问题
I'm reading JEP 317. It says that Graal (a new experimental Java-based JIT compiler) will be part of JDK 10, but then it says that is already available in JDK 9. So, what's the point of JEP 317 then? Does Java 9 include Graal or not?
回答1:
From one of my memos, on linux-x64 (out-of-the-box) to use Graal on JDK 9, you can enable it using :
-XX:+UnlockExperimentalVMOptions -XX:+EnableJVMCI -XX:+UseJVMCICompiler
Source: Tweet from Chris.
If you're explicitly willing to make use of the org.graalvm.* classes, they are not present in the JDK-9 build except for the Linux distribution here and the JEP-317#Experimental Java-Based JIT Compiler's status also reads something similar.
Status Integrated Scope JDK Release 10
Update:- To use Graal on JDK10, one can enable it using:-
-XX:+UnlockExperimentalVMOptions -XX:+EnableJVMCI -XX:+UseJVMCICompiler -Djvmci.Compiler=graal
回答2:
Graal is included in 9 but it used in a different way.
In Java 9 Graal is used as an AOT(Ahead of time) compiler which allows users to use the jaotc tool to create manually compiled .so libraries.
These libraries can then be loaded at JVM startup and directly used.
A detailed description can be found at the related Java 9 JEP: http://openjdk.java.net/jeps/295
In Java 10 Graal, as described in JEP 317, can be used as the JIT compiler instead of HotSpot.
This means that classes will be compiled in the JVM "on the fly" instead of requiring manual compilation beforehand.
来源：https://stackoverflow.com/questions/48252830/does-java-9-include-graal