Java 8中Lambda表达式的阴暗面

blog_lambada_2

这篇文章可能不会使我成为任何新朋友。哦，好吧，无论如何我从来没有真正在学校受到欢迎。但是，让我们说清楚。就语言而言，Java 8的最大特色无疑是Lambda表达式。几年来，它一直是功能语言（例如Scala和Clojure）的旗舰功能，现在Java终于加入了。

第二大功能（当然取决于您问谁）是Nashorn –新的JVM JavaScript引擎，可以使Java与其他JS引擎（例如V8和它的node.js容器）相提并论。

但是这些新功能对他们不利。

我会解释。 Java平台由两个主要组件构建而成。 JIT编译并执行字节码的JRE，JDK包含开发工具和javac源代码编译器。这两个组件是完全（但不是完全）分离的，这使人们能够编写自己的JVM语言，而Scala在最近几年中日益受到关注。其中存在一些问题。

JVM被构建为与语言无关，只要它可以转换为字节码，就可以执行以任何语言编写的代码。字节码规范本身是完全面向对象的，旨在与Java语言紧密匹配。这意味着从Java源代码编译的字节码在结构上将非常类似于它。

但是与Java的距离越远，距离就越远。当您查看Scala是一种功能语言时，源代码与执行的字节码之间的距离就很大。编译器添加了大量合成类，方法和变量，以使JVM执行该语言所需的语义和流控制。

当您查看完全动态的语言（例如JavaScript）时 ，这种距离变得很大。

而现在有了Java 8，它也开始渗透到Java中。

那我为什么要在乎呢？

我希望这是一个理论性的讨论，尽管有趣，但对我们的日常工作没有实际意义。不幸的是，它确实以很大的方式做到了。随着向Java中添加新元素的推动，代码与运行时之间的距离越来越大，这意味着您正在编写的内容和正在调试的内容将是两件事。

要了解让我们如何（重新）访问下面的示例。

Java 6和7

这是传统的方法，通过该方法我们可以遍历一串字符串以映射其长度。

// simple check against empty strings
public static int check(String s) {if (s.equals("")) {throw new IllegalArgumentException();}return s.length();
}//map names to lengthsList lengths = new ArrayList();for (String name : Arrays.asList(args)) {lengths.add(check(name));
}

如果传递一个空字符串，这将引发异常。堆栈跟踪看起来像–

at LmbdaMain.check(LmbdaMain.java:19)
at LmbdaMain.main(LmbdaMain.java:34)

在这里，我们看到了所看到的堆栈跟踪与所编写的代码之间的1：1关系，这使得调试此调用堆栈非常简单。这就是大多数Java开发人员所习惯的。

现在让我们看一下Scala和Java 8。

斯卡拉

让我们看看Scala中的相同代码。在这里，我们有两个重大变化。第一个是使用Lambda表达式映射长度，第二个是迭代是由框架执行的（即内部迭代）。

val lengths = names.map(name => check(name.length))

在这里，我们真的开始注意到您编写的代码外观与JVM（以及您）在运行时如何看到它们之间的区别。如果抛出异常，则调用堆栈会长一个数量级 ，并且很难理解。

at Main$.check(Main.scala:6)
at Main$$anonfun$1.apply(Main.scala:12)
at Main$$anonfun$1.apply(Main.scala:12)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
at scala.collection.AbstractTraversable.map(Traversable.scala:105)
at Main$delayedInit$body.apply(Main.scala:12)
at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
at scala.App$$anonfun$main$1.apply(App.scala:71)
at scala.App$$anonfun$main$1.apply(App.scala:71)
at scala.collection.immutable.List.foreach(List.scala:318)
at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:32)
at scala.App$class.main(App.scala:71)
at Main$.main(Main.scala:1)
at Main.main(Main.scala)

*请记住，此示例非常简单。使用现实世界中的嵌套Lambda和复杂结构，您将需要更长的综合调用堆栈，您需要从中了解发生了什么。

长期以来，Scala一直是一个问题，这也是我们构建Scala Stackifier的原因之一。

现在在Java 8中

直到现在，Java开发人员都对此免疫。随着Lambda表达式成为Java不可或缺的一部分，这种情况将会改变。让我们看一下相应的Java 8代码以及生成的调用堆栈。

Stream lengths = names.stream().map(name -> check(name));at LmbdaMain.check(LmbdaMain.java:19)
at LmbdaMain.lambda$0(LmbdaMain.java:37)
at LmbdaMain$$Lambda$1/821270929.apply(Unknown Source)
at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
at java.util.Spliterators$ArraySpliterator.forEachRemaining(Spliterators.java:948)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:512)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:502)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.LongPipeline.reduce(LongPipeline.java:438)
at java.util.stream.LongPipeline.sum(LongPipeline.java:396)
at java.util.stream.ReferencePipeline.count(ReferencePipeline.java:526)
at LmbdaMain.main(LmbdaMain.java:39)

这与Scala变得非常相似。我们为更短，更简洁的代码，更复杂的调试和更长的综合调用堆栈付出了代价。

原因是，尽管javac已扩展为支持Lambda函数，但JVM仍然对它们不了解 。这是Java人士的一项设计决策，目的是保持JVM在较低级别运行，而又不会在其规范中引入新的元素。

尽管您可以辩论此决定的优缺点，但这意味着作为Java开发人员，无论我们是否愿意，当我们拿到罚单时弄清楚这些调用堆栈的成本现在都落在我们肩上。

Java 8中JavaScript

Java 8引入了全新JavaScript编译器。现在，我们终于可以以高效，直接的方式集成Java + JS。但是，我们编写的代码与我们调试的代码之间的不协调之处比这里更大。

这与Nashorn中的功能相同–

ScriptEngineManager manager = new ScriptEngineManager();
ScriptEngine engine = manager.getEngineByName("nashorn");String js = "var map = Array.prototype.map \n";
js += "var a = map.call(names, function(name) { return Java.type(\"LmbdaMain\").check(name) }) \n";
js += "print(a)";
engine.eval(js);

在这种情况下，字节代码是在运行时使用Lambda表达式的嵌套树动态生成的。我们的源代码与由JVM执行的结果字节码之间几乎没有关联。现在，调用堆栈要长两个数量级 。用T先生的凄美的话语- 我可怜那些需要调试调用堆栈的傻瓜，您将到达这里。

有任何疑问吗？（假设您可以一直滚动到此调用堆栈下方）。在评论部分让我知道。

LmbdaMain [Java Application]
LmbdaMain at localhost:51287
Thread [main] (Suspended (breakpoint at line 16 in LmbdaMain))
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LmbdaMain.main(String[]) line: 44