如何用Java 11生成代码，但要针对Java 8及更高版本？ [英] How to produce code in Java 11, but target Java 8 and above?
问题描述
我正在开发一个小型库，出于明显的原因，我想使用Java 11的所有功能（我现在猜想的模块除外）来生成代码，但是我希望该库与Java 8及更高版本兼容。
I am working on a small library and for obvious reasons I would like to produce code using all the Java 11 features (except modules I guess for now), but I would like the library to be compatible with Java 8 and above.
当我尝试以下操作时：
javac -source 11 -target 1.8 App.java
我收到以下消息：
warning: source release 11 requires target release 11
...当我查看字节码时，我看到该类的版本为 0x37 （Java 11）：
...and when I look at the byte code I see that the version of the class is 0x37 (Java 11):
$ xxd App.class
00000000: cafe babe 0000 0037 ...
Java 8无法加载它：
And Java 8 cannot load it:
Exception in thread "main" java.lang.UnsupportedClassVersionError: App has been
compiled by a more recent version of the Java Runtime (class file version 55.0),
this version of the Java Runtime only recognizes class file versions up to 52.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)
at java.net.URLClassLoader.access$100(URLClassLoader.java:74)
at java.net.URLClassLoader$1.run(URLClassLoader.java:369)
at java.net.URLClassLoader$1.run(URLClassLoader.java:363)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:362)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)
如何操作le提供这样的兼容性吗？我对所有构建工具都开放。
How do people provide such compatibility? I am open to all build tools.
对我来说，将高级语言（Java）转换为低级语言（字节码）似乎很容易。在我看来，当高级语言更改时，低级语言应保持不变。这就是为什么我认为有可能的原因。
For me it seems easy just to transform high-level language (Java) into low-level (bytecode). It appears to me that when the high-level language changes, the low-level should stay the same. That is why I thought it was possible.
更新
我是不要以为这个答案重复了移至OpenJDK-11却可以在Java 8中编译，因为在那里OP询问如何继续使用 Java 8 功能生成代码，但以Java 11为目标（这是著名的向后兼容性）。我的问题是相反的：我想用Java 11生成代码，但要针对Java8。我在提出问题之前研究该主题时遇到了这个问题。我发现它不适用于我的情况。
Guys, I don't think that this answer duplicates Move to OpenJDK-11 but compile in Java 8, because there the OP asks how to keep producing the code with Java 8 features, but target Java 11 (which is just a famous backward compatibility). My question is the other way around: I want to produce the code in Java 11, but target Java 8. I came across that question when I was researching the topic before posing the question. I didn't find it applicable to my situation.
另一个问题可以Java 8代码被编译为可以在Java 7 JVM上运行的确与我的问题相似，但是在2013年被问到，字节码显然在Java 7和Java 8之间发生了变化。
The other question Can Java 8 code be compiled to run on Java 7 JVM does look similar to my question, but it was asked in 2013 and the bytecode obviously changed between Java 7 and Java 8.
自Java 8以来，我认为字节码变化不大，这就是为什么我问这个问题。
I didn't think the bytecode changed that much since Java 8 that is why I asked this question.
推荐答案
尽管从理论上讲，使用复杂的工具可以将为JDK 11编译的类转换为JDK 8，但这并非微不足道。二进制级别有重大变化。
While conversion of classes compiled for JDK 11 to JDK 8 would be theoretically possible with a sophisticated tool, it’s not trivial. There are significant changes on the binary level.
首先，JDK 11引入了嵌套类型，这样就无需在访问 private 时生成综合访问器方法内部/外部类的成员。当然，这种访问在旧版本中将失败。
First, JDK 11 introduced nest types, which eliminates the need to generate synthetic accessor methods when accessing private members of inner/outer classes. Of course, such access would fail in older versions.
它还引入了动态常量，尽管我不知道Java语言是否在任何地方都利用了该功能。
It also introduced dynamic constants, though I don’t know whether the Java language exploits that feature anywhere. This is mainly intended for future versions.
然后，由于JDK 9，使用 invokedynamic 引用 java Java 8中不存在的.lang.invoke.StringConcatFactory 。
Then, since JDK 9, string concatenation gets compiled using invokedynamic referring to java.lang.invoke.StringConcatFactory which is not present in Java 8.
一个可行的功能是 private 接口中的方法，作为语言功能在Java 9中引入，但已经在Java 8中以二进制级别进行了处理。
A feature that could work, is private methods in interfaces, introduced in Java 9 as a language feature, but already handled on the binary level in Java 8.
Java 8也将无法处理模块定义，但我想，它们将被忽略。
Java 8 would also be unable to process module definitions, but I suppose, they would be ignored.
这篇关于如何用Java 11生成代码，但要针对Java 8及更高版本？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
