JML

JML, the Java Modeling Language, is a behavioral specification language for Java that enables programmers to state contracts for Java methods and classes without altering Java syntax. It allows preconditions, postconditions, invariants, and frame conditions to be expressed as part of the source.

Specifications are written in Java comments, using constructs such as requires, ensures, invariant, and assignable. For example, a method that returns a non-negative result might declare ensures \result >= 0, and a state-changing method may specify an assignable clause. JML also supports model fields, ghost variables, and quantifiers for expressing abstract properties.

JML specifications can be checked at runtime or by static analysis. Runtime assertion checking inserts checks during execution; static verifiers attempt to prove that Java code conforms to its JML specification. Tools include ESC/Java2, Krakatoa (historical), and OpenJML, which integrates with the Java toolchain and IDEs.

JML originated in the late 1990s as part of research into formal methods for object-oriented software. It is widely used in academia and has influenced approaches to specification-driven development. Open-source OpenJML remains a primary implementation for verification and testing.

JML is not part of the Java language specification; it is a separate language that complements Java by providing a formal contract framework. It is intended to improve program reliability and facilitate verification and documentation of Java code.