JPlag is a mature source-code plagiarism detection engine delivered as a Java library, a picocli-based CLI, and an embedded Vue.js report viewer. In production it parses submissions into language-specific token streams (via ANTLR, JavaCC, or custom parsers), compares token sequences pairwise using greedy string tiling with rolling hashes, merges fragmented matches, applies frequency-based weighting, optionally clusters submissions, and writes a .jplag ZIP report containing JSON models consumed by the viewer. The CLI can also start a local HTTP server to browse results. Architecturally it is a multi-module Maven project: core contains the comparison pipeline and reporting; language-api and language-antlr-utils define a plugin SPI for adding languages; languages/* provides 20+ modules; cli assembles the executable; report-viewer is an npm workspace with model/parser/ui-components packages. Key entry points are de.jplag.cli.CLI and de.jplag.JPlag.run(). External dependencies include ANTLR, picocli, Jackson, JGraphT, Apache Commons Math, and the Vue/TypeScript ecosystem. Although primarily an application, it embeds research-grade components such as spectral and agglomerative clustering with Bayesian optimization, token-sequence normalization, and match-frequency weighting, and it is backed by academic publications. It is not a data pipeline; the inputs are source-code submissions and the output is a similarity report. The repository is actively maintained with extensive CI, end-to-end gold-standard tests, and dependabot, though some language grammars lag language evolution and a few deprecated APIs remain.