Chicory Runtime

Chicory is a JVM native WebAssembly runtime. It allows you to run WebAssembly programs with zero native dependencies or JNI. Chicory can run Wasm anywhere that the JVM can go. It is designed with simplicity and safety in mind. See the development section for a better idea of what we are trying to achieve and why.

Reach out to us: Chicory is very early in development and there will be rough edges. We're hoping to talk to some early adopters and contributors before we formally announce it a beta to the world. Please join our team Zulip chat with this invite link if you're interested in providing feedback or contributing. Or just keeping up with development.

Getting Started (as a user)

Install Dependency

To use the runtime, you need to add the com.dylibso.chicory:runtime dependency to your dependency management system.

Maven

<dependency>
  <groupId>com.dylibso.chicory</groupId>
  <artifactId>runtime</artifactId>
  <version>0.0.9</version>
</dependency>

Gradle

implementation 'com.dylibso.chicory:runtime:0.0.9'

Loading and Instantiating Code

First your Wasm module must be loaded from disk and then "instantiated". Let's download a test module . This module contains some code to compute factorial:

Download from the link or with curl:

curl https://raw.githubusercontent.com/dylibso/chicory/main/runtime/src/test/resources/wasm/iterfact.wat.wasm > factorial.wasm

Now let's load this module and instantiate it:

import com.dylibso.chicory.runtime.ExportFunction;
import com.dylibso.chicory.wasm.types.Value;
import com.dylibso.chicory.runtime.Module;
import com.dylibso.chicory.runtime.Instance;
import java.io.File;

// point this to your path on disk
Module module = Module.builder(new File("./factorial.wasm")).build();
Instance instance = module.instantiate();

You can think of the module as the inert code and the instance as a virtual machine loaded with the code and ready to execute.

Invoking an Export Function

Wasm modules, like all code modules, can export functions to the outside world. This module exports a function called "iterFact". We can get a handle to this function using Instance#export(String):

ExportFunction iterFact = instance.export("iterFact");

iterFact can be invoked with the apply() method. We must map any java types to a wasm type and do the reverse when we want to go back to Java. This export function takes an i32 argument. We can use a method like Value#asInt() on the return value to get back the Java integer:

Value result = iterFact.apply(Value.i32(5))[0];
System.out.println("Result: " + result.asInt()); // should print 120 (5!)

Note: Functions in Wasm can have multiple returns but here we're just taking the first returned value.

Memory and Complex Types

Wasm only understands basic integer and float primitives. So passing more complex types across the boundary involves passing pointers. To read, write, or allocate memory in a module, Chicory gives you the Memory class. Let's look at an example where we have a module count_vowels.wasm, written in rust, that takes a string input and counts the number of vowels in the string:

curl https://raw.githubusercontent.com/dylibso/chicory/main/runtime/src/test/resources/wasm/count_vowels.rs.wasm > count_vowels.wasm

Build and instantiate this module:

Instance instance = Module.builder(new File("./count_vowels.wasm")).build().instantiate();
ExportFunction countVowels = instance.export("count_vowels");

To pass it a string, we first need to put the string in the module's memory. To make this easier and safe, the module gives us some extra exports to allow us to allocate and deallocate memory:

ExportFunction alloc = instance.export("alloc");
ExportFunction dealloc = instance.export("dealloc");

Let's allocate Wasm memory for a string and put in the instance's memory. We can do this with Memory#put:

Memory memory = instance.memory();
String message = "Hello, World!";
int len = message.getBytes().length;
// allocate {len} bytes of memory, this returns a pointer to that memory
int ptr = alloc.apply(Value.i32(len))[0].asInt();
// We can now write the message to the module's memory:
memory.writeString(ptr, message);

Now we can call countVowels with this pointer to the string. It will do it's job and return the count. We will call dealloc to free that memory in the module. Though the module could do this itself if you want:

Value result = countVowels.apply(Value.i32(ptr), Value.i32(len))[0];
dealloc.apply(Value.i32(ptr), Value.i32(len));
assertEquals(3, result.asInt()); // 3 vowels in Hello, World!

Host Functions

On its own, Wasm can't do anything but compute. It cannot affect the outside world. This might seem like a weakness but it's actually Wasm's greatest strength. By default, programs are sandboxed and have no capabilities. If you want a program to have capabilities, you must provide them. This puts you in the seat of the operating system. A module can ask for a capability by listing an "import" function in it's bytecode format. You can fulfill this import with a host function written in Java. Regardless of the language of the module, it can call this Java function when it needs. If it helps, you can think of host functions like syscalls or a languages standard library but you decide what they are and how they behave and it's written in Java.

Let's download another example module to demonstrate this:

curl https://raw.githubusercontent.com/dylibso/chicory/main/runtime/src/test/resources/wasm/host-function.wat.wasm > logger.wasm

This module expects us to fulfil an import with the name console.log which will allow the module to log to the stdout. Let's write that host function:

var func = new HostFunction(
    (Instance instance, Value... args) -> { // decompiled is: console_log(13, 0);
        var len = args[0].asInt();
        var offset = args[1].asInt();
        var message = instance.memory().readString(offset, len);
        System.out.println(message);
        return null;
    },
    "console",
    "log",
    List.of(ValueType.I32, ValueType.I32),
    List.of());

Again we're dealing with pointers here. The module calls console.log with the length of the string and the pointer (offset) in its memory. We again use the Memory class but this time we're pulling a string out of memory. We can then print that to stdout on behalf of our Wasm program.

Note that the HostFunction needs 3 things:

A lambda to call when the Wasm module invokes the import
The namespace and function name of the import (in our case it's console and log respectively)
The Wasm type signature (this function takes 2 i32s as arguments and returns nothing)

Now we just need to pass this host function in during our instantiation phase:

var funcs = new HostFunction[] {func};
var instance = Module.builder(new File("./logger.wasm")).build().instantiate(funcs);
var logIt = instance.export("logIt");
logIt.apply();
// should print "Hello, World!" 10 times

Development

Why is this needed?

There are a number of mature Wasm runtimes to choose from to execute a Wasm module. To name a few v8, wasmtime, wasmer, wasmedge, etc.

Although these can be great choices for running a Wasm application, embedding them into your existing Java application has some downsides. Because these runtimes are written in C/C++/Rust/etc, they must be distributed and run as native code. This causes two main friction points:

1. Distribution

If you're distributing a Java library (jar, war, etc), you must now distribute along with it a native object targeting the correct architecture and operating system. This matrix can become quite large. This eliminates a lot of the simplicity and original benefit of shipping Java code.

2. Runtime

At runtime, you must use FFI to execute the module. While there might be performance benefits to doing this for some modules, when you do, you're effectively escaping the safety and observability of the JVM. Having a pure JVM runtime means all your security and memory guarantees, and your tools, can stay in place.

Goals

Be as safe as possible
- In that we are willing to sacrifice things like performance for safety and simplicity
Make it easy to run Wasm in any JVM environment without native code, including very restrictive environments.
Fully support the core Wasm spec
Make integration with Java (and other host languages) easy and idiomatic.

Non-Goals:

Be a standalone runtime
Be the fastest runtime
Be the right choice for every JVM project

Roadmap

Wasm binary parser link
Simple bytecode interpreter
Generate JUnit tests from wasm test suite link
Make all tests green with the interpreter
Implement validation logic
Performance improvements
AOT compiler (generate JVM bytecode from Wasm module)

Some nice to have but probably separate items:

Prior Art

Building the Runtime

Contributors and other advanced users may want to build the runtime from source. To do so, you'll need to have Maven installed.

Basic steps:

mvn clean install to run all of the project's tests and install the library in your local repo
mvn spotless:apply to autoformat the code
./scripts/compile-resources.sh will recompile and regenerate the resources/compiled folders

NOTE: The install target relies on the wabt library to compile the test suite. This is not currently released for ARM (e.g. new Macs with Apple Silicon). However, wabt is available from Homebrew, so brew install wabt before running mvn clean install should work.

logging

For maximum compatibility and to avoid external dependencies we use, by default, the JDK Platform Logging (JEP 264). You can configure it by providing a logging.properties using the java.util.logging.config.file property and here you can find the possible configurations.

For more advanced configuration scenarios we encourage you to provide an alternative, compatible, adapter:

slf4j
log4j2

It's also possible to provide a custom com.dylibso.chicory.log.Logger implementation if JDK Platform Logging is not available or doesn't fit.

muvalika/chicory