WebAssembly Host Runtimes — More than just a J2EE Evolution

Kevin Hoffman
6 min readDec 12, 2019


WebAssembly Logo

Collectively, our industry has a long and difficult history of re-inventing things as something that may still not solve the original problem. Those who adhere to the not invented here (NIH) philosophy are constantly re-building things as their own. All too often, we end up re-inventing the wheel as a square simply so that we can claim this new thing as our wheel.

Are we doing that with WebAssembly outside the browser? If you take only a cursory glance at some of the features of wasm host runtimes in the cloud, it might indeed seem like we’re just re-inventing J2EE.

J2EE is a specification. It is an agreement between an application and a supporting runtime that the runtime will satisfy some set of requirements. As the word “enterprise” is actually in the spec name, J2EE runtimes were designed for the enterprise. They sought to facilitate things like the creation of web services and distributed computing, and took care of things like scalability, distribution, and concurrency. J2EE is, in my opinion, very much an inspiration for modern-day workload scheduling platforms like Kubernetes.

J2EE had reference runtimes–products made by different vendors (some open source, some not) that purported to conform to the J2EE specification. The WebAssembly community has something similar to this. We have a specification called WASI that is a contract between a host runtime and a wasm file guaranteeing that certain low-level, POSIX-like system calls will be available. J2EE runtimes guaranteed that various enterprise-level services would be available.

Sandboxes and Sidecars

The first and likely most important difference between J2EE and host runtimes is that a host runtime is a true, isolated sandbox whereas J2EE is actually more akin to today’s concept of a sidecar that provides a suite of supporting, opt-in services to an application.

A J2EE runtime does not constrain(there are exceptions that vary by runtime) the applications whereas with WebAssembly, it’s physically impossible for the wasm code to do anything beyond the sandbox. Even the system-level WASI calls are imports, and as such must go through the host runtime and are only permitted at the behest of that host. I’ve seen dozens of applications running in production that made their own outbound HTTP (or message broker, database, etc) calls without going through the J2EE container, essentially circumventing the sandbox.


Security was one of the biggest motivating factors in our work to build waSCC and it’s definitely one that weighs heavily on the minds of anyone who has had to roll out newly built docker images to production because of discovered vulnerabilities, or those who have had rogue workloads scheduled into their clusters to do all kinds of nefarious things.

J2EE reference runtimes from Oracle and IBM and open source implementations all had security. In general, you had the ability to tag Java application bundles with a set of roles, granting it certain privileges or restrictions within the runtime. However, it was pretty easy to tamper with application bundles and poorly configured host runtimes were a popular attack vector.

This is precisely what runtimes like waSCC aim to fix. We can embed cryptographic signatures inside a wasm file that can securely prove the unique identity of the module, its capabilities (in J2EE terms, that would be the roles the application is allowed), and other important metadata that not only travels with the module, but cannot be separated from it without invalidating it.

Java Bytecode vs WebAssembly Bytecode

Again, on the surface, these things look pretty similar. Both .wasm files and .jar files contain bytecode, which is basically a low-level format designed to be interpreted (or interpreted and then compiled into a non-portable native format).

Java’s portability was one of the biggest promises of the language when it came out, and, with some exceptions, it delivered on that promise. The difference again lies within the host runtime implementations. While the J2EE specification was pretty clear on which features were supported, every vendor had their own special implementation with subtle differences and, more importantly, extra features. Enterprise developers wrote code tightly coupled to those features and suddenly you had portable Java code that was not portable across J2EE runtimes, negating many of the benefits of both the language and the runtime.

WASI is a low-level specification, and waPC is a higher-level spec upon which waSCC is built. Portability of bytecode here depends entirely on the implementation of the host runtimes and well both the runtimes and the guest modules conform to these specifications without adding hidden or extra tentacles to encourage tight coupling.

Another feature we get with wasm is the ability to send compiled code to a live process. This is also something that we could (sort-of) do with J2EE, and again the comparison is one of evolution. With a runtime like waSCC we can ship in-place, zero-downtime upgrades without the need to bring up transient resources (like we need to do with Kubernetes rolling upgrades). You could push an update to a J2EE application while it was live, but it wasn’t fast, it didn’t scale well, and it rarely ever qualified as “zero downtime”.

Performance, Size, and Workload Density

waSCC (and I suspect other similar runtimes) is smaller, faster, and more efficient than J2EE. It is designed to support modern applications, services, or functions running dynamically anywhere they are needed.

Our experiments show that even some of the larger microservices built on waSCC are only 2MB compiled wasm files. Today’s enterprises are rightfully concerned with workload density–the number of workloads they can squeeze onto a single host to maximize utilization and reduce cost overhead.

J2EE was massive. The installation footprint of reference runtimes could be enormous, as could the demands these runtimes made on their hosted applications. They were designed to be monoliths. With just a few MB of RAM overhead from the waSCC runtime, you can literaly stuff hundreds of services into the same footprint we can run a single standalone application written in modern Java with Spring Boot or a handful of Go applications.


Are out-of-browser WebAssembly host runtimes just another J2EE? No. However, J2EE and runtimes like waSCC endeavor to solve the same kinds of problems. We need a way to reduce boilerplate, speed time to production, remove friction from every step of the development process, and we need to be able to build applications that can run anywhere we deploy them, regardless of the host characteristics.

Developing with J2EE was a high-friction activity with an error-prone, brittle deployment process and an aged configuration mechanism steeped in the world of pets for computing instead of cattle. That said, Fortune 500 companies relied on J2EE runtimes as the bedrock of their server-side operations and did so for years. For all its faults, J2EE solved real problems.

So perhaps WebAssembly host runtimes that deliver enterprise capabilities are just an evolution of J2EE–but I think we’ve learned from our mistakes, and if we do this right, we can build the cloud native version of what J2EE should have been. With WebAssembly the workloads are smaller, faster, arguably more portable, easier to deploy, more secure, and benefit from a stronger sandbox.

I think we have the potential to deliver solutions to the new problems we face as we move to the cloud, to the edge, and to embedded devices in a way that was better than J2EE. However, whether we succeed or are remembered as another failed re-invention is entirely up to us and the community, the tooling, and the technology we deliver. It’s our game to lose.



Kevin Hoffman

In relentless pursuit of elegant simplicity. Tinkerer, writer of tech, fantasy, and sci-fi. Converting napkin drawings into code for @CapitalOne