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Introduction

An Online Judge (OJ) is a web-based system used in competitive programming. It presents algorithmic problems, accepts code submissions in various languages, automatically compiles and runs the code against hidden test cases, and immediately tells the user whether their solution is correct, all without any human involvement in the grading process. Platforms like LeetCode and Codeforces are well-known examples.

Verwandlung Online Judge is a self-hostable, open-source OJ built for running your own contests or practice environment. Its main distinguishing feature at the time of release was cross-platform support: most open-source OJs were Linux-only due to their reliance on Linux-specific sandboxing APIs, whereas Verwandlung runs natively on both Windows and Linux.

The name Verwandlung traces back through a chain of references: LinkedIn’s distributed message queue Kafka was named after the author Franz Kafka; Alibaba’s MetaQ was built on top of Kafka and took its name from Kafka’s most famous work, Metamorphosis. Since a message queue is a core component of this project, the name was chosen as Verwandlung, the German title of Metamorphosis.

I shared a more detailed write-up of the design on Zhihu: 如何实现一个在线评测系统（Online Judge）？ and Online Judge 系统是怎么判断用户提交的代码是否正确运行的？.

Architecture

The system is split into two independent modules: Web and Judge Worker, connected through an ActiveMQ message queue.

Web is built on Spring MVC and handles all user-facing functionality: problem management, submission history, contest administration, and result display. When a user submits code, the web server does not evaluate it directly; instead, the submission is pushed onto the message queue, keeping the web layer responsive and making it easy to scale out judging capacity by running multiple judge workers in parallel.

Judge Worker consumes submissions from the queue, compiles and executes the code, and reports results back. The full stack is Spring MVC + MySQL + MyBatis + ActiveMQ. NoSQL was deliberately omitted to keep deployment simple.

Cross-Platform Support

Cross-platform judging is the core technical challenge of the project and is achieved through JNI. Each judge worker calls native OS APIs to enforce memory limits and measure execution time:

• On Windows, the judge uses the Windows API (e.g., GetProcessMemoryInfo, GetProcessTimes)
• On Linux, it uses the equivalent POSIX API (e.g., getrusage)

Everything above that OS boundary is implemented in pure Java, so the same codebase compiles and runs on both platforms without modification.

Real-Time Results

Rather than waiting for all test cases to finish before showing a verdict, Verwandlung streams results back one test case at a time using Server-Sent Events (SSE). As the judge worker finishes each test case, it posts a result message back onto the ActiveMQ queue. The web server listens on the same queue and immediately forwards each message to the waiting browser, so users see their results appear progressively.

Screenshots