The Java compilers that you worked with until now were usually written in C/C++. What would you think about a Java compiler written in Java?

Graal, a Java-based just in-time (JIT) compiler, was introduced in Java 9. Java 10 enables the use of Graal as an experimental JIT compiler on Linux/x64 platforms. Eventually, Oracle will explore the possibility of using Graal as a Java-based JIT for the JDK.

Graal uses the JVM compiler interface, introduced in JDK 9. The objective of Graal is not to compete with the existing JIT compilers. It is a part of the project Metropolis, which explores and incubates Java-on-Java implementation techniques for HotSpot, the open JDK implementation of the JVM.

Since Graal is written in Java, fears of using Graal are related to lower startup performance of applications and increased heap usage.

The following command-line compiler option can be used to enable Graal as your JIT compiler:

-XX:+UnlockExperimentalVMOptions -XX:+UseJVMCICompiler