Working with WebAssembly and Java: From A to Z

Working with WebAssembly and Java: From A to Z

WebAssembly (Wasm) is a low-level binary instruction format designed for execution in the browser and other environments. It allows running code written in high-level languages (C, C++, Rust, Go) with near-native performance. For Java developers, WebAssembly opens new horizons: JVM code can now be compiled into Wasm and executed on the client side, in server environments, or in isolated sandboxes. In this article, we will explore how to work with WebAssembly from Java, what tools exist for this, and review practical examples.

What is WebAssembly and why does a Java developer need it?

WebAssembly is a virtual machine with a compact binary representation of code. The main advantages of Wasm:

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• Performance — execution close to native.
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• Security — isolated sandbox.
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• Portability — runs in the browser, Node.js, on the server.
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• Cross-platform — works everywhere Wasm is supported.
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For a Java developer, Wasm is useful in the following scenarios:

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• Offloading heavy computations from server to client (rendering, data processing).
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• Using existing Java libraries in web applications.
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• Creating plugins and extensions with high performance.
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• Running Java code in environments where JVM is unavailable (IoT, blockchain).
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Tools for compiling Java to WebAssembly

Currently, there are several approaches for converting Java code to Wasm. Let's review the main ones.

GraalVM and AOT (Ahead-of-Time) Compilation

GraalVM is a high-performance virtual machine that supports compiling Java bytecode into native executables. Starting from version 22.0, GraalVM includes experimental support for compiling to WebAssembly. The process looks like this:

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1. Install GraalVM with Wasm support.
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2. Compile a Java class into a Wasm module.
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3. Run the resulting .wasm file in a runtime environment (wasmtime, wasmer).
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Example of compiling a simple Java class:

// HelloWasm.java\npublic class HelloWasm {\n    public static int add(int a, int b) {\n        return a + b;\n    }\n\n    public static void main(String[] args) {\n        System.out.println("Result: " + add(5, 3));\n    }\n}\n

Command to compile to Wasm using GraalVM:

javac HelloWasm.java\nnative-image --wasm --output=hello.wasm HelloWasm\n

After this, you will get a hello.wasm file that can be run:

wasmtime hello.wasm\n# Output: Result: 8\n

TeaVM Project

TeaVM is a compiler of Java bytecode to JavaScript and WebAssembly. It is focused on web development and allows translating Java classes into Wasm modules. TeaVM supports most of the standard Java library and can work with Maven/Gradle.

Example of integrating TeaVM into a Maven project:

<dependency>\n    <groupId>org.teavm</groupId>\n    <artifactId>teavm-core</artifactId>\n    <version>0.8.1</version>\n</dependency>\n

Java code for compilation:

import org.teavm.jso.JSBody;\n\npublic class Calculator {\n    @JSBody(params = {"a", "b"}, script = "return a + b;")\n    public static native int add(int a, int b);\n\n    public static void main(String[] args) {\n        System.out.println(add(10, 20));\n    }\n}\n

TeaVM generates a Wasm file that can be linked to an HTML page.

CheerpJ Project

CheerpJ is a commercial product that converts Java bytecode