User user = new User();
 user.setAccountName(email.split("@")[0]); //default value, user can change it later
 user.setEmail(email);
 user.setPassword(password);
 user.setRegistrationDate(DateFormatUtils.format(new Date(), "dd.MM.yyyy"));
 user.setCharacters(new Character[1]);
 user.setStash(new InventoryItem[10]);
 dynamoDBMapper.save(user); //thread blocks here

@DynamoDBTable(tableName="Users")
public class User {
    private String email;
    private String accountName;
    private String password;
    private String registrationDate;
    private Character[] characters;
    private InventoryItem[] stash;

    @DynamoDBHashKey(attributeName = "email")
    public String getEmail() { return email; }
    public void setEmail(String email) { this.email = email; }

    @DynamoDBAttribute(attributeName = "accountName")
    public String getAccountName() { return accountName; }
    public void setAccountName(String accountName) { this.accountName = accountName; }

    @DynamoDBAttribute(attributeName="password")
    public String getPassword() { return password; }
    public void setPassword(String password) { this.password = password;}

    @DynamoDBAttribute(attributeName = "registrationDate")
    public String getRegistrationDate() { return registrationDate; }
    public void setRegistrationDate(String registrationDate) { this.registrationDate = registrationDate; }

    @DynamoDBTypeConverted(converter = CharacterConverter.class)
    @DynamoDBAttribute(attributeName = "characters")
    public Character[] getCharacters(){
        return this.characters;
    }
    @DynamoDBAttribute(attributeName = "characters")
    public void setCharacters(Character[] characters) {
        this.characters = characters;
    }

    @DynamoDBTypeConverted(converter = InventoryItemConverter.class)
    @DynamoDBAttribute(attributeName = "stashItems")
    public InventoryItem[] getStash() {
        return this.stash;
    }
    @DynamoDBAttribute(attributeName = "stashItems")
    public void setStash(InventoryItem[] stash) {
        this.stash = stash;
    }
}

public class InventoryItem {
    private int itemId;
    private boolean isInStash;
    private int slotNumber;

    public int getItemId() { return itemId; }
    public void setItemId(int itemId) { this.itemId = itemId; }

    public boolean getIsInStash() { return isInStash; }
    public void setIsInStash(boolean isInStash) { this.isInStash = isInStash; }

    public int getSlotNumber() { return slotNumber; }
    public void setSlotNumber(int slotNumber) { this.slotNumber = slotNumber; }
}

public class Character {
    private int characterSlotNumber;
    private int level;
    private int currentExp;
    private String characterClass;
    private InventoryItem[] inventoryItems;

    private int baseHp;
    private int baseMp;
    private int baseDexterity;
    private int baseIntelligence;
    private int baseDefense;
    private int baseStamina;
    private int baseRegeneration;
    private int baseSpeed;

    public int getCharacterSlotNumber() { return characterSlotNumber; }
    public void setCharacterSlotNumber(int characterSlotNumber) { this.characterSlotNumber = characterSlotNumber; }

    public int getLevel() { return level; }
    public void setLevel(int level) { this.level = level; }

    public int getCurrentExp() { return currentExp; }
    public void setCurrentExp(int currentExp) { this.currentExp = currentExp; }

    public String getCharacterClass() { return characterClass; }
    public void setCharacterClass(String characterClass) { this.characterClass = characterClass;}

    public InventoryItem[] getInventoryItems() { return this.inventoryItems; }
    public void setInventoryItems(InventoryItem[] inventoryItems) { this.inventoryItems = inventoryItems; }

    public int getBaseHp() { return baseHp; }
    public void setBaseHp(int baseHp) { this.baseHp = baseHp; }

    public int getBaseMp() { return baseMp; }
    public void setBaseMp(int baseMp) { this.baseMp = baseMp; }

    public int getBaseDexterity() { return baseDexterity; }
    public void setBaseDexterity(int baseDexterity) { this.baseDexterity = baseDexterity; }

    public int getBaseIntelligence() { return baseIntelligence; }
    public void setBaseIntelligence(int baseIntelligence) { this.baseIntelligence = baseIntelligence; }

    public int getBaseDefense() { return baseDefense; }
    public void setBaseDefense(int baseDefense) { this.baseDefense = baseDefense; }

    public int getBaseStamina() { return baseStamina; }
    public void setBaseStamina(int baseStamina) { this.baseStamina = baseStamina; }

    public int getBaseRegeneration() { return baseRegeneration; }
    public void setBaseRegeneration(int baseRegeneration) { this.baseRegeneration = baseRegeneration; }

    public int getBaseSpeed() { return baseSpeed; }
    public void setBaseSpeed(int baseSpeed) { this.baseSpeed = baseSpeed; }
    }
}

public class InventoryItemConverter implements DynamoDBTypeConverter<String, InventoryItem[]> {

    public String convert(InventoryItem[] inventoryItems) {
        StringBuilder stringBuilder = new StringBuilder();
        for (InventoryItem inventoryItem : inventoryItems) {
            stringBuilder
                    .append(inventoryItem.getItemId()).append("-")
                    .append(inventoryItem.getIsInStash()).append("-")
                    .append(inventoryItem.getSlotNumber()).append("=");
        }
        return stringBuilder.toString();
    }

    public InventoryItem[] unconvert(String s) {
        String[] inventoryItemsString = s.split("=");
        InventoryItem[] inventoryItems = new InventoryItem[inventoryItemsString.length];
        for (int i = 0; i < inventoryItemsString.length; i++) {
            String[] inventoryItemFields = inventoryItemsString[i].split("-");
            InventoryItem inventoryItem = new InventoryItem();
            inventoryItem.setItemId(Integer.valueOf(inventoryItemFields[0]));
            inventoryItem.setIsInStash(Boolean.valueOf(inventoryItemFields[1]));
            inventoryItem.setSlotNumber(Integer.valueOf(inventoryItemFields[2]));
            inventoryItems[i] = inventoryItem;
        }
        return inventoryItems;
    }
}

public class CharacterConverter implements DynamoDBTypeConverter<String, Character[]> {
    public String convert(Character[] characters) {
        StringBuilder stringBuilder = new StringBuilder();
        InventoryItemConverter inventoryItemConverter = new InventoryItemConverter();
        for (Character character : characters) {
            stringBuilder
                    .append(character.getCharacterSlotNumber()).append(",")
                    .append(character.getLevel()).append(",")
                    .append(character.getCurrentExp()).append(",")
                    .append(character.getCharacterClass()).append(",")
                    .append(inventoryItemConverter.convert(character.getInventoryItems())).append(",")
                    .append(character.getBaseHp()).append(",")
                    .append(character.getBaseMp()).append(",")
                    .append(character.getBaseDexterity()).append(",")
                    .append(character.getBaseIntelligence()).append(",")
                    .append(character.getBaseDefense()).append(",")
                    .append(character.getBaseStamina()).append(",")
                    .append(character.getBaseRegeneration()).append(",")
                    .append(character.getBaseSpeed()).append(";");
        }
        return stringBuilder.toString();
    }

    public Character[] unconvert(String s) {
        InventoryItemConverter inventoryItemConverter = new InventoryItemConverter();

        String[] charactersStrings = s.split(";");
        Character[] characters = new Character[charactersStrings.length];
        for (int i = 0; i < characters.length; i++) {
            String[] characterFields = charactersStrings[i].split(",");
            Character character = new Character();
            character.setCharacterSlotNumber(Integer.valueOf(characterFields[0]));
            character.setLevel(Integer.valueOf(characterFields[1]));
            character.setCurrentExp(Integer.valueOf(characterFields[2]));
            character.setCharacterClass(characterFields[3]);
            character.setInventoryItems(inventoryItemConverter.unconvert(characterFields[4]));
            character.setBaseHp(Integer.valueOf(characterFields[5]));
            character.setBaseMp(Integer.valueOf(characterFields[6]));
            character.setBaseDexterity(Integer.valueOf(characterFields[7]));
            character.setBaseIntelligence(Integer.valueOf(characterFields[8]));
            character.setBaseDefense(Integer.valueOf(characterFields[9]));
            character.setBaseStamina(Integer.valueOf(characterFields[10]));
            character.setBaseRegeneration(Integer.valueOf(characterFields[11]));
            character.setBaseSpeed(Integer.valueOf(characterFields[12]));
        }
        return characters;
    }
}