Multitenancy with Spring Boot

Reading Time: 7 minutes

Why should you consider implementing multitenancy in your project?

  • Cost: Multi-tenant architecture allows the sharing of resources, databases, and the application itself, thus the cost to run the system is fixed.
  • Maintenance: Users do not have to pay a considerable amount of fees to keep the software up to date. This reduces the overall cost of maintenance for each tenant.
  • Performance: Easier to assess and optimize speed, utilization, response time across the entire system, and even update the technology stack when needed.

In this blog we will implement multitenancy in our Spring Boot project.

Let’s create a simple Spring Boot project from, with only basic dependencies (Spring Web, Spring Data JPA, Spring Configuration Processor, MySQL Driver).

The good thing for implementing multitenancy is that we do not need additional dependencies.
We will split this example into two parts. In the first one, we will explain the idea/logic behind it and split the approach into 7 configuration steps, and explain every step. In the second part, we will see how it’s implemented in real life and we will test the solution.

1. Let’s start with creating Tenant Storage. We will use it for keeping the tenant value while the request is executing.

public class TenantStorage {

    private static ThreadLocal<String> currentTenant = new ThreadLocal<>();

    public static void setCurrentTenant(String tenantId) {

    public static String getCurrentTenant() {
        return currentTenant.get();

    public static void clear() {

2. Next, we will create the Tenant Interceptor. For every request, we will set the value at the beginning and clear it at the end. As you can see, in Tenant Interceptor, I decided for this demo to fetch the value of the tenant from request header (X-Tenant), this is up to you. Just keep an eye on data security when using this in production. Maybe you want to fetch from a cookie or some other header name.

public class TenantInterceptor implements WebRequestInterceptor {

    private static final String TENANT_HEADER = "X-Tenant";

    public void preHandle(WebRequest request) {

    public void postHandle(WebRequest webRequest, ModelMap modelMap) {

    public void afterCompletion(WebRequest webRequest, Exception e) {


3. Next thing is to add the tenant Interceptor in the interceptor registry. For that purpose, I will create WebConfiguration that will implement WebMvcConfigurer.

public class WebConfiguration implements WebMvcConfigurer {

    private final TenantInterceptor tenantInterceptor;

    public WebConfiguration(TenantInterceptor tenantInterceptor) {
        this.tenantInterceptor = tenantInterceptor;

    public void addInterceptors(InterceptorRegistry registry) {

4. Now, let’s update the application.yml file with some properties for the database connections.

      jdbcUrl: jdbc:mysql://localhost:3306/n47schema1?verifyServerCertificate=false&amp;useSSL=false&amp;requireSSL=false
      driverClassName: com.mysql.cj.jdbc.Driver
      username: root
      jdbcUrl: jdbc:mysql://localhost:3306/n47schema2?verifyServerCertificate=false&amp;useSSL=false&amp;requireSSL=false
      driverClassName: com.mysql.cj.jdbc.Driver
      username: root
    database-platform: org.hibernate.dialect.MySQL5Dialect

5. Following, we will wrap the tenant’s values to map with key = tenant name, value = data source in DataSourceProperties.

@ConfigurationProperties(prefix = "tenants")
public class DataSourceProperties {

    private Map<Object, Object> dataSources = new LinkedHashMap<>();

    public Map<Object, Object> getDataSources() {
        return dataSources;

    public void setDataSources(Map<String, Map<String, String>> datasources) {
        datasources.forEach((key, value) -> this.dataSources.put(key, convert(value)));

    public DataSource convert(Map<String, String> source) {
        return DataSourceBuilder.create()

6. Afterwards, we should create DataSource Bean, and for that purpose, I will create DataSourceConfig.

public class DataSourceConfig {

    private final DataSourceProperties dataSourceProperties;

    public DataSourceConfig(DataSourceProperties dataSourceProperties) {
        this.dataSourceProperties = dataSourceProperties;

    public DataSource dataSource() {
        TenantRoutingDataSource customDataSource = new TenantRoutingDataSource();
        return customDataSource;

7. At last, we will extend the AbstractRoutingDataSource and implement our lookup key.

public class TenantRoutingDataSource extends AbstractRoutingDataSource {

    protected Object determineCurrentLookupKey() {
        return TenantStorage.getCurrentTenant();


And we are done with the first part.

Let’s see how it looks in the real world:

For this example, we will use two schemas from the same database instance, we will create a user and get all users. Also, I will show you how you can implement Flyway and test the solution.

First, let’s configure our databases. In my local instance of MySQL server, we will create two schemas: n47schema1 and n47schema2.

Next step is to execute this CREATE statement on both schemas:

CREATE TABLE `users` (
	`name` VARCHAR(64) NOT NULL DEFAULT '' COLLATE 'utf8_general_ci',

Then, we will create two APIs, one for creating a user, and the other one to fetch all users.

public class UserController {

    private final UserRepository userRepository;

    public UserController(UserRepository userRepository) {
        this.userRepository = userRepository;

    public UserDomain addUser(@RequestBody UserRequestBody userRequestBody) {
        UserDomain userDomain = new UserDomain(userRequestBody.getName());

    public List<UserDomain> getAll() {
        return userRepository.findAll();

Also we need to create UserDomain, UserRepository and UserRequestBody.

@Table(name = "users")
public class UserDomain {

    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;

    public UserDomain() {

    public UserDomain(String name) { = name;

    public Long getId() {
        return id;

    public void setId(Long id) { = id;

    public String getName() {
        return name;

    public void setName(String name) { = name;
public interface UserRepository extends JpaRepository<UserDomain, Long> {
public class UserRequestBody {
    private String name;

    public UserRequestBody() {

    public UserRequestBody(String name) { = name;

    public String getName() {
        return name;

    public void setName(String name) { = name;

And we are done with coding.

We can run our application and start making a request.

First, let’s create some users with a POST request to http://localhost:8080/users. The most important thing not to forget is that we need to provide header X-Tenant with the value n47schema1 or n47schema2.

We will create two users for tenant n47schema1: Antonie and John. Example:

After that, we will change the X-Tenant header value to n47schema2 and create two users: William and Joseph.

You will notice that the ids retrieved in the response are the same as the first tenant value. Now let’s fetch the users by the API.

When you make a GET request to http://localhost:8080/users with header X-Tenant having value n47schema1 you will fetch the users from the n47schema1 schema, and when you make a request with a header value n47schema2 you will fetch from the n47schema2 schema.

You can also check the data in the database to be sure that it is stored correctly.

You can always set fallback if the X-Tenant header is not provided, or it’s with a wrong value.

As the last thing, I will show you how you can implement Flyway with multitenancy. First, you need to add flyway as a dependency and disable it in the application.yml

    enabled: false

Add PostConstruct method in DataSourceConfig configuration:

public void migrate() {
        for (Object dataSource : dataSourceProperties.getDataSources().values()) {
            DataSource source = (DataSource) dataSource;
            Flyway flyway = Flyway.configure().dataSource(source).load();

And we are done, hope this blog helps you to understand what multitenancy is and how it’s implemented in Spring Boot project.

Download the source code

The project is freely available on our Bitbucket repository. Feel free to fix any mistakes and to comment here if you have any questions or feedback.

Did we forget the Immutable classes in Java?

Reading Time: 5 minutes

Well, I did. In everyday work we can hear discussions about microservices, containers, beans, entities etc. but it is very hard and rare to hear some talk about immutable or mutable classes. Why is it like that?

Let’s first refresh our memories of what an Immutable class is.

Immutable class means, that once an object is initialized we cannot change its content.

To be more clear, let’s see how we can write Immutable classes in Java.

Basic rules to write some immutable classes are:
  1. Don’t provide “setter” methods — methods that modify fields or objects referred to by fields.
  2. Make all fields final and private.
  3. Don’t allow subclasses to override methods. The simplest way to do this is to declare the class as final. A more sophisticated approach is to make the constructor private and construct instances in factory methods.
  4. If the instance fields include references to mutable objects, don’t allow those objects to be changed:
    • Don’t provide methods that modify the mutable objects.
    • Don’t share references to the mutable objects. Never store references to external, mutable objects passed to the constructor; if necessary, create copies, and store references to the copies. Similarly, create copies of your internal mutable objects when necessary to avoid returning the originals in your methods.

How to make Immutable classes

After defining these basic rules there are several different solutions to write Immutable classes.

Basic one without using external libraries:
final public class ImmutableBasicExample {

   final private Long accountNumber;
   final private String accountCurrency;

   private void check(String accountCurrency, Long accountNumber) {
       // Some constructor rules
       // throw new IllegalArgumentException()

   public ImmutableBasicExample(
           String accountCurrency, Long accountNumber) {
       check(accountCurrency, accountNumber);
       this.accountCurrency = accountCurrency;
       this.accountNumber = accountNumber;

   public String getAccountCurrency() {
       return accountCurrency;

   public Long getAccountNumber() {
       return accountNumber;

Use final class, make fields final and set them in the constructor. Don’t write setter for fields, just getter.

We can use Lombok:
import lombok.Value;

public class LombokImmutable {
   Long accountNumber;
   String accountCurrency;

Much shorter, with just one annotation we have done our job. Value is variant of data and is immutable because: all fields are private and final, the class is made final, the getter for each field, additional some basic methods like toString(), equals() and hasCode().

Or the newest way with using a record (Java 14 preview features):
public record RecordRequstBody(
       Long accountNumber,
       String accountCurrency) {

By now the most sophisticated way to make an Immutable class. Small,  readable and useful code. Without using external libraries. Compiler auto generates following code: private final field, public read assessor, public constructor with signature same like state description and implementation of following methods: toString(), hashCode() and equals().

I’m sure that there are other ways to write Immutable classes, but these are enough to understand how much code and effort we need to write one.

Use of Immutable classes

We already use Immutable classes every day in our work. All primitives wrapper classes are immutable. Here is one everyday practical example:

Integer integerExample = 6;

where changeInteger is:

private void changeInteger(Integer integerExample) {
   integerExample = integerExample + 1;

The output will be:


It is because the line

integerExample = integerExample + 1;

creates a new reference to the new object for integerExample and the integerExample in the main code is still referencing the old Integer object, which is not changed.

This also applies to all other primitive wrappers that are immutable: Byte, Short, Integer, Long, Float, Double, Character, Boolean. Additionally BigDecimal, BigInteger and LocalDate are immutable.

Other one interesting immutable class in Java is String. If we write the following code:

String string1 = "first string";
string1 += "concatenation";

Concatenation of two Strings with “+” will produce new String. It is fine if we do this with two or a few Strings, but if we build one String with more concatenations then we will initialize more objects. (That is why here it is better to use StringBuilder)

We can create and use Immutable classes for RequestBody (DTO) in our rest controllers. It is a good idea because the state will be validated when the request is created, once, and will be valid all time after that. We will not have any need to change the state of the request. If we are changing the request then we are doing something wrong.

Another scenario where we can use them is when we need to have some business classes (where we will process some data) where the state should be unchanged. We can find a few examples for this, using them for Currency, Account information etc…

How often should we use them?

Well we see that there are some benefits using them:

  • They are thread-safe
  • Safer because their state can not be changed
  • They are simple for construct, test and use
  • It is good to understand them and use them if you work more functional and concurrent programming

But there are disadvantages too:

At first, you can’t change fields in them. To do that you should create a copy of them with changed values. It means that you will have more objects initialized in the VM and for that, you should add some code to copy objects.
To be sure that you will make some classes immutable you should be sure that the state of the object would not be changed. These days developers don’t spend time analyzing where that class will be or will not be changed.

There is one general concept from Effective Java, which describes the use of immutability:

Classes should be immutable unless there’s a very good reason to make them mutable… If a class cannot be made immutable, limit its mutability as much as possible.

Hibernate techniques for mapping sets, lists and enumerations

Reading Time: 4 minutes

As we all know, Hibernate is an Object Relational Mapping (ORM) framework for the Java programming language. This blog post will teach you how to use advanced hibernate techniques for mapping sets, lists and enums in simple and easy steps.

Mapping sets

Set is a collection of objects in which duplicate values are not allowed and the order of the objects is not important. Hibernate uses the following annotation for mapping sets:

  • @ElementCollection – Declares an element collection mapping. The data for the collection is stored in a separate table.
  • @CollectionTable – Specifies the name of a table that will hold the collection. Also provides the join column to refer to the primary table.
  • @Column – The name of the column to map in the collection table.

@ElementCollection is used to define the following relationships: One-to-many relationship to an @Embeddable object and One-to-many relationship to a Basic object, such as Java primitives (wrappers): int, Integer, Double, Date, String, etc…

Now you’re probably asking yourself: Hmmm… How does this compare to @OneToMany?

@ElementCollection is similar to @OneToMany except that the target object is not an @Entity. These annotations give you an еasy way to define a collection with simple/basic objects. But, you can’t query, persist or merge target objects independently of their parent object. ElementCollection does not support a cascade option, so target objects are ALWAYS persisted, merged, removed with their parent object.

Mapping lists

Lists are used when we need to keep track of order position and duplicates of the elements are allowed. Additional annotation that we are going to use here is @OrderColumn, that specified the name of the column to track the element order/position (name defaults to <property>_ORDER):

Mapping maps

When you want to access data via a key rather than integer index, you should probably decide to use maps. Additional annotation used for maps is @MapKeyColumn which helps us to define the name of the key column for a map. Name defaults to <property>_KEY :

Mapping sorted sets

As we mentioned before, the set is an unsorted collection with no duplicates. But what if we don’t need duplicates and the order of retrieval is also important? In that case, we can use @OrderBy and specify the ordering of the elements when a collection is retrieved.

Syntax: @OrderBy(“[field name or property name] [ASC |DESC]”)

Mapping sorted maps

@OrderBy can be also used in maps. In that case, the default value is a key column, ascending.

Mapping Enums

By default, Hibernate maps an enum to a number. This mapping is very efficient, but there is a high risk that adding or removing a value from your enum will change the ordinal of the remaining values. Because of that, you should map the enum value to a String with the @Enumerated annotation. This annotation is used to reference an Enum type and save the field in database as String.


In this article, we have taken a look in the simple techniques for mapping sets, lists and enumerations when we are using Hibernate. I hope you enjoyed reading it and have found it helpful.

Crypto Trading Bot

Reading Time: 6 minutes

Every year, N47 as a tech family celebrates a tech festival as Hackdays at the end of the year. In December 2019 we were in Budapest, Hungary for Hackdays. There were five different teams and each team had created some cool projects in a short time. I was also part of a team and we implemented a simple Trading Bot for Crypto. In this blog post, I want to share my experiences.

Trading Platform

To create a Trading Bot, you first need to find the right trading platform. We selected Binance DEX, which can offer a good volume for selected trading pairs, testnet for test purposes and was a Decentralized EXchange (DEX). Thanks to DEX, we can connect the wallet directly and use the credit directly from it.

Binance Chain is a new blockchain and peer-to-peer system developed by Binance and the community. Binance DEX is a secure, native marketplace that is based on the Binance Chain and enables the exchange of digital assets that are issued and listed in the DEX. Reconciliation takes place within the blockchain nodes, and all transactions are recorded in the chain, creating a complete, verifiable activity book. BNB is the native token in the Binance Chain, so users are charged the BNB for sending transactions.

Trading fees are subject to a complex logic, which can lead to individual transactions not being calculated exactly at the rates mentioned here, but instead between them. This is due to the block-based matching engine used in the DEX. The difference between Binance Chain and Ethereum is that there is no idea ​​of gas. As a result, the fees for the remaining transactions are set. There are no fees for a new order.

The testnet is a test environment for Binance Chain network, run by the Binance Chain development community, which is open to developers. The validators on the testnet are from the development team. There is also a web wallet that can directly interact with the DEX testnet. It also provides 200 testnet BNB so that you can interact with the Binance DEX testnet.

For developers, Binance DEX has also provided the REST API for testnet and main net. It also provides different Binance Chain SDKs for different languages like GoLang, Javascript, Java etc. We used Java SDK for the Trading Bot with Spring Boot.

Trading Strategy

To implement a Trading Bot, you need to know which pair and when to buy and sell Crypto for these pairs. We selected a very simple trading strategy for our project. First, we selected the NEXO / BINANCE trading pair (Nexo / BNB) because this pair has the highest trading volume. Perhaps you can choose a different trading pair based on your analysis.

For the purchase and sale, we made a decision based on Candlestick count. We considered the size of Candlestick for 15 minutes. If three are still red (price drops), buy Nexo and if three are still green (price increase), sell Nexo. Once you’ve bought or sold, you’ll have to wait for the next three. Continue with the red or green Candlestick. The purchase and sales volume is always 20 Nexo. You can also choose this based on your analysis.

Let’s Code IT

We have implemented the frontend (Vue.Js) and the backend (Spring Boot) for the Trading Bot, but here I will only go into the backend application as it contains the main logic. As already mentioned, the backend application was created with Spring Boot and Binance Chain Java SDK.

We used ThreadPoolTaskScheduler for the application. This scheduler runs every 2 seconds and checks Candlestick. This scheduler has to be activated once via the frontend app and is then triggered automatically every 2 seconds.

ThreadPoolTaskScheduler.scheduleAtFixedRate(task, 2000);

Based on the scheduler, the execute() method is triggered every two seconds. This method first collects all previous Candlestick for 15 minutes and calculates the green and red Candlestick. Based on this, it will buy or sell.

private double quantity = 20.0;
private String symbol = NEXO-A84_BNB; 
public void execute() {
        List<Candlestick> candleSticks = binanceDexApiRestClient.getCandleStickBars(this.symbol, CandlestickInterval.FIFTEEN_MINUTES);
        List<Candlestick> lastThreeElements = candleSticks.subList(candleSticks.size() - 4, candleSticks.size() - 1);
        // check if last three candlesticks are all red (close - open is negative)
        boolean allRed =
                .filter(cs -> Double.parseDouble(cs.getClose()) - Double.parseDouble(cs.getOpen()) < 0.0d).count() == 3;
        // check if last three candlesticks are all green (close - open is positive)
        boolean allGreen =
                .filter(cs -> Double.parseDouble(cs.getOpen()) - Double.parseDouble(cs.getClose()) < 0.0d).count() == 3;
        Wallet wallet = new Wallet(privateKey, binanceDexEnvironment);

        // open and closed orders required to check last order creation time
        OrderList closedOrders = binanceDexApiRestClient.getClosedOrders(wallet.getAddress());
        OrderList openOrders = binanceDexApiRestClient.getOpenOrders(wallet.getAddress());

        // order book required for buying and selling price
        OrderBook orderBook = binanceDexApiRestClient.getOrderBook(symbol, 5);
        Account account = binanceDexApiRestClient.getAccount(wallet.getAddress());

        if ((openOrders.getOrder().isEmpty() || openOrders.getOrder().get(0).getOrderCreateTime().plusMinutes(45).isBeforeNow()) &amp;&amp; (closedOrders.getOrder().isEmpty() || closedOrders.getOrder().get(0).getOrderCreateTime().plusMinutes(45).isBeforeNow())) {
            if (allRed) {
                if (Double.parseDouble(account.getBalances().stream().filter(b -> b.getSymbol().equals(symbol.split("_")[1])).findFirst().get().getFree()) >= (quantity * Double.parseDouble(orderBook.getBids().get(0).getPrice()))) {
                    order(wallet, symbol, OrderSide.BUY, orderBook.getBids().get(0).getPrice());
                    System.out.println("Buy Order Placed  Quantity:" + quantity + "  Symbol:" + symbol + "  Price:" + orderBook.getAsks().get(0).getPrice());
                } else {
                    System.out.println("do not have enough Token: " + symbol + " in wallet for buy");

            } else if (allGreen) {
                if (Double.parseDouble(account.getBalances().stream().filter(b -> b.getSymbol().equals(symbol.split("_")[0])).findFirst().get().getFree()) >= quantity) {
                    order(wallet, symbol, OrderSide.SELL, orderBook.getAsks().get(0).getPrice());
                    System.out.println("Sell Order Placed  Quantity:" + quantity + "  Symbol:" + symbol + "  Price:" + orderBook.getAsks().get(0).getPrice());
                } else {
                    System.out.println("do not have enough Token:" + symbol + " in wallet for sell");

            } else System.out.println("do nothing");
        } else System.out.println("do nothing");


    private void order(Wallet wallet, String symbol, OrderSide orderSide, String price) {
        NewOrder no = new NewOrder();

        TransactionOption options = TransactionOption.DEFAULT_INSTANCE;

        try {
            List<TransactionMetadata> resp = binanceDexApiRestClient.newOrder(no, wallet, options, true);
  "TransactionMetadata", resp);
        } catch (Exception e) {
            log.error("Error occurred while order", e);

At first glance, the strategy looks really simple, I agree. After this initial setup, however, it’s easy to add more complex logic with some AI.


Since 12th December 2019, this bot is running on Google Cloud and did 1130 transactions (buy/sell) until 14th April 2020. Initially, I started this bot with 2.6 BNB. On 7th February 2020, the balance was 2.1 BNB in the wallet, but while writing this blog on 14th April 2020, it looks like the bot has recovered the loss and the balance is 2.59 BNB. Hopefully, in future it will make some profit💰🙂.

Let me know your suggestions in a comment on this bot and I would also like to answer your questions if you have anything on this topic. Thanks for the time.

JHipster with Google App Engine and Cloud MySQL

Reading Time: 5 minutes

How does it sound to set up a complete spring application, with front-end and database? With all the models, repositories and controllers? Even with Unit and Integration tests, with mocked data? All within a few hours? Your solution is JHipster!


JHipster or “Java Hipster” is a handy application generator, a development platform, to develop and deploy web applications. JHipster has become popular in a short time, and it has been featured in many conferences all around the globe – Montreal, Omaha, Taipei, Richmond, Frankfurt, Paris, London. It supports:

  • Spring Boot (Back-end)
  • Angular/React/Vue (Front-end)
  • Spring microservices

JHipster is used for generating complete applications, it will create for you a Spring Boot and Angular/React/Vue application, high-quality application with most of the things pre-configured, using Java as back-end technology and an extensive set of Spring technologies: Spring Security, Spring Boot, Spring MVC (providing a framework for web-sockets, REST and MVC), Spring Data, etc. and Angular/React/Vue front-end and a suite of pre-configured development tools like Yeoman, Maven, Gradle, Grunt, Gulp.js and Bower.

JHipster gives you a head start in creating Spring Boot application with a set of pre-defined screens for user management, monitoring, and logging. The generated Spring Boot application is specifically tailored to make working with Angular/React/Vue a smoother experience. At the top of all that, JHipster also gives you the tools to update, manage and package the resulting application.

By now you may think it sounds too good to be true… But it is not everything that JHipster offers. If you are a web developer, by now probably you have a lot of questions. 🙂
One important question we will answer in this blog post: is it supported by today’s cloud solutions, is it compatible with all of them? The answer is yes, it is compatible with the popular cloud solutions from Google, Amazon, Microsoft, and Heroku. Let’s see what it takes to make a complete integration in Google’s cloud platform, the app engine.

Compatibility Test - NEXCOM

Google App Engine

Google App Engine is a cloud solution provided by Google, a platform for developing and hosting web applications in data centres managed by Google; Platform as a Service (PaaS). Applications are sandboxed and run across multiple servers. The App Engine supports Java or Python, uses the Google query language and stores data in Google BigTable.

It is free of usage up to a certain amount of resource usage. After the user is exceeding the limited usage rates for storage, CPU resources, requests or number of API calls and concurrent requests can pay for more of these resources.

It is fully compatible with the JHipster generated projects. What it takes to host your application is just to follow the official how-to guide from Google App Engine documentation, as normal Spring Boot Application. To make things easier, Google offers a database which works closely with the Google App Engine, the Cloud SQL.

Cloud SQL

Cloud SQL is a database service offered by Google for their cloud solutions, fully-managed that makes it easy to configure, manage, maintain, and operate your relational databases on Google Cloud Platform.

It offers three database options to integrate with:

  • MySQL
  • PostgreSQL
  • SQL Server

Let’s get into details of integrating with Cloud SQL for MySQL:

  1. The first step is to create a Cloud SQL instance on the Google Cloud Platform, which requires few things like instance ID, password and etc. to be set and it gives you the option to choose the MySQL database version.
  2. The following step is to create the database in the newly created instance. It is possible to have more databases in one instance.
  3. Now, our application, in the case to be able to communicate with the Cloud SQL, without any permission blockers, we need to register the application in the Cloud SQL and manually configure the service account roles.
  4. The final step is connecting your application to the created Cloud SQL instance. It is done through JDBC. All the required properties can be found in the overview of the Cloud SQL, instance connection name, credentials and etc.

So the conclusion: don’t be afraid to invest some time in new technologies, be curious, you never know where they may lead you. Thank you for reading. 🙂

JHipster, is it worth it?

Reading Time: 7 minutes

JHipster is an open-source platform to generate, develop and deploy Spring Boot + Angular / React / Vue web applications. And with over 15 000 stars on Github, it is the most popular code generation framework for Spring Boot. But is it worth the hype or is the generated code too difficult to maintain and not production-ready?

How does it work?

The first thing to note is that JHipster is not a separate framework by itself. It uses yeoman and .jdl files in order to generate code in Spring Boot for backend and Angular or React or Vue for frontend. And after the initial generation of the project, you have the option to use the generated code without ever running JHipster commands again or to use JHipster in order to incrementally grow the projects and develop new features.

What exactly is JDL?

JDL is a JHipster-specific domain language where you can describe all your applications, deployments, entities and their relationships in a single file (or more than one) with a user-friendly syntax.

You can use our online JDL-Studio or one of the JHipster IDE plugins/extensions, which support working with JDL files.

Example of simple JDL file for Blog application:

entity Blog {
  name String required minlength(3)
  handle String required minlength(2)

entity Post {
  title String required
  content TextBlob required
  date Instant required

entity Tag {
  name String required minlength(2)

relationship ManyToOne {
  Blog{user(login)} to User
  Post{blog(name)} to Blog

relationship ManyToMany {
  Post{tag(name)} to Tag{entry}

paginate Post, Tag with infinite-scroll

Which technologies are used?

On the backend we have the following technologies:

  • Spring Boot as the primary backend framework
  • Maven or Gradle for configuration
  • Spring Security as a Security framework
  • Spring MVC REST + Jackson for REST communication
  • Spring Data JPA + Bean Validation for Object Relational Mapping
  • Liquibase for Database updates
  • MySQL, PostgreSQL, Oracle, MsSQL or MariaDB as SQL databases
  • MongoDB, Counchbase or Cassandra as NoSQL databases
  • Thymleaf as a templating engine
  • Optional Elasticsearch support if you want to have search capabilities on top of your database
  • Optional Spring WebSockets for Web Socket communication
  • Optional Kafka support as a publish-subscribe messaging system

On the frontend side these technologies are used:

  • Angular or React or Vue as a primary frontend framework
  • Responsive Web Design with Twitter Bootstrap
  • HTML5 Boilerplate compatible with modern browsers
  • Full internationalization support
  • Installation of new JavaScript libraries with NPM
  • Build, optimization and live reload with Webpack
  • Testing with Jest and Protractor
  • Optional Sass support for CSS design

How to get started?

  1. Pre-requirements: JavaGit and Node.js.
  2. Install JHipster npm install -g generator-jhipster
  3. Create a new directory and go into it mkdir myApp && cd myApp
  4. Run JHipster and follow instructions on the screen jhipster
  5. Model your entities with JDL Studio and download the resulting jhipster-jdl.jh file
  6. Generate your entities with jhipster import-jdl jhipster-jdl.jh
  7. Run ./mvnw to start generated backend
  8. Run npm start to start generated frontend with live reload support

How does the generated code and application look like?

In case you only want to see a sample generated application without starting the whole framework you can check this official Github repo for the latest up-to-date sample code:

Following are some screen from my up and running JHipster application:

Welcome screen jhipster homepageThis is the initial screen when you open your JHipster app

Create a user screenjhipster user create screenWith this form you can create a new user in the app

View all users screenjhipster user management screenIn this screen you have the option to manage all your existing users

Monitoring of your JHipster application screenjhipster monitoring screenMonitoring of JVM metrics, as well as HTTP requests statistics

What are the pros and cons

The important thing to remember is that JHipster is not a “magic bullet” that will solve all your problems and is not an optimal solution for all the new projects. As a good software engineer, you will have to weigh in the pros and cons of this platform and decide when it makes sense to use and when it’s better to go with a different approach. Having used JHipster for production projects these are some of the pros and cons that I’ve experienced:


  • Easy bootstrap of a new project with a lot of technologies preconfigured
  • JHipster almost always follows best practices and latest trends in backend and frontend development
  • Login, register, management of users and monitoring comes out-of-the-box
  • Wizard for generating your project, only the technologies that you select are included in the project
  • After defining your own JDL file, all of the models, repository, service and controllers classes for your entities are generated, together with integration tests. This is saving a lot of time in the begging of the project when you want to get to feature development as soon as possible


  • If you are not familiar with technologies that are being used in the generated project it can be overwhelming and it’s easy to get lost into this mix of lots of different technologies
  • Using JHipster after the initial project is not a smooth experience. Classes and Liquibase scripts are being overwritten and you have to be very careful with changing the initial JDL model. Or you can decide to continue without using JHipster after the initial generation of projects
  • REST responses that are returned from endpoints will not always correspond to business requirements, very often you will have to manually modify your initial JHipster REST responses
  • Not all of the options that are available are at the same level, some technologies that JHipster is using and configuring are more polished than the others. Especially true if you decide to use community modules

What kind of projects are a good fit?

Having said all of this, it’s important to understand that there are projects which can benefit a lot from JHipster and projects that are better without using this platform.

In my experience, a good candidate is a greenfield project where it’s expected to deliver a lot of features fast. JHipster will help a lot to be productive from day one and to cut on the boilerplate code that you need to write. So you will be able, to begin with, feature development really fast. This works well with new projects with tight deadlines, proof of concepts, internal projects, hackathons, and startups.

On the other hand, a not so ideal situation is if you have an already started and up and running project, there is not much a JHipster can do in this case. Or another case would if the application has a lot of specific business logic and its not a simple CRUD application, for example, an AI project, a chatbot or a legacy ecosystem where these new technologies are not suitable or supported.

JHipster, is it worth it?

There is only one sure way to decide if JHipster is worth it for your next project or not and that is to try it out yourself and play around with the different features and configuration that JHipster offers.

At best, you will find a new framework for your next project and save a lot of effort next time you have to start a project. At worst, you will get to know the latest trends in both backend and frontend and learn some of the best practices from a very large community.

Testing asynchronous code in a concise and easy to read manner

Reading Time: 7 minutes

We live in a fast-paced world where a standard project delivery strategy is agile or it is a direction which people tend to follow. If you have been part of an agile software delivery practice then somewhere in your coding career you have met with some form of tests. Whether they might be unit or integration ( system ) or some form of E2E test.

You might be familiar with the testing pyramid and with the benefits and scopes of the different types of tests presented in the pyramid.

Let’s take a quick look at the pyramid:


As shown on the image above tests that we write are grouped into layers from which the pyramid is built. The foundation layer which is the biggest. It shows us their quantity. Meaning we need more of them on our application. They are also called Unit Tests because of the scope which they are testing. A small unit e.g. an if clause.


The tests belonging to the middle layer are called Integration tests and their purpose is to test integration between one or more elements inside an application and in quantitative representation we need fewer tests of this type than Unit tests.


The last layer is the smallest one meaning that the quantity of those tests should be the smallest. Those types of tests are also called UI or E2E tests. Here a test has the biggest scope meaning that it is checking more interconnected parts of your application i.e whole register scenario from UI perspective.

As we go from the bottom to the top costs for maintenance are increasing, respectively their speed is decreasing. Confidence is also a crucial part. If a test higher in the pyramid passes we are more confident that our application works or some part of it at least.

Our focus is on the middle layer. So-called Integration tests lay there. As we mentioned above those are the tests that check the interconnection between one or more modules inside an application e.g tests which check that a user can be registered by pinging an endpoint. The scope of this test is to prepare data, send a request to the corresponding endpoint and also check whether the user has been successfully created in the underlying datastore. Testing integration between controller and repository layer, therefore, their name “An integration test”.
In my opinion, I think that tests are a must-have for every application.

Therefore we are writing integration tests for asynchronous code.

With multi-threaded data processing systems and increased popularity of reactive programming in Java, we are puzzled with writing proficient tests for asynchronous code.
Writing high-value tests is hard, but writing high-value tests for asynchronous code is harder.


Let’s take a look at this example where we have a small system that exposes several endpoints for updating a person. We have created several tests each is updating a person with different names. When a test is running it tries to update a person by sending a request via an endpoint. The system receives the request and returns ok status. In the meantime, it spans a different thread for the actual person update. On the side of the tests, we don’t know how much time does it gonna take for the update to happen so the naive approach is to wait for a specific time after which we are going to verify whether the actual update has happened.

We have several tests which ping a different endpoint. The endpoints are differing in the wait time that would be needed to process each request

In order for our tests to pass, I used the naive approach by adding a function waitForCompetition() which is nothing else than some sleep of the test thread. Thread.sleep() in Java.


The first execution of tests with a timeout of 1 second. The total execution is 4 seconds but not all tests have passed.

The second execution of tests with a timeout of 3 seconds. The total execution is 12 seconds but not all tests have passed.

Third execution of tests with a timeout of 5 seconds. The total execution is 20 seconds where all tests have passed.

But in order for all the tests to pass, we would need a max of 5-second sleep wait which is executed after each test. This way we are guaranteeing that every test will pass. However, we add an unnecessary wait of 4 seconds for the first test and respectively add wait time for other tests. This results increased execution time, hence optimum wait time is not guaranteed.


As stated in the official documentation Awaitility is a small java library for synchronizing asynchronous operation. Which helps expressing expectations in a concise and easy to read manner. Which is a smart option for checking the outcome of some async operation.
It’s fairly easy to incorporate this library into your codebase.

You just need to add the library into pom.xml:


And add the import in your test:
import static org.awaitility.Awaitility.await;

Let’s take a look at an example before using this library:

    public void testDelay1Second() throws Exception {
        Person person = new Person();
        person.setAddress("New York");

        ObjectMapper mapper = new ObjectMapper();


        this.mockMvc.perform(put("/api/endpoint1/" + person.getId())
                .andExpect(content().string(containsString("Request received")));


An example with Awaitility:

    public void testDelay1Second() throws Exception {
        Person person = new Person();
        person.setAddress("New York");

        ObjectMapper mapper = new ObjectMapper();


        this.mockMvc.perform(put("/api/endpoint1/" + person.getId())
                .andExpect(content().string(containsString("Request received")));

        await().atMost(Duration.FIVE_SECONDS).untilAsserted(() -> assertThat(personRepository.findById(person.getId()).get().getName())

Example of the executed test suite with the library:

As we can see the execution time is greatly reduced from 20 seconds for all tests to pass in just under 10 seconds.
As you can spot the function waitForCompletition() is removed and a new wait is introduced from the library as await().atMost(Duration.FIVE_SECONDS).untilAsserted()

You can also configure the library using static methods from the Awaitility class:
Awaitility.setDefaultPollInterval(10, TimeUnit.MILLISECONDS);


In this article, we have taken a look at how to improve tests when dealing with asynchronous code using an interesting library. I hope this post helps benefit you and adds to your knowledge. You can find a working example with all of the tests with and without the Awaitility library on this repository.
Also, you can find more about the library here.

ReactiveX in Android with an example – RxJava

Reading Time: 5 minutes

What is Reactive Programming?

Reactive programming is programming with asynchronous data streams. It enables to create streams of anything – events, fails, variables, messages and etc. By using reactive programming in your application, you are able to create streams which you can then perform actions while the data emitted by those created streams.

Observer Pattern

The observer pattern is a software design pattern which defines a one-to-many relationship between objects. It means if the value/state of the observed object is changed/modified, the other objects which are observing are getting notified and updated.


ReactiveX is a polyglot implementation of reactive programming which extends observer pattern and provides a bunch of data manipulation operators, threading abilities.


RxJava is the JVM implementation of ReactiveX.

  • Observable – is a stream which emits the data
  • Observer – receives the emitted data from the observable
    • onSubscribe() – called when subscription is made
    • onNext() – called each time observable emits
    • onError() – called when an error occurs
    • onComplete() – called when the observable completes the emission of all items
  • Subscription – when the observer subscribes to observable to receive the emitted data. An observable can be subscribed by many observers
  • Scheduler – defines the thread where the observable emits and the observer receives it (for instance: background, UI thread)
    • subscribeOn(
    • observeOn(AndroidSchedulers.mainThread())
  • Operators – enable manipulation of the streamed data before the observer receives it
    • map()
    • flatMap()
    • concatMap() etc.

Example usage on Android

Tools, libraries, services used in the example:

  • Libraries:
    • ButterKnife – simplifying binding for android views
    • RxJava, RxAndroid – for reactive libraries
    • Retrofit2 – for network calls
  • Fake online rest API:
  • Java object generator from JSON file

What we want to achieve is to fetch users from 1. show in RecyclerView and load todo list to show the number of todos in the same RecyclerView without blocking the UI.

Here we define our endpoints. Retrofit2 supports return type of RxJava Observable for network calls.

    Observable<List<User>> getUsers();

    Observable<List<Todo>> getTodosByUserID(@Path("id") int id);

    Observable<List<Todo>> getTodos();

Let’s fetch users:

  • .getUsers – returns observable of a list of users
  • .subscribeOn( – make getUser() performs on background thread
  • .observeOn(AndroidSchedulers.mainThread()) – we switch to UI thread
  • flatMap – we set data to RecyclerView and return Observable user list which will be needed in fetching todo list
    private Observable<User> getUsersObservable() {
        return ServicesProvider.getDummyApi()
                .flatMap((Function<List<User>, ObservableSource<User>>) users -> {
                    return Observable.fromIterable(users);

Now, fetch todo list of users using the 2nd endpoint.

Since we are not going to make another call, we don’t need Observable type in return of this method. So, here we use map() instead of flatMap() and we return User type.

    private Observable<User> getTodoListByUserId(User user) {
        return ServicesProvider.getDummyApi()
                .map(todoList -> {
                    return user;

Now, fetch todo list of users using the 3rd endpoint.

The difference to the 2nd endpoint is that this returns a list of todos for all users. Here we can see the usage of filter() operator.

    private Observable<User> getAllTodo(User user) {
        return ServicesProvider.getDummyApi()
                .flatMapIterable((Function<List<Todo>, Iterable<Todo>>) todoList -> todoList)
                .filter(todo -> todo.getUserId().equals(user.getId()) &amp;&amp; todo.getCompleted())
                .map(todoList -> {
                    return user;
  • .flatMapIterable() – is used to convert Observable<List<T>> to Observable<T> which is needed for filter each item in list
  • .filter() – we filter todos to get each user’s completed todo list
  • .toList().toObservable() – for converting back to Observable<List<T>>
  • .map() – we set filtered list to user object which will be used in next code snippet

Now, the last step, we call the methods:

                .concatMap((Function<User, ObservableSource<User>>) this::getTodoListByUserId) // operator can be concatMap()
                .subscribe(new Observer<User>() {
                    public void onSubscribe(Disposable d) {

                    public void onNext(User user) {

                    public void onError(Throwable e) {
                        Log.e(TAG, e.getMessage());

                    public void onComplete() {
                        Log.d(TAG, "completed!");
  • subscribeOn() – makes the next operator performed on background
  • concatMap() – here we call one of our methods getTodoListByUserId() or getAllTodo()
  • .observeOn(), .subscribe() – every time the user’s todo list is fetched from api in background thread, it emits the data and triggers onNext() so we update RecyclerView in UI thread
  • Left
    • getTodoListByUserId()
    • flatMap()
  • Right
    • concatMap()
    • getAllTodo() – filter usage

Difference between flatMap and concatMap is that the former is done in an arbitrary order but the latter preserves the order


When an observer subscribes to an observable, a disposable object is provided in onSubscribe() method so it can later be used to terminate the background process to avoid it returning from callback to a dead activity.

private CompositeDisposable disposables = new CompositeDisposable();

observableobject.subscribe(new Observer() {
    public void onSubscribe(Disposable d) {

protected void onDestroy() {


In this post, I tried to give brief information about reactive programming, observer pattern, ReactiveX library and a simple example on android.

Why should you use RxJava in your projects?

  • less boilerplate code
  • easy thread management
  • thread-safety
  • easy error handling

Gitlab Repository

Example sourcecode:


Project Lombok explained

Reading Time: 4 minutes

In this article, I want to present a very powerful tool called Project Lombok. It acts as an annotation processor that allows us to modify the classes at compile time. Project Lombok enables us to reduce the amount of boilerplate code that needs to be written. The main idea is to give the users an option to put annotation processors into the build classpath.

Add Project Lombok to the project

  • using gradle
 compileOnly "org.projectlombok:lombok:1.16.16"
  • using maven

Project Lombok

Project Lombok provides the following annotations:

  • @Getter and @Setter: create getters and setters for your fields
  • @EqualsAndHashCode: implements equals() and hashCode()
  • @ToString: implements toString()
  • @Data: uses the four previous features
  • @Cleanup: closes your stream
  • @Synchronized: synchronize on objects
  • @SneakyThrows: throws exceptions
    and many more. Check the full list of available annotations:

Common object methods

In this example, we have a class that represents User and holds five attributes, for which we want to have an additional constructor for all attributes, toString representation, getters, and setters and overridden equals and hashCode in terms of the email attribute:

 private String email;
    private String firstName;
    private String lastName;
    private String password;
    private int age;

    // empty constructor
    // constructor for all attributes
    // getters and setters
    // toString
    // equals() and hashCode()

With some help from Lombok, the class now looks like this:

import lombok.AllArgsConstructor;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.ToString;

@EqualsAndHashCode(of = {"email"})
public class User {

    private String email;
    private String firstName;
    private String lastName;
    private String password;
    private int age;

As you can see, the annotations are replacing the boilerplate code that needs to be written for all the fields, constructor, toString, etc. The annotations do the following:

  • using @Getter and @Setter Lombok is instructed to generate getters and setters for all attributes
  • using @NoArgsConstructor and @AllArgsConstructors Lombok created the default empty constructor and an additional one for all the attributes
  • using @ToString generates toString() method
  • using @EqualsAndHashCode we get the pair of equals() and hashCode() methods defined for the email field (Note that more than one field can be specified here)

Customize Lombok Annotations

We can customize the existing example with the following:

  • in case we want to restrict the visibility of the default constructor we can use AccessLevel.PACKAGE
  • in case we want to be sure that the method fields won’t get null values assigned to them, we can use @NonNull annotation
  • in case we want to exclude some property from toString generated code, we can use excludes argument in @ToString annotation
  • we can change the access level of the setters from public to protected with AccessLevel.PROTECTED for @Setter annotation
  • in case we want to do some kind of checks in case the field gets modified we can implement the setter method by yourself. Lombok will not generate the method because it already exists

Now the example looks like the following:

import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.NonNull;
import lombok.Setter;
import lombok.ToString;

@NoArgsConstructor(access = AccessLevel.PACKAGE)
@ToString(exclude = {"age"})
@EqualsAndHashCode(of = {"email"})
public class User {

    private @NonNull String email;
    private @NonNull String firstName;
    private @NonNull String lastName;
    private @NonNull String password;
    private @NonNull int age;

    protected void setEmail(String email) {
        // Check for null and valid email code = email;

Builder Annotation

Lombok offers another powerful annotation called @Builder. Builder annotation can be placed on a class, or on a constructor, or on a method.

In our example, the User can be created using the following:

User user = User


Looking at our example the code can be further improved. If we want to follow the rule of composition over inheritance, we can create a new class called ContactInformation. The object can be modelled via an interface:

public interface HasContactInformation {
    String getEmail();
    String getFirstName();
    String getLastName();

The class can be defined as the following:

public class ContactInformation implements HasContactInformation {

    private String email;
    private String firstName;
    private String lastName;

In the end, our User example will look like the following:

import lombok.AccessLevel;
import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.NonNull;
import lombok.Setter;
import lombok.ToString;
import lombok.experimental.Delegate;

@NoArgsConstructor(access = AccessLevel.PACKAGE)
@ToString(exclude = {"password"})
@EqualsAndHashCode(of = {"contactInformation"})
public class User implements HasContactInformation {

    @Delegate(types = {HasContactInformation.class})
    private final ContactInformation contactInformation = new ContactInformation();

    private @NonNull byte[] password;

    private @NonNull Instant registrationTs;

    private boolean payingCustomer = false;


This article covers some basic features and there is a lot more that can be investigated. I hope you have found a motivation to give Lombok a chance in your project if you find it applicable.

CQRS and Event Sourcing with Axon Framework

Reading Time: 5 minutes

What is CQRS?

CQRS stands for Command Query Responsibility Segregation. It is a pattern where allowed actions in the application are divided into two groups: Commands and Queries. A command changes the state of an object but does not return any data, while a query returns data and does not change any state. This design style comes from the need for different strategies when scaling the reading part and the writing part of our application. By dividing methods into these two categories, you can use a different model to update information than the model you use to read information. By separate models we most commonly mean different object models, probably running in different logical processes, perhaps on separate hardware. A web example would see a user looking at a web page that’s rendered using the query model. If they initiate a change that change is routed to the separate command model for processing, the resulting change is communicated to the query model to render the updated state.

Event Sourcing

Event Sourcing is a specialized pattern for data storage. Instead of storing the current state for an entity, every change of state is stored as a separate event that makes sense to a business user. The current state is calculated by applying all events that changed the state of an entity. In terms of CQRS, the events stored are the results of executing a command against an aggregate on the right side. The event store also transmits events that it saves. The read side can process these events and builds the targeted data sets it needs for queries.

AXON Framework

Axon is “CQRS Framework” for Java. It is an Open-source framework that provides the building blocks that CQRS requires and helps to create scalable and extensible applications while maintaining application consistency in distributed systems. It provides basic building blocks for writing aggregates, commands, queries, events, sagas, command handlers, event handlers, query handlers, repositories, communication buses and so on. Axon Framework comes with a Spring Boot Starter, making using it as easy as adding a dependency. Axon will automatically configure itself based on best practices and other dependencies you have set. Providing explicit configuration is a matter of adding a bean of the component that needs to be configured differently. Furthermore, Axon Framework integrates with Spring Cloud Discovery, Spring Messaging and Spring Cloud AMQP.

AXON components

  • Command – a combination of expressed intent (which describes what you want to be done) as well as the information required to undertake action based on that intent. The Command Model is used to process the incoming command, to validate it and define the outcome. Commands are typically represented by simple and straightforward objects that contain all data necessary for a command handler to execute it
  • Command Bus – receives commands and routes them to the Command Handlers
  • Command Handler – the component responsible for handling commands of a certain type and taking action based on the information contained inside it. Each command handler responds to a specific type of command and executes logic based on the contents of the command
  • Event bus – dispatches events to all interested event listeners. This can either be done synchronously or asynchronously. Asynchronous event dispatching allows the command execution to return and hand over control to the user, while the events are being dispatched and processed in the background. Not having to wait for event processing to complete makes an application more responsive. Synchronous event processing, on the other hand, is simpler and is a sensible default. By default, synchronous processing will process event listeners in the same transaction that also processed the command
  • Event Handler – are the components that act on incoming events. They typically execute logic based on decisions that have been made by the command model. Usually, this involves updating view models or forwarding updates to other components, such as 3rd party integrations
  • Query Handler – components act on incoming query messages. They typically read data from the view models created by the Event listeners
  • Query Bus receives queries and routes them to the Query Handlers. A query handler is registered at the query bus with both the type of query it handles as well as the type of response it provides. Both the query and the result type are typically simple, read-only DTO objects

Will each application benefit from Axon?

Unfortunately not. Simple CRUD (Create, Read, Update, Delete) applications which are not expected to scale will probably not benefit from CQRS or Axon. Fortunately, there is a wide variety of applications that do benefit from Axon.
Axon platform is being used by a wide range of companies in highly demanding sectors such as healthcare, banking, insurance, logistics and public sector. Axon Framework is free and the source code is available in a Git repository: