Espresso ☕

A meta-circular Java bytecode interpreter for the GraalVM.
Espresso is a fully meta-circular implementation of The Java Virtual Machine Specification, Java SE 8 and 11 Edition, written in Java, capable of running non-trivial programs at speed.
A Java bytecode interpreter at its core, turned Just-In-Time (JIT) compiler by leveraging Truffle and the Graal compiler on the GraalVM.
It highlights the sublime potential of the GraalVM as a platform for implementing high-performance languages and runtimes.

Status

Espresso is still an early prototype, but it already passes the Java Compatibility Kit (a.k.a. the JCK or TCK for Java SE) 8c and 11 runtime suite.
Espresso can compile itself with both javac and (the Eclipse Java Compiler) ecj.
It features complete meta-circularity: it can run itself any amount of layers deep, preserving all the capabilities (Unsafe, JNI, Reflection...) of the base layer. Running HelloWorld on three nested layers of Espresso takes ~15 minutes.

Espresso is similar to HotSpot Express, the same codebase can run either an 8 or 11 guest JVM, on either an 8 or 11 host JVM.

The development of Espresso happens mostly on HotSpot, but this configuration (Espresso on HotSpot) is only supported on Linux, see Limitations .
Espresso's native image runs on Linux, MacOS and Windows.

Building Espresso

Set your (JVMCI-enabled) JDK via mx argument e.g. mx --java-home /path/to/java/home ... or via export JAVA_HOME=/path/to/java/home.

mx build-ing Espresso creates a GraalVM with Espresso included.

To build the default configuration (interpreter-only), on the espresso repository:

mx build

Other configurations are provided:

mx --env jvm build       # GraalVM CE + Espresso jars (interpreter only)
mx --env jvm-ce build    # GraalVM CE + Espresso jars (JIT)
mx --env native-ce build # GraalVM CE + Espresso native (JIT)

# Use the same --env argument used to build.
export ESPRESSO=`mx --env native-ce graalvm-home`/bin/espresso

Configuration files: mx.espresso/{jvm,jvm-ce,native-ce} and mx.espresso/native-image.properties

Running Espresso

mx espresso runs Espresso (from jars or native) from within a GraalVM. It mimics the java (8|11) command. Bare mx espresso runs Espresso on interpreter-only mode.

mx --env jvm-ce build # Always build first
mx --env jvm-ce espresso -cp my.jar HelloWorld

To build and run Espresso native image:

mx --env native-ce build # Always build first
mx --env native-ce espresso -cp my.jar HelloWorld

The mx espresso launcher adds some overhead, to execute Espresso native image directly use:

mx --env native-ce build # Always build first
export ESPRESSO=`mx --env native-ce graalvm-home`/bin/espresso
time $ESPRESSO -cp my.jar HelloWorld

mx espresso-standalone ...

To run Espresso on a vanilla JDK (8|11) and/or not within a GraalVM use mx espresso-standalone ..., it mimics the java (8|11) command. The launcher adds all jars and properties required to run Espresso on any vanilla JDK (8|11).

To debug Espresso:

mx build
mx -d espresso-standalone -cp mxbuild/dists/jdk1.8/espresso-playground.jar com.oracle.truffle.espresso.playground.HelloWorld

It can also run on a GraalVM with JIT compilation:

mx build
mx --dy /compiler espresso-standalone -cp my.jar HelloWorld

Dumping IGV graphs

mx -v --dy /compiler -J"-Dgraal.Dump=:4 -Dgraal.TraceTruffleCompilation=true -Dgraal.TruffleBackgroundCompilation=false" espresso-standalone -cp  mxbuild/dists/jdk1.8/espresso-playground.jar com.oracle.truffle.espresso.playground.TestMain

Running Espresso cross-versions

By default, Espresso runs within a GraalVM and it reuses the jars and native libraries shipped with GraalVM. But it's possible to specify a different Java home, even with a different version; Espresso will automatically switch versions regardless of the host JVM.

mx build
mx espresso -version
mx espresso --java.JavaHome=/path/to/java/8/home -version
mx espresso --java.JavaHome=/path/to/java/11/home -version

Limitations

Espresso relies on glibc's dlmopen to run on HotSpot, but this approach has limitations that lead to crashes e.g. libnio.so: undefined symbol: fstatat64 . Some of these limitations can be by avoided by defining LD_DEBUG=unused e.g.

LD_DEBUG=unused mx espresso -cp mxbuild/dists/jdk1.8/espresso-playground.jar com.oracle.truffle.espresso.playground.Tetris

Espressoⁿ Java-ception

Self-hosting requires a Linux distribution with an up-to-date glibc. Use mx espresso-meta to run programs on Espresso². Ensure to prepend LD_DEBUG=unused to overcome a known glibc bug.
To run HelloWorld on Espresso² execute the following:

mx build
LD_DEBUG=unused mx --dy /compiler espresso-meta -cp my.jar HelloWorld

It takes some time for both (nested) VMs to boot, only the base layer is blessed with JIT compilation. Enjoy!