```
@Documented
@Retention(RUNTIME)
@Target({METHOD,CONSTRUCTOR})
public @interface Impure
```
Impure is a method annotation that means the method might have side effects and/or might be nondeterministic. Conceptually, it completes the "lattice" of purity annotations by serving as a top element. That is, any @Pure method can be treated as @SideEffectFree or @Deterministic, and any @SideEffectFree or @Deterministic method can be treated as @Impure.
This annotation should not be written by a programmer, because leaving a method unannotated is equivalent to writing this annotation.
The purpose of this annotation is for use by tools. A tool may distinguish between unannotated methods (that the tool has not yet examined) and @Impure methods (that the tool has determined to be neither @SideEffectFree nor @Deterministic).

See the Checker Framework Manual:
Side effects, determinism, purity, and flow-sensitive analysis

Annotation Type Impure