Synthia is a versatile, modular and extensible Java-based data structure generation library. It is centered on the notion of “pickers”, which are objects producing values of a given type on demand. Pickers are stateful and can be given as input to other pickers; this chaining principle can generate objects whose structure follows a complex pattern. The paper describes the core principles and key features of the library, including test input shrinking, provenance tracking, and object mutation.