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Running Java in Node.js with WebAssembly

Many organisations have a lot of Java code. As they adopt newer technologies such as Node.js or Cloudflare Workers, maintaining interoperability with this existing code is important. WebAssembly is an instruction set for a stack-based virtual machine. This is the same type of abstract machine the Java Virtual Machine (JVM) uses making it possible to convert existing Java code to WebAssembly. This would allow Java code to be "imported" directly without rewrites, meaning it could be executed anywhere WebAssembly was supported. In this talk, I'll describe a research project that does just that. I’ll speak about the history of WebAssembly, what Java bytecode looks like, how to interpret it, decompilation techniques and implementing polymorphic objects.

This talk has been presented at Node Congress 2025, check out the latest edition of this JavaScript Conference.

FAQ

The main topic is running Java in Node.js with WebAssembly.

Running Java in Node.js is significant for maintaining interoperability with existing Java code as organizations adopt newer technologies such as Node.js or CloudFloat workers.

WebAssembly is important because it allows Java or any other JVM targeting languages to run in Node.js or the browser by compiling JVM bytecode to WebAssembly, which is highly optimizable and supported in various environments.

WebAssembly addresses the limitations of Asm.js by providing a new binary format, supporting streaming compilation, and modern CPU features like SIMD and 64-bit integers.

WebAssembly improves performance by allowing direct compilation to optimized machine code, thanks to its typed nature, without needing to observe execution first.

WebAssembly allows other languages to run in JavaScript engines by acting as a compilation target, thanks to its stack machine-based nature and optimizable binary format.

Java class files are parsed to extract method descriptors and bytecode, which are then transformed into WebAssembly types and instructions for compilation.

Virtual method tables are used to manage method calls in object-oriented programming by storing method offsets for each class, allowing for dynamic method dispatch and inheritance handling.

The two types of loops discussed are pre-tested loops (while loops) and post-tested loops (do-while loops).

The source code for the project is available on Brendan's GitHub.

node.js

Brendan Coll

19 min

17 Apr, 2025

Comments

Video Summary and Transcription

Hello, everyone. Welcome to my talk on running Java in Node.js with WebAssembly. Many organisations have a lot of Java code, and as they adopt newer technologies such as Node.js or CloudFloat workers, maintaining interoperability with this existing code becomes important. WebAssembly is a stack machine-based thing, like the JVM, but with a different instruction set. It improves over Asm.js with a new binary format and supports streaming compilation, SIMD, and 64-bit integers. Compiling Java code to WebAssembly involves converting JVM instructions, handling local slots, recovering if statements, and understanding control flow. Short circuit conditionals and loops are important to consider in the conversion process. Additionally, memory allocation, object creation, and program memory are key concepts. Overall, this talk explores the challenges and techniques involved in running Java in Node.js with WebAssembly.

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