The Java Modeling Language (JML) is a specification language for Java programs, using Hoare style pre- and postconditions and invariants, that follows the design by contract paradigm. Specifications are written as Java annotation comments to the source files, which hence can be compiled with any Java compiler.
Various verification tools, such as a runtime assertion checker and the Extended Static Checker (ESC/Java) aid development.
JML is a behavioural interface specification language for Java modules. JML provides semantics to formally describe the behavior of a Java module, preventing ambiguity with regard to the module designers' intentions. JML inherits ideas from Eiffel, Larch and the Refinement Calculus, with the goal of providing rigorous formal semantics while still being accessible to any Java programmer. Various tools are available that make use of JML's behavioral specifications. Because specifications can be written as annotations in Java program files, or stored in separate specification files, Java modules with JML specifications can be compiled unchanged with any Java compiler.
JML specifications are added to Java code in the form of annotations in comments. Java comments are interpreted as JML annotations when they begin with an @ sign. That is, comments of the form
requires : Defines a precondition on the method that follows.ensures : Defines a postcondition on the method that follows.
signals : Defines a postcondition for when a given Exception is thrown by the method that follows.
signals_only : Defines what exceptions may be thrown when the given precondition holds.
assignable : Defines which fields are allowed to be assigned to by the method that follows.
pure : Declares a method to be side effect free (like assignable \nothing but can also throw exceptions). Furthermore, a pure method is supposed to always either terminate normally or throw an exception.
invariant : Defines an invariant property of the class.
loop_invariant : Defines a loop invariant for a loop.
also : Combines specification cases and can also declare that a method is inheriting specifications from its supertypes.
assert : Defines a JML assertion.
spec_public : Declares a protected or private variable public for specification purposes.
Basic JML also provides the following expressions
\result : An identifier for the return value of the method that follows.\old(<expression>) : A modifier to refer to the value of the <expression> at the time of entry into a method.
(\forall <decl>; <range-exp>; <body-exp>) : The universal quantifier.
(\exists <decl>; <range-exp>; <body-exp>) : The existential quantifier.
a ==> b : a implies b
a <== b : a is implied by b
a <==> b : a if and only if b
as well as standard Java syntax for logical and, or, and not. JML annotations also have access to Java objects, object methods and operators that are within the scope of the method being annotated and that have appropriate visibility. These are combined to provide formal specifications of the properties of classes, fields and methods. For example, an annotated example of a simple banking class may look like
A variety of tools provide functionality based on JML annotations. The Iowa State JML tools provide an assertion checking compiler jmlc which converts JML annotations into runtime assertions, a documentation generator jmldoc which produces Javadoc documentation augmented with extra information from JML annotations, and a unit test generator jmlunit which generates JUnit test code from JML annotations.
Independent groups are working on tools that make use of JML annotations. These include: