Edit:

As written in the comments the old code is not “best practice”.
You should use a keygeneration algorithm like PBKDF2 with a high iteration count.
You also should use at least partly a non static (meaning for each “identity” exclusive) salt. If possible randomly generated and stored together with the ciphertext.

    SecureRandom sr = SecureRandom.getInstanceStrong();
    byte[] salt = new byte[16];
    sr.nextBytes(salt);

    PBEKeySpec spec = new PBEKeySpec(password.toCharArray(), salt, 1000, 128 * 8);
    SecretKey key = SecretKeyFactory.getInstance("PBKDF2WithHmacSHA1").generateSecret(spec);
    Cipher aes = Cipher.getInstance("AES");
    aes.init(Cipher.ENCRYPT_MODE, key);

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Old Answer

You should use SHA-1 to generate a hash from your key and trim the result to 128 bit (16 bytes).

Additionally don’t generate byte arrays from Strings through getBytes() it uses the platform default Charset. So the password “blaöä” results in different byte array on different platforms.

byte[] key = (SALT2 + username + password).getBytes("UTF-8");
MessageDigest sha = MessageDigest.getInstance("SHA-1");
key = sha.digest(key);
key = Arrays.copyOf(key, 16); // use only first 128 bit

SecretKeySpec secretKeySpec = new SecretKeySpec(key, "AES");

Edit:
If you need 256 bit as key sizes you need to download the “Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files” Oracle download link, use SHA-256 as hash and remove the Arrays.copyOf line.
“ECB” is the default Cipher Mode and “PKCS5Padding” the default padding.
You could use different Cipher Modes and Padding Modes through the Cipher.getInstance string using following format: “Cipher/Mode/Padding”

For AES using CTS and PKCS5Padding the string is: “AES/CTS/PKCS5Padding”