Running Java in Node.js with WebAssembly

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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.

Brendan Coll
Brendan Coll
19 min
17 Apr, 2025

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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.

1. Introduction to Java in Node.js with WebAssembly

Short description:

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. Let's start from the basics. This is a simple Java program that prints to the console. The JVM is a stack machine. The JVM provided a very portable runtime. So, there was this thing called Asm.js, which was designed as a compilation target for other languages. This is where WebAssembly comes in.

Hello, everyone. Welcome to my talk on running Java in Node.js with WebAssembly. I'm Brendan. I'm currently a research engineer at Expo, but before that, I worked on developer tools for CloudFloat workers.

So, before we begin, why would you want to do this? 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. Additionally, lots of interactive web experiences used to be powered by Java, but modern browsers block Java applets, so lots of sites no longer work. It would be nice if we didn't have to rewrite everything.

Let's start from the basics. This is a simple Java program that prints to the console. You compile it with the Java compiler that produces a class file and you can run it with the Java command. Java here is a runtime, which is slightly different from C and Go programs, where you're compiling for a specific architecture. As I mentioned, this runtime is the Java virtual machine, which takes these class files, which are portable and platform-independent, containing JVM instructions as opposed to architecture-specific instructions, and then runs them. The important thing here is that it doesn't have to be just Java. You can compile other languages like Scala and Kotlin to the same JVM bytecode as well.

The JVM is a stack machine, so let's have a quick look at what that looks like for this simple add program. So, the parameters to the function are stored as locals, and we start the first instruction, which pushes a local onto the stack, we move on to the next instruction, and then this add instruction pops two values off the stack, adds them together, and then pushes the result back onto the stack.

As I mentioned, the JVM provided a very portable runtime. You used to be able to run applets in browser via a plugin. Browsers started to disable applets, they introduced this thing called Java Web Start instead, but we've still got lots of, like, not very nice security warnings here. You still have to install the Java runtime, and it wasn't built into browsers. Nowadays, of course, we do all of this interactive stuff with JavaScript instead, and JavaScript engines are becoming very powerful. So, modern JavaScript engines will use just-in-time compilation. This is a simplified pipeline of Chrome's V8 engine. So, what happens is you first interpret the JavaScript and then you observe what types functions are called with on a frequent basis, and then you generate optimized code as a result of that. With all of these, sort of, efficient JavaScript engines, it becomes possible to potentially run other languages in the browser as well using the same engine.

So, there was this thing called Asm.js, which was designed as a compilation target for other languages. It was an extremely optimizable subset of JavaScript, and browsers that didn't support this Asm.js thing could still treat it as regular JavaScript and run it without, like, additional runtime. And what people found is when, like, you compiled C programs to this optimizable subset, you could usually, sort of, get near native performance with still, sort of, integrating with JavaScript. But there was more that could be done here. So, this is where WebAssembly comes in.

2. WebAssembly: Introduction and Compatibility

Short description:

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. WebAssembly has two interchangeable formats: binary and human-readable text. It can run everywhere, including browsers, Node.js, and other JavaScript runtimes.

WebAssembly is a stack machine-based thing, like the JVM, but with a different instruction set. So, it's typed, so it's highly optimizable, but it improves over Asm.js with this new binary format, and it also supports streaming compilation as well, and modern CPU features like SIMD and 64-bit integers.

WebAssembly has two interchangeable formats. You've got the binary format on the left and this human-readable text format on the right. And in all future examples, we're just gonna use the text format because it's much easier to read.

So, if we go back to the pipeline, because we now know these types ahead of time, we can go straight to the optimized machine code. So, we don't have to, like, watch the executions and see what's going on first. WebAssembly runs everywhere these days. So, browsers, of course, Node.js, other JavaScript runtimes on the server, like CloudFlow Workers and all that, and embedded runtimes as well, so you can sort of run it within your Rust or Go programs or whatever. Yeah. So, that's what this project is going to be about. We're going to be compiling JVM bytecode to WebAssembly so that Java or any other JVM targeting languages can run Node.js or the browser or anywhere else that supports WebAssembly.

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