Java元编程

Question

我正在做我的第一个真正的Java项目。我开始适应这种语言，尽管我对动态语言有更多的经验。

我有一个类，它的行为类似于以下代码：

class Single
{
    public void doActionA() {}
    public void doActionB() {}
    public void doActionC() {}
}

然后我有一个SingleList类，它充当这些类的集合(具体地说，它用于2D Sprite库，“动作”是所有类型的转换:旋转、剪切、缩放等)。我希望能够做到以下几点：

class SingleList
{
    public void doActionA() {
        for (Single s : _innerList) {
            s.doActionA();
        }
    }

    ... etc ...
}

有没有办法简单地将一个方法(或已知的方法列表)推迟到内部列表的每个成员？有没有办法不专门列出每个方法，然后遍历每个内部成员并手动应用它？

让事情变得更难的是，这些方法的大小各不相同，但都是返回类型"void“。

Answer 1

不幸的是，Java不支持在运行时创建类，而这正是您需要的:为了匹配Single类，需要使用必要的存根方法自动更新SingleList。

我可以想到以下方法来解决这个问题：

使用Java reflection的

- Pros:   
    - It's readily available in the Java language and you can easily find documentation and examples.

​

- Cons:  
    - The `SingleList` class would not be compatible with the `Single` class interface any more.
    - The Java compiler and any IDEs are typically unable to help with methods called via reflection - errors that would be caught by the compiler are typically transformed into runtime exceptions.
    - Depending of your use case, you might also see a noticeable performance degradation.

​

​

1. 使用构建系统和某种源代码生成器自动创建SingleList.java文件。

- Pros:   
    - Once you set it up you will not have to deal with it any more.

​

- Cons:  
    - Setting this up has a degree of difficulty.
    - You would have to separately ensure that the `SingleList` class loaded in any JVM - or your IDE, for that matter - actually matches the loaded `Single` class.

​

​

1. 手动解决了这个问题-创建一个接口(例如SingleInterface)或一个基础抽象类以供这两个类使用应该会有所帮助，因为任何像样的集成开发环境都会指出未实现的方法。正确的类体系结构将最大限度地减少重复代码，并且您的集成开发环境也许能够帮助生成样板部分。

- Pros:  
    - There is no setup curve to get over.
    - Your IDE will always see the right set of classes.
    - The class architecture is usually improved afterwards.

​

- Cons:  
    - Everything is manual.

​

​

1. 使用字节码生成库(如Javassist或BCEL )动态生成/修改SingleList类。

- Pros:   
    - This method is extremely powerful and can save a lot of time in the long term.

​

- Cons:  
    - Using bytecode generation libraries is typically _not_ trivial and _not_ for the faint-hearted.
    - Depending on how you write your code, you may also have issues with your IDE and its handling of the dynamic classes.

​

​