder">
    <replaced-method name="record" replacer="recordDevice"/>
    <replaced-method name="playback" replacer="playbackDevice"/>
  </bean>
      
  <bean id="recordDevice"
      class="com.habuma.wiring.TapeRecorder"/>
  <bean id="playbackDevice"
      class="com.habuma.wiring.SpeakerPlayback"/>
</beans>

As wired here, I've declared the "recorder" bean to be a simple tape recorder. It records to tape and plays back to a speaker. If I decide later that I need the playback to be over the radio instead a speaker, I just switch the "playbackDevice" bean's class to be "com.habuma.wiring.RadioPlayback". Likewise, I can swap out the "recordDevice" bean's class with "com.habuma.wiring.Mp3Recorder" to record to MP3.

第一个例子看不出什么作用，第二个却是很有趣。
这篇blog让我想起了荣耀先生网站上的一篇文章《委托和事件》

通过这种方法，我可以只实现我关心的接口，比如例子中record，而不必理会playback的存在。
实际的测试证明，如果我只用一个Replacer实现了record，只要我不去调用playback，万事OK。

例子完全可以再改进一下，用接口，连抽象类都省略，这样看得更清楚。

这是又是一种利用Cglib动态生成类的做法，对于动态生成技术的运用倒是一天天多了起来。

不过，正如femto所说，这种东西不太容易理解，很容易造成误用。
还有，使用这种方法，就会将代码绑定在Spring上，看看Replacer实现的接口就知道了。而且从代码上很难看出这个Replacer同Recorder的关联，这样也会增加理解的复杂程度。
看来随着Spring运用的日益广泛，使用得越是高级，被绑定的可能性越大，那样的话，Spring宣扬的不依赖API就成了鬼话。

public class RecordOnTape implements UnaryFunction {
  public Object evaluate(Object arg0); {
    String content = (String); arg0;
    return new Tape(content);;
  }
}

public Object reimplement(Object target, Method method, Object[] args); {
  return new RecordOnTape();.evaluate(args[0]);;
}

http://jroller.com/comments/habuma/Weblog/spring_method_replacement



这个东西看起来很好玩，但是我一时还没想到有什么用。大家来探讨一下吧。