Java AES encryption output length

What is described in the question is Format Preserving Encryption. However, expect a steep learning curve, and a severe lack of ready made implementations.

Depending on what security you are trying to achieve, this might not be possible: A certain amount of output expansion is necessary to provide protection against certain attacks.

Consider the case of a database that stores health records in the form of a list of illnesses that a person is affected by; also, consider that "high blood pressure" encrypts to 0x1234abcd using "plain" AES (or any block cipher) in ECB mode. (ECB has other problems besides being deterministic, but I'll focus on that aspect here.)

An attacker might not know the meaning of 0x1234abcd, but he can easily identify all patients sharing that record!

More elaborate attacks are possible if the attacker has write access to the database: They could, for example, insert random illnesses, look at the corresponding ciphertexts and build a table with plaintext-ciphertext correspondences.

This is why block ciphers are almost exclusively used in a mode of operation that expands the input by a certain length: The expansion is (among other things) necessary to allow multiple message encryption under a single key.

Depending on the content of your database, the mentioned attack might or might not be a problem, but in the overwhelming majority of all applications, it is desirable ECBto use a CCA-secure cryptosystem, which is necessarily probabilistic, and therefore also length-extending.

You could consider using a cipher in CTR mode, which essentially turns it into a stream cipher. The output length should equal the input length, but you still need to come up with a unique nonce for each record. If you have a unique identifier for each record, you can use that to generate your nonce for the counter. Be careful: using the same nonce twice with the same key allows an attacker to trivially recover the plaintext.