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 bitSecretKeySpec 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"

You should use a KeyGenerator to generate the Key,

AES key lengths are 128, 192, and 256 bit depending on the cipher you want to use.

Take a look at the tutorial here

Here is the code for Password Based Encryption, this has the password being entered through System.in you can change that to use a stored password if you want.

        PBEKeySpec pbeKeySpec;        PBEParameterSpec pbeParamSpec;        SecretKeyFactory keyFac;        // Salt        byte[] salt = {            (byte)0xc7, (byte)0x73, (byte)0x21, (byte)0x8c,            (byte)0x7e, (byte)0xc8, (byte)0xee, (byte)0x99        };        // Iteration count        int count = 20;        // Create PBE parameter set        pbeParamSpec = new PBEParameterSpec(salt, count);        // Prompt user for encryption password.        // Collect user password as char array (using the        // "readPassword" method from above), and convert        // it into a SecretKey object, using a PBE key        // factory.        System.out.print("Enter encryption password:  ");        System.out.flush();        pbeKeySpec = new PBEKeySpec(readPassword(System.in));        keyFac = SecretKeyFactory.getInstance("PBEWithMD5AndDES");        SecretKey pbeKey = keyFac.generateSecret(pbeKeySpec);        // Create PBE Cipher        Cipher pbeCipher = Cipher.getInstance("PBEWithMD5AndDES");        // Initialize PBE Cipher with key and parameters        pbeCipher.init(Cipher.ENCRYPT_MODE, pbeKey, pbeParamSpec);        // Our cleartext        byte[] cleartext = "This is another example".getBytes();        // Encrypt the cleartext        byte[] ciphertext = pbeCipher.doFinal(cleartext);

import java.security.Key;import javax.crypto.Cipher;import javax.crypto.spec.SecretKeySpec;import sun.misc.*;import java.io.BufferedReader;import java.io.FileReader;public class AESFile {private static String algorithm = "AES";private static byte[] keyValue=new byte[] {'0','2','3','4','5','6','7','8','9','1','2','3','4','5','6','7'};// your key    // Performs Encryption    public static String encrypt(String plainText) throws Exception     {            Key key = generateKey();            Cipher chiper = Cipher.getInstance(algorithm);            chiper.init(Cipher.ENCRYPT_MODE, key);            byte[] encVal = chiper.doFinal(plainText.getBytes());            String encryptedValue = new BASE64Encoder().encode(encVal);            return encryptedValue;    }    // Performs decryption    public static String decrypt(String encryptedText) throws Exception     {            // generate key             Key key = generateKey();            Cipher chiper = Cipher.getInstance(algorithm);            chiper.init(Cipher.DECRYPT_MODE, key);            byte[] decordedValue = new BASE64Decoder().decodeBuffer(encryptedText);            byte[] decValue = chiper.doFinal(decordedValue);            String decryptedValue = new String(decValue);            return decryptedValue;    }//generateKey() is used to generate a secret key for AES algorithm    private static Key generateKey() throws Exception     {            Key key = new SecretKeySpec(keyValue, algorithm);            return key;    }    // performs encryption & decryption     public static void main(String[] args) throws Exception     {        FileReader file = new FileReader("C://myprograms//plaintext.txt");        BufferedReader reader = new BufferedReader(file);        String text = "";        String line = reader.readLine();    while(line!= null)        {            text += line;    line = reader.readLine();        }        reader.close();    System.out.println(text);            String plainText = text;            String encryptedText = AESFile.encrypt(plainText);            String decryptedText = AESFile.decrypt(encryptedText);            System.out.println("Plain Text : " + plainText);            System.out.println("Encrypted Text : " + encryptedText);            System.out.println("Decrypted Text : " + decryptedText);    }}