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Spring WebFlux: A Basic CRUD Application (Part 2)

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Spring WebFlux: A Basic CRUD Application (Part 2)

We continue building out our Spring web application by refactoring our code in order to implement a MongoDB database and populate that database.

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In Part 2 of this series, we will refactor the application written in Part 1 in order to use a database. We will take a short look at the choices we have when selecting a database in combination with Spring WebFlux, use an embedded version of the database, refactor the sources and find solutions for the problems we encounter. The code can be found at GitHub in branch mongodb.

Select a Database

When we take a look at Spring Initializr, we notice that only NoSQL database have reactive support. At the time of writing these are:

  • Reactive Redis
  • Reactive MongoDB
  • Reactive Cassandra
  • Reactive Couchbase

Why do only NoSQL databases have reactive support and not the traditional relational databases? The answer is quite simple: relational databases make use of JDBC and JDBC is a blocking API. Also, we use database transactions and these block resources which is not very reactive. However, there are some initiatives for developing an async driver for relational databases. But for now, only NoSQL databases are officially supported because they fit best in the reactive world. As a consequence, there is no reactive support for Hibernate or JPA.

In our example, we will choose Reactive MongoDB and, more specifically, for the embedded version, as this will allow us to run and test the application without needing to install MongoDB.

In the following sections, we will make small refactor steps and check whether the application still compiles and starts without any errors.

Adapt pom.xml

First of all, we are going to remove the dependency on spring-data-commons. We added this dependency in order to make use of the ReactiveCrudRepository interface.


Next, we add a dependency for Reactive MongoDB which includes spring-data-mongodb and the reactive driver:


In order to make use of embedded MongoDB, we add the dependency on Embedded MongoDB:


Run the application with spring-boot:run and the application still starts successfully.

Adapt the Repository

For the repository, we can now make use of the ReactiveMongoRepository interface. Our code was the following:

public class ReactiveShowRepository implements ReactiveCrudRepository<Show, String> {
  // a lot of code

And now becomes:

public interface ReactiveShowRepository extends ReactiveMongoRepository<Show, String> {


So, we just extend the interface and then, automagically, our repository works? Yes, it does. Spring Boot will automatically plug in an implementation of the ReactiveShowRepository based on the class SimpleReactiveMongoRepository at runtime. This class provides the most common CRUD methods.

We also added the following to the module-info.java:

requires spring.data.mongodb;

Run the application, our URL, http://localhost:8080/, returns nothing, which we expected because we have nothing in our database.

Adapt the Show Domain Object

We annotate our Show domain object as a Document and remove the constructor we made.

public class Show {
  private String id;
  private String title;

Fill the Database

Everything is up-and-running now, but we need some data in our database. In order to do so, I will create a class DataImportConfiguration as is demonstrated in the Spring Boot webinar by Phil Webb. This class reads a YAML file which contains the data, converts it into properties, and converts it into a list of Show objects. When the application is started, the data is injected into the database.

Because we are using the YamlPropertiesFactoryBean, we also add the dependency on spring.beans in our module-info.java.

The DataImportConfiguration class is the following:

public class DataImportConfiguration {

  public CommandLineRunner initData(MongoOperations mongo) {
    return (String... args) -> {

  private List<Show> getShows() {
    Properties yaml = loadShowsYaml();
    MapConfigurationPropertySource source = new MapConfigurationPropertySource(yaml);
    return new Binder(source).bind("shows", Bindable.listOf(Show.class)).get();

  private Properties loadShowsYaml() {
    YamlPropertiesFactoryBean properties = new YamlPropertiesFactoryBean();
    properties.setResources(new ClassPathResource("shows.yml"));
    return properties.getObject();


We create the following show.yml file into our resources directory:

- title: "Title 1"
- title: "Title 2"
- title: "Title 3"
- title: "Title 4"
- title: "Title 5"

Run the application and invoke http://localhost:8080/shows. The following output is shown in our browser:

[{"id": "5aad24d7c568b82764d592e8","title": "Title 1"},
{"id": "5aad24d7c568b82764d592e9","title": "Title 2"},
{"id": "5aad24d7c568b82764d592ea","title": "Title 3"},
{"id": "5aad24d7c568b82764d592eb","title": "Title 4"},
{"id": "5aad24d7c568b82764d592ec","title": "Title 5"}]

Invoke the URL http://localhost:8080/shows/5aad24d7c568b82764d592e8 in order to retrieve the data for one show. The output is:

"id": "5aad24d7c568b82764d592e8",
"title": "Title 1"

And we can invoke the URL for retrieving events for a show: http://localhost:8080/shows/5aad24d7c568b82764d592e8/events. Every second an event is added to the output:






At this point, we have refactored our application in order to use embedded MongoDB and our endpoints are still working as expected.

Adapt the Unit Test

The last thing to check is whether our unit test is still working. Run the unit test MySpringWebfluxCrudPlanetApplicationTests from within your IDE. I would have expected that the unit test was still working since we didn't change anything to the application logic. However, 100 errors were displayed! Below is a snippet, but all the given errors were similar to those shown below:

Error:java: the unnamed module reads package com.mongodb from both mongodb.driver.core and mongodb.driver
Error:java: the unnamed module reads package com.mongodb.client from both mongodb.driver.core and mongodb.driver

After some investigating, it turns out that these errors occur because we are using Java 9 modules. Disable the usage of Java 9 modules by renaming the module-info.java to module-info.java_ and run the test again. The test result is now successful.

So, what happened? The reason for the error is called 'split packages': two packages with the same name exist in different modules and this is not allowed, except for unnamed modules.

Let's take a look at the problem. The mongodb-driver-3.6.3.jar contains the following packages and we notice that this jar does not have a module descriptor, thus an automatic module is created.

No module descriptor found. Derived automatic module.

mongodb.driver@3.6.3 automatic
requires java.base mandated
contains com.mongodb
contains com.mongodb.client
contains com.mongodb.client.gridfs
contains com.mongodb.client.jndi
contains com.mongodb.client.model
contains com.mongodb.gridfs
contains com.mongodb.util
contains org.bson
contains org.bson.io
contains org.bson.types
contains org.bson.util

We get a similar output for the mongodb-driver-core-3.6.3.jar.

No module descriptor found. Derived automatic module.

mongodb.driver.core@3.6.3 automatic
requires java.base mandated
contains com.mongodb
contains com.mongodb.annotations
contains com.mongodb.assertions
contains com.mongodb.async
contains com.mongodb.binding
contains com.mongodb.bulk
contains com.mongodb.client
contains com.mongodb.client.gridfs.codecs
contains com.mongodb.client.gridfs.model
contains com.mongodb.client.model
contains com.mongodb.client.model.changestream
contains com.mongodb.client.model.geojson
contains com.mongodb.client.model.geojson.codecs
contains com.mongodb.client.result
contains com.mongodb.connection
contains com.mongodb.connection.netty
contains com.mongodb.diagnostics.logging
contains com.mongodb.event
contains com.mongodb.internal
contains com.mongodb.internal.async
contains com.mongodb.internal.authentication
contains com.mongodb.internal.connection
contains com.mongodb.internal.dns
contains com.mongodb.internal.event
contains com.mongodb.internal.management.jmx
contains com.mongodb.internal.session
contains com.mongodb.internal.thread
contains com.mongodb.internal.validator
contains com.mongodb.management
contains com.mongodb.operation
contains com.mongodb.selector
contains com.mongodb.session

We can see that the packages com.mongodb and com.mongodb.client are present in the automatic modules mongodb.driver and mongodb.driver.core. But, they are unnamed modules and, in that case, split packages are allowed. Therefore, it is strange that the errors occur when running the test from the IDE.

Running the test with a module-info.java and by means of the Maven target test is successful. Therefore, I guess something is wrong with my run configuration of IntelliJ, but I haven't found out what.


In this post, we refactored the basic CRUD application from Part 1 in order to use an embedded MongoDB database. We used the YamlPropertiesFactoryBean in order to initially fill the database and ran up against an issue when running the unit test.

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spring web flux ,reactive streams ,web dev ,java web development ,crud application

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