PICO is a type system that supports class level and object level immutability based on Checker Framework.
Haifeng Shi: Context-Sensitive Optional Type Systems Meet Generics: A Uniform Treatment and Formalization
Lian Sun: An Immutability Type System for Classes and Objects: Improvements, Experiments, and Comparisons
Mier Ta: Context Sensitive Typechecking And Inference: Ownership And Immutability