OpenJML

Free#25 of 35 in Formal Verification Tools

OpenJML: A focused open-source toolkit for checking Java contracts, proofs, and runtime assertions. Ranked #25 of 35 in Formal Verification Tools by our editors (7.0/10); pricing: Open source; best for java and JML contract checking.

7.0/10★★★★☆Editor score

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At a glance

• Editor score
  7.0 / 10
• Pricing
  Open source
• Best for
  Java and JML contract checking
• Paid from
  Free
• Verification method
  Deductive
• Facts checked
  20 Sep 2026

• Where it wins

  • Checks Java and JML types, contracts, invariants, and assertions
  • Uses SMT solvers for static deductive verification
  • Reports counterexamples and execution traces for failed proofs

• Where it doesn't

  • Focused on Java and JML rather than multiple input languages
  • Self-hosted deployment requires teams to manage the local environment
  • IDE coverage centers on Eclipse and Visual Studio Code

Our verdict on OpenJML

OpenJML is an open-source suite for specifying and verifying Java programs with the Java Modeling Language. It is aimed at software practitioners, students, and researchers who work with formal contracts, invariants, preconditions, and postconditions. The toolkit parses and type-checks Java plus JML programs, performs static deductive verification, and supports runtime assertion checking. Its focus is narrow by design: teams seeking formal specification workflows for Java can work within a dedicated contract-based toolset, while projects written in other input languages are outside its stated scope.

The verification workflow combines several useful layers. Static deductive verification uses SMT solvers to assess whether specified behavior can be proved, while runtime assertion checking provides a way to evaluate assertions during execution. OpenJML also checks JML invariants, preconditions, and postconditions, and reports counterexamples and execution traces when proofs fail. These reports can help users investigate why a specification does not hold instead of receiving only a pass-or-fail result. This depth makes OpenJML a fit for codebases where contracts and proof obligations are central, rather than for teams looking only for general-purpose testing or code analysis.

OpenJML supports Windows, macOS, and Linux through a self-hosted deployment model. The project provides a standalone command-line tool, a Language Server Protocol server, an Eclipse plugin integration, and a Visual Studio Code extension. That combination supports both command-line workflows and editor-based development, with LSP support extending compatibility to suitable language-server clients. Teams should choose OpenJML when Java and JML are an intentional part of their verification approach and when they can manage a self-hosted toolchain. Projects needing broader language coverage or a different formalism should consider alternatives designed beyond Java contracts.

Questions about OpenJML

What does OpenJML run on? Windows, macOS and Linux. It runs self-hosted.

What does OpenJML integrate with? 3 integrations: Eclipse, Visual Studio Code and Language Server Protocol.

Who is OpenJML built for? Solo users, small businesses and mid-market companies (our editorial estimate).

Your product? Our editors can test OpenJML in depth and publish a review, labelled as paid · from $149/mo →

OpenJML pricing

OpenJML fact sheet

OpenJML integrations

OpenJML lists 3 integrations on its own site.

• Eclipse
• Visual Studio Code
• Language Server Protocol

Alternatives to OpenJML

• IIsabelleA free, open-source proof assistant for broad mathematical and systems verification.9.4
• RocqA free, kernel-checked theorem prover with dependent types, automation, and certified extraction.9.3
• PPVSA broad, self-hosted environment for hybrid formal verification workflows.9.2

See all OpenJML alternatives →

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Featured on iTechGuides

Reviewed by iTechGuides Editors · Editorial team · Updated Nov 2026

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Last updated 14 Nov 2026 · How we research and update