Scala中的语言特性是如何实现的(3) -- Trait

我的新博客地址：http://cuipengfei.me/blog/2013/10/13/scala-trait/

 

我在Coursera上跟了一门叫做Functional Programming Principles in Scala的课程，是由Scala的作者Martin Odersky讲授的。其中第三周的作业中使用到了Scala的trait这个语言特性。

我以前熟知的语言都没有类似的特性（Ruby的mixin和Scala的trait很像，但是Ruby我不熟），所以这周的博客就分析一下这个语言特性是如何实现的。

trait

在讲trait的实现机制之前，先看一个使用trait的例子。 假设我们有以下几个类：

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abstract class Plant { def photosynthesis = println("Oh, the sunlight!") } class Rose extends Plant { def smell = println("Good!") def makePeopleHappy = println("People like me") } class Ruderal extends Plant { def grow = println("I take up all the space!") } abstract class Animal { def move = println("I can move!") } class Dog extends Animal { def bark = println("Woof!") def makePeopleHappy = println("People like me") } class Snake extends Animal { def bite = println("I am poisonous!") }

植物家族有玫瑰和杂草。

动物家族有狗和毒蛇。

仔细观察可以发现，玫瑰和狗有一个共同的行为，它们都可以取悦人类，这个行为是用完全一样的代码实现的。

如何把Rose和Dog中的重复代码消除掉呢？有一种潜在的解决方案： 把makePeopleHappy提取到一个类中去，让植物和动物都继承自它。

这么做虽然消除了重复代码但有两个明显的缺点：

1. 植物和动物继承自同一个类，不太合理
2. 杂草和毒蛇也具有了取悦于人的能力，也不太合理

这时我们就可以使用trait，它没有上面提到的两个缺点。

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trait PeoplePleaser { def makePeopleHappy = println("People like me") } class Rose extends Plant with PeoplePleaser { def smell = println("Good!") } class Dog extends Animal with PeoplePleaser { def bark = println("Woof!") }

我们定义一个trait，把makePeopleHappy置于其中，让Rose和Dog都with这个trait。然后就可以写这样的代码来调用它们了：

1 2	new Rose().makePeopleHappy new Dog().makePeopleHappy

这样我们就解决了重复代码的问题，而且没有触及已存在的继承关系。

现在看看trait的实现机制吧，我们开始反编译！

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public abstract interface PeoplePleaser { public abstract void makePeopleHappy(); } public abstract class PeoplePleaser$class { public static void makePeopleHappy(PeoplePleaser $this) { Predef..MODULE$.println("People like me"); } public static void $init$(PeoplePleaser $this) { } } public class Rose extends Plant implements PeoplePleaser { public void makePeopleHappy() { PeoplePleaser$class.makePeopleHappy(this); } public void smell() { Predef..MODULE$.println("Good!"); } public Rose() { PeoplePleaser.class.$init$(this); } } public class Dog extends Animal implements PeoplePleaser { public void makePeopleHappy() { PeoplePleaser$class.makePeopleHappy(this); } public void bark() { Predef..MODULE$.println("Woof!"); } public Dog() { PeoplePleaser.class.$init$(this); } }

真相大白了，PeoplePleaser被编译成了一个接口加一个抽象类。Rose和Dog实现这个接口，并通过调用抽象类中的静态方法来实现了makePeopleHappy。

很有趣的一点是Rose和Dog在调用静态方法时都把this传了进去，为什么呢？我们把原来的代码改成这样来看：

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trait PeoplePleaser { val moreMessage = "" def makePeopleHappy = println("People like me. " + moreMessage) } class Rose extends Plant with PeoplePleaser { override val moreMessage = "Because I smell nice." def smell = println("Good!") } class Dog extends Animal with PeoplePleaser { override val moreMessage = "Because I fetch balls." def bark = println("Woof!") }

我们给makePeopleHappy加上一段额外的信息。 现在再次反编译。

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public abstract interface PeoplePleaser { public abstract void objsets$PeoplePleaser$_setter_$moreMessage_$eq(String paramString); public abstract String moreMessage(); public abstract void makePeopleHappy(); } public abstract class PeoplePleaser$class { public static void makePeopleHappy(PeoplePleaser $this) { Predef..MODULE$.println(new StringBuilder() .append("People like me. ") .append($this.moreMessage()).toString()); } public static void $init$(PeoplePleaser $this) { $this.objsets$PeoplePleaser$_setter_$moreMessage_$eq(""); } } public class Rose extends Plant implements PeoplePleaser { private final String moreMessage; public void objsets$PeoplePleaser$_setter_$moreMessage_$eq(String x$1) { } public void makePeopleHappy() { PeoplePleaser$class.makePeopleHappy(this); } public String moreMessage() { return this.moreMessage; } public void smell() { Predef..MODULE$.println("Good!"); } public Rose() { PeoplePleaser.class.$init$(this); this.moreMessage = "Because I smell nice."; } } public class Dog extends Animal implements PeoplePleaser { private final String moreMessage; public void objsets$PeoplePleaser$_setter_$moreMessage_$eq(String x$1) { } public void makePeopleHappy() { PeoplePleaser$class.makePeopleHappy(this); } public String moreMessage() { return this.moreMessage; } public void bark() { Predef..MODULE$.println("Woof!"); } public Dog() { PeoplePleaser.class.$init$(this); this.moreMessage = "Because I fetch balls."; } }

现在就清楚了，抽象类中的静态方法可能会依赖于各个实例不同的状态，所以需要把this传递进去。 这样我们才能够给makePeopleHappy加上一段额外的信息。