Needed: Java library to calculate text readability/complexity
I looked for this as well and found only the unmaintained RAT library.
But basically all of these algorithms (most are described as formulas in this paper) can be directly translated from the single Python file with no extra dependencies.
One exception was the Linsear Write formula because of its needing a syllable counting method. But if you use the CMU pronunciation dictionary or equivalent, you can easily build the countSyllables method and the rest of the algorithm is trivial.