blocks|key|5098669|text|我想知道JVM如何区分Scala字节码和Java字节码。|type|blockquote|depth|inlineStyleRanges|entityRanges|data|5098670|没有，没有Scala字节码这样的东西。Scala编译器编译成JVM字节码。就像Java编译器也编译成JVM字节码一样。|unstyled|5098671|JVM对Scala一无所知。它也不了解Java。它也不知道任何关于Groovy、Clojure、Kotlin、锡兰、扇汤姆、Ruby、Python、ECMAScript或在JVM上有实现的任何其他400种编程语言的知识。|5098672|JVM只知道一种语言:+JVM字节码。|5098673|请注意，这实际上与任何其他机器并没有什么不同，不管是虚拟的还是非虚拟的。CLR只知道CIL，对C#、VB.NET或F#一无所知。Intel只知道AMD64和x86机器代码，对C、C%2B%2B、Objective、Swift、Go、Java、Python一无所知。CPython+VM只知道CPython字节码，对CPython一无所知。|5098674|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$]]|$1|E|3|F|5|D|7|O|8|@]|9|@]|A|$]]|$1|G|3|H|5|D|7|P|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Q|8|@]|9|@]|A|$]]|$1|K|3|-4|5|D|7|R|8|@]|9|@]|A|$]]]|L|$]]

<blockquote>
 I am wondering How JVM distinguish between Scala bytecode and Java bytecode.
</blockquote>

It doesn't. There is no such thing as Scala bytecode. The Scala compiler compiles to JVM bytecode. Just like the Java compiler also compiles to JVM bytecode.

The JVM doesn't know anything about Scala. It doesn't know anything about Java, either. Nor does it know anything about Groovy, Clojure, Kotlin, Ceylon, Fantom, Ruby, Python, ECMAScript, or any of the other ~400 programming languages for which there are implementations on the JVM.

The JVM only knows about one language: JVM bytecode.

Note that this is really no different from any other machine, virtual or not. The CLR only knows about CIL, it knows nothing about C#, VB.NET, or F#. An Intel Core CPU knows only about AMD64 and x86 machine code, it knows nothing about C, C++, Objective-C, Swift, Go, Java, Python. The CPython VM only knows about CPython bytecode, it knows nothing about Python.

blocks|key|1435879|text|Scala编译成与Java相同的字节码。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1435880|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

It doesn't. Scala compiles to the same bytecode as Java.

blocks|key|5098707|text|一幅画胜过千言万语|type|unstyled|depth|inlineStyleRanges|entityRanges|data|5098708|​|5098709|📷|atomic|offset|length|5098710|5098711|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/fa97856447e03fd31ca15a29e4821780.png|imageAlt^0|0|0|0|1|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@$G|V|H|W|1|X]]|A|$]]|$1|I|3|C|5|6|7|Y|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|Q|R|-4]]]]

A picture is worth a thousand words

<a href="https://i.stack.imgur.com/XW1Gv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/XW1Gv.png" alt="Scala Runtime"></a>

blocks|key|400521|text|scalac和javac都生成字节码。JVM并不关心字节码是如何产生的，对于JVM来说是一样的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|400522|但是，scala和java设置启动CLASSPATH的方式不同，所以如果您的代码包含Scala调用，并且很可能会，那么它需要由scala而不是java运行。|400523|如果绝对需要的话，您可以使用java手动设置启动CLASSPATH，但是当scala将为您完成这些额外的工作时，为什么还要进行这些额外的工作呢？|400524|entityMap^0|0|6|7|5|0|3|5|9|4|1R|5|1Z|4|0|E|4|11|5|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|Z|8|@$9|10|A|11|B|C]|$9|12|A|13|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|14|8|@]|D|@]|E|$]]]|K|$]]

Both <code>scalac</code> and <code>javac</code> generate bytecode. The JVM doesn't care how the bytecode was produced, it's all the same to the JVM.

However, <code>scala</code> and <code>java</code> sets up the boot CLASSPATH differently, so if your code contains Scala Runtime Library calls, and it very likely will, it needs to be run by <code>scala</code>, not <code>java</code>.

You can setup the boot CLASSPATH manually using <code>java</code>, if you absolutely have to, but why go through all that extra work, when <code>scala</code> will do it for you?

blocks|key|400544|text|Scala编译成普通的Java字节码，因此JVM似乎没有什么区别。Java不具备的Scala的额外特性是通过编译时传递和运行时帮助函数的组合来实现的。如果您拆解生成的Scala类，您可能会看到大量对Scala运行时的调用，用于装箱和取消装箱参数。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|400545|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Scala compiles to normal Java bytecode, so the JVM doesn't seem any difference. The extra features of Scala that Java doens't have are implemented through a combination of compile time passes and runtime helper functions. If you disassemble generated Scala classes, you'll probably see tons of calls to the Scala runtime for stuff like boxing and unboxing arguments.

blocks|key|1435946|text|从执行器的角度来看，标准中定义的字节代码是相同的。但是，在使用调试器时，需要更多信息来匹配正确的语言和源文件。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1435947|指示生成JVM字节码的源代码的唯一方法是通过可选类属性SourceFile+(来自Java1.0.2)和SourceDebugExtension+(来自Java5.0)。https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.10和11|offset|length|style|CODE|1435948|一个附加信息将给出源LineNumberTable中各种对象的行号，这也是可选的。https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.12|1435949|您可以找到其他JVM规范的等价物。|1435950|IDE使用它们获取JVM类的上下文，但它们是可选值，调试信息取决于编译器。|1435951|因此，它可以是一个信息，知道什么是编译器，但你不能依赖它作为确定的信息。如果定义了它，那么它应该在编译器定义中。|1435952|使用scalac，您可以使用-g选项切换信息存储，与javac相同，但notc不管理TCO：|1435953|g:{none,source,line,vars,notc}++++"none"+generates+no+debugging+info,++++"source"+generates+only+the+source+file+attribute,++++"line"+generates+source+and+line+number+information,++++"vars"+generates+source,+line+number+and+local+variable+information,++++"notc"+generates+all+of+the+above+and+will+not+perform+tail+call+optimization.|1435954|默认情况下line|1435955|要获得从类文件中检索信息的ClassLoader，可以使用开放源代码库ASM：http://asm.ow2.org/|1435956|您还可以自己解析类文件或内部存储，检索类属性并不是很困难。|1435957|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.10|1|https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.12|2|http://asm.ow2.org/^0|0|R|A|1G|K|2D|22|0|0|A|F|15|22|1|0|0|0|0|Z|4|0|0|99|0|5|4|0|13|J|2|0|0^^$0|@$1|2|3|4|5|6|7|1B|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1C|8|@$D|1D|E|1E|F|G]|$D|1F|E|1G|F|G]]|9|@$D|1H|E|1I|1|1J]]|A|$]]|$1|H|3|I|5|6|7|1K|8|@$D|1L|E|1M|F|G]]|9|@$D|1N|E|1O|1|1P]]|A|$]]|$1|J|3|K|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|L|3|M|5|6|7|1R|8|@]|9|@]|A|$]]|$1|N|3|O|5|6|7|1S|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1T|8|@$D|1U|E|1V|F|G]]|9|@]|A|$]]|$1|R|3|S|5|6|7|1W|8|@$D|1X|E|1Y|F|G]]|9|@]|A|$]]|$1|T|3|U|5|6|7|1Z|8|@$D|20|E|21|F|G]]|9|@]|A|$]]|$1|V|3|W|5|6|7|22|8|@]|9|@$D|23|E|24|1|25]]|A|$]]|$1|X|3|Y|5|6|7|26|8|@]|9|@]|A|$]]|$1|Z|3|-4|5|6|7|27|8|@]|9|@]|A|$]]]|10|$11|$5|12|13|14|A|$15|16]]|17|$5|12|13|14|A|$15|18]]|19|$5|12|13|14|A|$15|1A]]]]

The byte code as defined in the standard is the same in the point of view of the executor. But when you work with a debugger, more information is needed to match the correct language and source files.

The only way to indicate what is the source code from which the JVM bytecode has been generated is through optional class attributes <code>SourceFile</code> (from Java 1.0.2) and <code>SourceDebugExtension</code> (from Java 5.0).
<a href="https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.10" rel="nofollow">https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.10</a> and 11 

An additional information will give the line numbers of the various objects in the source <code>LineNumberTable</code>, this also is optional.
<a href="https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.12" rel="nofollow">https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.12</a>

You can find the equivalent for other JVM specs.

These are used by IDEs to get the context of JVM class, but are optional values and debug information depends on the compiler.

So it can be an info to know what the compiler was, but you cannot rely on it as sure info. If it's defined, it should be in the compiler definition.

With scalac you can switch the information storage with the -g option, the same as javac except the <code>notc</code> as Java doesn't manage TCO:

<code>g:{none,source,line,vars,notc}
 "none" generates no debugging info,
 "source" generates only the source file attribute,
 "line" generates source and line number information,
 "vars" generates source, line number and local variable information,
 "notc" generates all of the above and will not perform tail call optimization.</code>

By default <code>line</code>

To get a ClassLoader to retrieve info from the class files you can use the Open Source library ASM : <a href="http://asm.ow2.org/" rel="nofollow">http://asm.ow2.org/</a> 

You can also parse the class file or internal storage by yourself, it is not very difficult retrieving class attributes.

As Scala also produces bytecode and executed by JVM. I am wondering How JVM distinguish between Scala bytecode and Java bytecode. Can anyone please explain?

<pre><code> Scalac Myprogram.scala
 java Myprogram
</code></pre>

So this statements are perfectly fine?

How JVM distinguish between Scala bytecode and Java bytecode?

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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由于Scala还生成字节码并由JVM执行。我想知道JVM如何区分Scala字节码和Java字节码。有人能解释一下吗？ Scalac Myprogram.scala java Myprogram所以这些陈述都很好吗？

问JVM如何区分Scala字节码和Java字节码？
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回答 6

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