Java8 Lambda表达式性能问题

不念不忘少年蓝@ 2022-06-08 02:06 241阅读 0赞

# Java8 Lambda表达式和流操作如何让你的代码变慢5倍 #

有许许多多关于 Java 8 中流效率的讨论，但根据 Alex Zhitnitsky 的测试结果显示：坚持使用传统的 Java 编程风格——iterator 和 for-each 循环——比 Java 8 的实现性能更佳。

Java 8 中的 Lambda 表达式和流（Stream）受到了热烈欢迎。这是 Java 迄今为止最令人激动的特征。这些新的语言特征允许采用函数式风格来进行编码，我们可以用这些特性完成许多有趣的功能。这些特性如此有趣以至于被认为是不合理的。我们对此表示怀疑，于是决定对这些特性进行测试。

我们创建一个简单的任务：从一个 ArrayList 找出最大值，将传统方式与 Java 8 中的新方式进行测试比较。说实话，测试的结果让我感到非常惊讶。

### 命令式风格与 Java 8 函数式编程风格比较 ###

我喜欢直接进入主题，所以先看一下结果。为了做这次基准测试，我们先创建了一个 ArrayList，并插入一个 100000 个随机整数，并通过 7 种不同的方式遍历所有的值来查找最大值。实现分为两组：Java 8 中引入的函数式风格与 Java 一直使用的命令式风格。

这是每个方法耗费的时长：

![Center][]  
最大错误记录是并行流上的 0.042，完整输出结果在这篇文章结尾部分可以看到。

### 小贴士： ###

哇哦！Java 8 中提供的任何一种新方式都会产生约 5 倍的性能差异。有时使用简单迭代器循环比混合 lambda 表达式和流更有效，即便这样需要多写几行代码，且需要跳过甜蜜的语法糖（syntactic suger）。

使用迭代器或 for-each 循环是遍历 ArrayList 最有效的方式，性能比采用索引值的传统 for 循环方式好两倍。

在 Java 8 的方法中，并行流的性能最佳。但是请小心，在某些情况下它也可能会导致程序运行得更慢。

Lambda 表达式的速度介于流与并行流之间。这个结果确实挺令人惊讶的，因为 lambda 表达式的实现方式是基于流的 API 来实现的。

不是所有的情况都如上所示：当我们想演示在 lambda 表达式和流中很容易犯错时，我们收到了很多社区的反馈，要求我们优化基准测试代码，如消除整数的自动装包和解包操作。第二次测试（已优化）的结果在这篇文章结束位置可以看到。

让我们快速看一下每个方法，按照运行速度由快到慢：

### 命令式风格 ###

iteratorMaxInteger()——使用迭代器遍历列表：

forEachLoopMaxInteger()——不使用迭代器，使用 For-Each 循环遍历列表（不要误用 Java 8 的 forEach）

forMaxInteger()——使用简单的 for 循环和索引遍历列表：

### 函数式风格 ###

parallelStreamMaxInteger()——使用 Java 8 并行流遍历列表：

lambdaMaxInteger()——使用 lambda 表达式及流遍历列表。优雅的一行代码：

forEachLambdaMaxInteger()——这个用例有点混乱。可能是因为 Java 8 的 forEach 特性有一个很烦人的东西：只能使用 final 变量，所以我们创建一个 final 包装类来解决该问题，这样我们就能访问到更新后的最大值。

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      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important"></code> 
     </div> 
     <pre><code>publicintforEachLambdaMaxInteger() {
    finalWrapper wrapper = newWrapper();
    wrapper.inner = Integer.MIN_VALUE;
 
    integers.forEach(i -&gt; helper(i, wrapper));
    returnwrapper.inner.intValue();
}
</code></pre> 
     <br> 
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     </div> 
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      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important"></code> 
     </div> 
     <pre><code>publicstaticclass Wrapper {
    publicInteger inner;
}
 
privateinthelper(inti, Wrapper wrapper) {
    wrapper.inner = Math.max(i, wrapper.inner);
    returnwrapper.inner;
}</code></pre> 
     <br> 
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      <code style="border:0px!important; margin:0px!important; padding:0px!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"></code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

顺便提一下，如果要讨论 forEach，我们提供了一些有趣的关于它的缺点的见解，答案参见 [StackOverflow][]。

streamMaxInteger()——使用 Java 8 的流遍历列表：

### 优化后的基准测试 ###

根据这篇文章的反馈，我们创建另一个版本的基准测试。源代码的不同之处可以在[这里][Link 1]查看。下面是测试结果：

![Center 1][]

修改总结：

列表不再用 volatile 修饰。  
新方法 forMax2 删除对成员变量的访问。  
删除 forEachLambda 中的冗余 helper 函数。现在 lambda 表达式作为一个值赋给变量。可读性有所降低，但是速度更快。  
消除自动装箱。如果你在 Eclipse 中打开项目的自动装箱警告，旧的代码会有 15 处警告。  
优化流代码，在 reduce 前先使用 mapToInt。  
非常感谢 Patrick Reinhart, Richard Warburton, Yan Bonnel, Sergey Kuksenko, Jeff Maxwell, Henrik Gustafsson 以及每个 Twitter 上评论的人，感谢你们的贡献。

### 测试基础 ###

我们使用 JMH(Java Microbenchmarking Harness) 执行基准测试。如果想知道怎么将其应用在你自己的项目中，可以参考[这篇文章][Link 2]，我们通过一个自己写的实例来演示 JMH 的主要特性。

基础测试的配置包含 2 个JVM、5 次预热迭代和 5 次测量迭代。该测试运行在 c3.xlarge Amazon EC2 实例上（CPU：4 核，内存：7.5G，存储：2 x 40 GB SSD），采用 Java 8u66 和 JMH 1.11.2。所有的源代码都在 [GitHub][] 上，你可以在[这里][Link 3]看到原始的输出结果。

顺便做一下免责申明：基准测试往往不是完全可信的，也很难保证绝对正确。虽然我们试图以最准确的方式来运行，但仍然建议接受结果时抱有怀疑的态度。

### 最后的思考 ###

开始使用 Java 8 的第一件事情是在实践中使用 lambda 表达式和流。但是请记住：它确实非常好，好到可能会让你上瘾！但是，我们也看到了，使用传统迭代器和 for-each 循环的 Java 编程风格比 Java 8 中的新方式性能高很多。

当然，这也不是绝对的。但这确实是一个相当常见的例子，它显示可能会有大约 5 倍的性能差距。如果这影响到系统的核心功能或成为系统一个新的瓶颈，那就相当可怕了。

原文链接：  [jaxenter][] 翻译：  [ImportNew.com ][ImportNew.com]\-  [paddx][]  
译文链接：  [http://www.importnew.com/17262.html][http_www.importnew.com_17262.html]  
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### 关于作者： [paddx][] ###

[Center]: /images/20220608/94a9d4d1bf31449bbd4e6564e8140fe1.png
[StackOverflow]: http://stackoverflow.com/questions/16635398/java-8-iterable-foreach-vs-foreach-loop/20177092#20177092
[Link 1]: https://github.com/takipi/loops-jmh-playground/commit/d91c55aee2d480c31fa7fad72f6f0b94fb59612e
[Center 1]: /images/20220608/7ff342fff8f3480a91e066f30afcff90.png
[Link 2]: https://jaxenter.com/a-hands-on-session-with-the-java-microbenchmarking-harness-122104.html
[GitHub]: https://github.com/takipi/loops-jmh-playground/blob/master/src/main/java/com/takipi/oss/benchmarks/jmh/loops/LoopBenchmarkMain.java
[Link 3]: http://pastebin.com/fB1jirqU
[jaxenter]: https://jaxenter.com/how-java-8-lambdas-and-streams-can-make-your-code-5-times-slower-122603.html
[ImportNew.com]: http://www.importnew.com/
[paddx]: http://www.importnew.com/author/paddx
[http_www.importnew.com_17262.html]: http://www.importnew.com/17262.html